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REVIEW OF SCIENTIFIC INSTRUMENTS 87, 023107 (2016) Double-arm three-dimensional ion imaging apparatus for the study of ion pair channels in resonance enhanced multiphoton ionization M. S. Poretskiy,1,a) A. I. Chichinin,2 C. Maul,1 and K.-H. Gericke1 1 Institut für Physikalische und Theoretische Chemie, Technische Universität Braunschweig, 38106 Braunschweig, Germany 2 Institute of Chemical Kinetics and Combustion and Novosibirsk State University, 630090 Novosibirsk, Russia (Received 1 October 2015; accepted 21 November 2015; published online 18 February 2016) We present a novel experimental configuration for the full quantitative characterization of the multichannel resonance enhanced multiphoton ionization (REMPI) of small molecules in cases when the ion-pair dissociation channel is important. For this purpose, a double-arm time-of-flight mass spectrometer with three-dimensional (3D) ion imaging detectors at both arms is constructed. The REMPI of HCl molecules is used to examine the constructed setup. The apparatus allows us to perform simultaneous measurements of the 3D velocity vector distributions of positive (H+, HCl+, and Cl+) and negative (Cl−) photoions. The characterization consists of the determination of “two-photon absorption cross sections” for the process HCl(X)+2hν → HCl∗, one-photon absorption cross sections for subsequent processes HCl∗ + hν → HCl∗∗, and the probability of the subsequent non-adiabatic transition HCl∗∗ → HCl(B) → H+ + Cl−, which leads to ionic pairs. All these data should be obtained from the analysis of the dependencies of the number of ions on the laser energy. The full characterization of the laser beam and the knowledge of the ion detection probability are necessary parts of the analysis. Detailed knowledge of losses of produced ions in the mass spectrometer before detection requires understanding and characterization of such processes like electron emission from metallic grids under ion bombardment or charge transfer between positive ions and the metal surface of the grids, like Cl+ + (grid) → Cl−. These important phenomena from surface science are rarely discussed in the imaging literature, and here, we try to compensate for this shortcoming. C 2016 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4936984] I. INTRODUCTION The resonance enhanced multiphoton excitation of molecules with subsequent fragmentation, photodissociation, or photoionization has for decades been in the focus of interest of many laboratories as an effective method for studying high-lying excited states and for monitoring chemical reaction products. The resonance enhanced multiphoton ionization (REMPI) of diatomic molecules, HCl, for example, may occur in several ways, see Refs. 1–4 and references therein, nhν hν HCl(X) −−−→ HCl∗(IS1) −−→ HCl+ + e−, + hν HCl −−→ Cl + H+ + e−, hν (2) ∗ hν HCl∗(IS1) −−→ HCl∗∗(IS2) → H + Cl −−→ Cl+ + H + e−, hν → H∗ + Cl −−→ Cl + H+ + e−, + HCl(B) → Cl + H . − (1) (3) (4) (5) Here, the non-radiative transition between different electronic states is shown by a wave arrow, and the decays within a single electronic state are shown by simple arrows. In the simplest case, the molecule absorbs two photons (n = 2), reaches an excited intermediate state IS1, and is then photoionized, see channel (1). In more complicated cases, there are photoinduced transitions to superexcited states IS2 with a)Electronic mail: [email protected] subsequent decay into channels with excited atoms (3) and (4) or into anion–cation pairs, also called “ion-pair” channel (5). Hereafter, the initial intermediate states IS1 are referred to as “excited states,” and the states IS2 are called “superexcited states.” Sometimes the latter are also called “gateway” states which are normally unknown. In the HCl molecule, the excited state IS1 accessed in the first excitation step is frequently the electronic B state having a double-minimum potential energy curve, being a mixture of the Rydberg E1Σ+ state and the valence V1Σ+ ion-pair state. The general objective of our research in the recent years has been to determine all IS2 states and quantitatively characterize all radiative and non-radiative transitions (1)–(5). In H2, HX (X=F,Cl,Br), CH3Br, and several other molecules, an ionpair state has a double-well term with two minima at internuclear distances r 1 and r 2, where r 1 ≈ r e and r 2 ≫ r e , where r e is the ground state internuclear distance. We use multiphoton excitation, which opens the door for photoabsorption from the ion-pair B state at large distances ∼r 2, which may be a new kind of dynamics and/or spectroscopy, because the internuclear distances near r 2 are unavailable to one-photon excitation from the ground state normally used in ion-pair studies. Thus, the present study was partly stimulated by the interest in the absorption dynamics at large internuclear distances and partly, it is an attempt to characterize processes (1)–(5) quantitatively. In this study, we focus on channel (5) because the ion-pair formation is a very specific probe for superexcited states. A full understanding of the channel mechanism is necessary for a 0034-6748/2016/87(2)/023107/15/$30.00 87, 023107-1 © 2016 AIP Publishing LLC Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-2 Poretskiy et al. quantitative description of processes (1)–(5), and it should also help us to predict probabilities of ion-pair channels in other molecules. Despite the long history and the large number of photoion-pair studies, see reviews of Refs. 5–8, the mechanism of the ion-pair formation is still being discussed controversially.9,10 Generally, typical probabilities of ion-pair formation are rather small, 10−3 to 10−6, see review of Simpson.8 One mechanism of the ion-pair formation is a direct photoinduced transition from the ground state into the ion-pair state. In this case, the low probability of the ion-pair channel arises from the small Franck–Condon factors, resulting from a poor overlap of the nuclear wave functions of the molecular ground state and the ion-pair state.8 Another mechanism involves non-adiabatic transitions from superexcited states into the ion-pair state.6 In this case, the small probability for ion-pair formation arises from the small probability of the non-adiabatic transition. What is intriguing and stimulating the present study is the existence of rather large probabilities of the ion-pair channel in some cases. For example, for the two-photon transition in REMPI of HCl X 1Σ+, v = 0 → → V 1Σ+, v ′ = 12, the relative probability of the ion-pair channel (5) is close to 3%.1,4 For this reason, this transition normally serves as a benchmark in ion-pair studies. For simultaneous detection of positive and negative photoproducts, ions or electrons, a double-arm time-of-flight (TOF) mass spectrometer is normally used, where positive and negative photoions and electrons are accelerated in opposite directions by electric fields. While first experiments with such mass spectrometers were already done in the 1970s,11 they became really efficient only after the application of microchannel plates (MCPs).12–14 A variety of configurations of such mass spectrometers is known in the literature. Most of them are used to detect photoelectrons and positive photoions. They may use homogeneous electric fields (HFs),15–20 one-side velocity map imaging (VMI),14,21–36 or two-side VMI;29,31,34,35,37,38 delay line (DL) detectors,14–31,34–36 or charge-coupled device (CCD) cameras,32,33,39 or both;38 pulsed lasers,15,17–20,29,30,32,34–36,38 or discharge lamps,14,21–26 or quasi-continuous synchrotron radiation,28,31,36 or free-electron-laser radiation;38 simultaneous32,33 or coincidence14–19,21–23,26,28–31,34–36,38 detection; the ionization by one-photon absorption,14,20–26 or REMPI,34,35 or by the strong electric field of femtosecond laser pulses.15–20,28–31,33 Only a limited (and surely incomplete) set of references to the studies published in the last 15 years is shown here. The first observation of ionic pairs has been reported in 1932 in the pioneering work of Terenin and Popov.40 Since then, a lot of other ionic pairs have been observed, with the list given in the review of Berkowitz6 including TlX, X2, (X=F,Cl,Br,I), ClF, H2, O2, NO, CO, H2O, N2O, CO2, COS, SO2, KrF2, XeF2, C2H2, CH4, CH3F, CH3Cl, CH3Br, CF4, CF3Cl, CF2Cl2, CFCl3, C2H6, C3H8, C4H10, C2H5Cl, SF6, cyclo-C6H11Cl, and CH3ONO2. Additional molecules are listed in the review of Simpson,8 including CX4, C2X4, C3X6, C3X8, (X=H,F), CHnCl4−n, where n = 0–4, CFmCl4−m, where m = 1–3, CHF3, CH3Br, CF3Br, CF3I, SF6, SF5Cl, and SF5CF3. All these systems have been studied at the Daresbury Synchrotron Radiation Source between 2005 and 2008. Also, after 1996, ion-pair channels have been studied in the Rev. Sci. Instrum. 87, 023107 (2016) photodissociation of ICl,41 CH3Cl,42,43 CH3Br, C2H5Cl, and C2H5Br.43 Recently, one interesting molecule, the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([E+mim][Tf2N−]), was added to the lists by Koh and Leone.33 However, a simultaneous or coincidence detection of positive and negative photoions is rather rare. Our summary, which is almost surely incomplete, includes H2,44 N2O,45–47 CH3F,48 CH3Cl,39 SF6,49 OCS,CO2,50 CH4,CD4,51 CF4,52 CH3 X (X=F,Cl,Br),53 SO2,54 H2S,55 CH3Br,32 O2,56 and the ionic liquid [E+mim][Tf2N−].33 Ion-pair production is also used in threshold ion-pair production spectroscopy (TIPPS), where highly excited metastable Rydberg-like states are selectively detected using dissociation induced by a weak Stark field. TIPPS is used to investigate thermochemistry, spectroscopy, and superexcited state decay dynamics at high resolution. For example, it was successfully used to measure the dissociation energies of HCl, H2, D2, and H2S.55 In the present work, a double-arm mass spectrometer was constructed which allowed us to simultaneously measure the 3D velocity vector distributions of positive and negative ions. The detection of negative ions is a much more difficult task than might be expected initially. The main difficulty consists of a large number of negatively charged particles appearing at different times of flight. Main contributions are from the electron emission from metal grids under ion bombardment and from charge transfer between positive ions and the metal surface of the grids. What is intriguing is the absence of the description of these phenomena in the imaging literature, although double-arm mass spectrometers are not a novelty. Surely, all these phenomena are well known in the surface sciences. For example, the emission of negative ions under the bombardment of a surface by positive ions is used in secondary ion mass spectrometry, a technique developed in the 1960s, which is used in materials and surface sciences to analyze the composition of solid surfaces. Also, searching for negative ions from ion-pair channels without imaging capability is very difficult, almost impossible, mainly because of these secondary negative particles and also because of some other problems. For example, we found that the largest number of negative ions from channel (5) occurs at some optimal laser energy, with their number not growing monotonically with the laser energy. Finally, the focus of the paper is on two areas. The first one is the adaptation of an appropriate configuration of a double-side mass spectrometer to characterize quantitatively radiative transitions (1), (3), and (5). Whereas double-side mass spectrometers and measurements of twophoton cross sections are nothing new, the analysis of the kinetics of competing transitions (one-photon, two-photon, and non-radiative) between different states (X, IS1, and IS2) leading to different ionic products, positive and negative is demonstrated here for the first time. The second concern of this work is to elucidate the origin of the secondary negative ions and electrons and quantitatively characterize the loss of produced ions of interest (H+, HCl+, Cl+, Cl−) before the detection. This characterization is necessary for the first aim, the measurements of one- and two-photon absorption cross sections. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-3 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) II. EXPERIMENTAL SETUP A. TOF mass spectrometer Our TOF mass spectrometer with 3D ion imaging capability was previously used in a single-arm version to detect positive ions only,2,3,57–59 it has previously been described in detail.60–62 In the present study, the setup was modified to enable the detection of negative ions as well. The extended setup differs from the original version by an additional, second TOF mass spectrometer which is the mirror reflection of the first, sharing the symmetry axis and equipped with two independent ion detectors. A pulsed nozzle (General Valve Series 9) is installed oriented perpendicularly to the mass-spectrometer axis. The nozzle injects a supersonic molecular beam (MB) between the two mass spectrometers, where it is intersected by a laser beam (LB) propagating perpendicularly to the molecular beam and to the mass-spectrometer axis likewise. Laser beam (X), molecular beam (Y ), and mass-spectrometer axis (Z) define the Cartesian coordinate system of the experimental setup. The principal electric scheme and the dimensions of our double-arm mass spectrometer are presented in Fig. 1. These two mass spectrometers have one common acceleration region, which is placed between two field-free drift regions and contains 10 metallic ring electrodes kept apart by glass rods. Thus, the acceleration region is a 100 mm tube with eleven 1 mm gaps. This is done in order to shield the spectrometer interior from external electric fields. All metal parts are covered by graphite spray (GRAPHIT 33) to decrease the scattering of light. In the homogeneous electric field configuration, the voltages on the ring electrodes change linearly from 0 to 2Ua , it was varied from 50 to 2000 V in different measurements. The elementary composition of the grids was studied by energy dispersive X-ray fluorescence (EDX) using a scanning electron microscope (JEOL JSM 6400). The results are as follows (mass, in %): Cr (18.52), Fe (73.32), and Ni (8.15). The diameter of the grid wires was 52 µm, each grid occupied 19% of the area of the TOF tube of the mass spectrometer. Our setup has a rare option:61 we can easily switch from outside between a space-focusing configuration63 and the VMI configuration of Eppink and Parker.64,65 By space focusing, we mean that the TOF is unaffected by a small uncertainty dZ in the position where the ion has been created on the Z axis (d TOF/dZ = 0). In the space-focusing configuration, the acceleration voltage 2Ua is split by 11 equal resistors in order to obtain a HF throughout the acceleration region of the spectrometer. The VMI results from adding an ionic lens (IL) which arises from application of 2.9Ua voltage to the 8th electrode, while the remaining voltages remain unchanged, with electrodes 7 and 9 now being connected by two resistors. In this case, the electrode 8 serves as Einzel lens and its field is superimposed by the homogeneous field. The first configuration is useful, when high time resolution is necessary, allowing us to resolve fine structures in the TOF profiles of negative particles. However, the VMI configuration was always used in order to obtain speed distributions of ions with high resolution. We should note that our electric field configuration is not that of Wiley and McLaren,66 as we wrongly wrote in our review.62 Both configurations use homogeneous electric fields and both are space-focusing. But in our configuration,63 there is one acceleration region and one drift region, while in the Wiley-McLaren configuration, there are two acceleration regions and one drift region. In the present work, we have installed the VMI option only at one side. When operating the spectrometer in VMI FIG. 1. Electrodes configuration, dimensions, and voltages to the double-arm mass spectrometer (not to scale). Constructions from metal and notations, dimensions, glass, plastic, and electric scheme details are shown in black, red, green, blue, and violet colors, respectively. The plane with the laser beam (LB) and molecular beam (MB) is shown by red dotted-dashed line, four grids are shown by black dashed lines. Details of the left- and right-side mass spectrometers are labeled by letters L and R, respectively. The units are volts and millimeters. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-4 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) mode, the TOF profile of the VMI side and the spatial image on the other side are distorted and unsuitable for a quantitative analysis. If double-side VMI is desirable, a second ionic lens should be installed on the other side.29,31,34,35 perpendicular (Σ → Π → Σ), and the value b2 = 2.01 ± 0.15 was obtained. In the experiments, we found IQ(0)/IQ(1) ≈ 2. Therefore, from Eqs. (6) and (7), we obtain the rotational temperature to be Trot = 17 K, and the relative population of the J = 0 state to be 0.66. B. Laser system, vacuum system, and ions detector Laser radiation for the excitation of HCl molecules (10 Hz, up to 100 µJ/pulse) in the spectral region 234–240 nm is produced by BBO frequency doubling of the output of a tunable Nd:YAG-pumped dye laser (Lambda Physik Scanmate). The laser polarization is linear and usually oriented along the mass-spectrometer axis. The vacuum chamber is pumped by two turbomolecular pumps (Pfeiffer, TMU 260 and TMH 521 P), background pressure is ∼10−6 mbar. The pumps are forepumped by a rotary vane pump filled with oil. When the nozzle is operating, the time-integrated pressure is ≈10−5 mbar. Positive and negative ions fly in opposite directions, pass the acceleration and the drift regions, and hit the two commercial 3D imaging detectors. Each detector consists of a double stage MCP assembly (Hamamatsu) with a DL detector (Roentdek).67,68 Each DL detector has 4 outputs; the signals which are recorded by a 500 MHz oscilloscope (LeCroy Waverunner 6050) and analyzed by a custom program which decomposes the signals on each line into individual pulses. Usually, each pulse has almost Gaussian shape, with known width and amplitude; this knowledge is used to distinguish ion signals and noise. The program is self-learning, from statistical analysis, it determines the average values for the widths, amplitudes, and TOF profile areas (A) of the pulses and uses them in consequent analysis. D. Determination of the ion number It is easy to measure the average number of ions projected on a detector per laser pulse at reduced laser energies, when the number of ions per laser pulse ñ is small, ñ ≤ 1. At higher laser energies, when ñ significantly exceeds unity, the ion signals on the delay lines overlap, the procedure of individual ion identification fails, and the value ñ cannot be determined directly. In this case, the value was determined as ñ = At /A, where At is the total area of the overlapped signal of the DL detector, and A ≈ 1 ns × V is the mean area of the signal of the individual ion obtained at reduced laser energy. E. Determination of the molecule concentration in the molecular beam For the determination of the absolute values of absorption cross sections, we need to know the concentration [HCl] on the molecular beam axis. This concentration was calculated from the equations [HCl(L, θ)] = [HCl(L, θ = 0)] cosn θ, π/2 NHCl = 2πL 2V t˜ [HCl(L, θ)] sin θdθ, In the present study, we show experimental results for only two transitions, the lower state was always X 1Σ+(v = 0, J = 0), and the upper I S1 states were V 1Σ+(v ′ = 12, 15; J ′ = 0); wavenumbers are 84 745.60 cm−1 for v ′ = 1269 and 86 401.6 cm−1 for v ′ = 15.70 The chlorine isotope shift is rather large, 4.5 cm−1 for v ′ = 12 and 6.1 cm−1 for v ′ = 15;70,71 therefore, only H35Cl was detected in the present study. The rotational temperature Trot of HCl in the molecular beam was determined from the relative line intensities of REMPI signals of rotational Q(0) and Q(1) transitions into the (V, v ′ = 12) excited state. For Q branches of two-photon electronic transitions with ∆Ω = 0, the rotational line intensities are IQ(J) = e−B0J (J +1)/kTrot S(J), (6) where B0 = 10.5 cm is the ground state rotational constant and S(J) are line strengths. For REMPI transitions they may be taken from our work,2 see also Refs. 72 and 73, J(J + 1) 2J + 1 2 b + , (7) S(J) = 9 5(2J − 1)(2J + 3) −1 where the parameter b depends on the nature of the transition. For the 2-photon transitions to the (V, v ′ = 12) state, it was measured previously,2 the transition was found to be (9) 0 NHCl = ⟨dNHCl/dt⟩/ν, V ≈ Vma x = {2kT0 ϱ/(ϱ − 1)MHCl}1/2, C. Spectroscopy of HCl (8) (10) (11) where L = 10 cm is the distance from the nozzle to the laser beam, θ is the azimuthal angle in the spherical coordinate system aligned with the molecular beam, NHCl is the number of HCl molecules ejected from the nozzle in one molecular beam pulse, ⟨dNHCl/dt⟩ is the long-term averaged flow of HCl molecules (typically 3 × 1017 s−1), t˜ = 165 µs is the duration of the molecular beam pulse, V is the speed of the molecular beam, Vmax = 696 m/s is the maximum speed of the molecular beam, when the internal temperature is zero, ν = 10 Hz is the laser pulse repetition rate, MHCl = 36 a.m.u. is the mass of the H35Cl molecule, k is the Boltzmann constant, ϱ ≡ CP/CV = 7/5, and T0 = 300 K is the temperature. The first formula (8) describes the angular distribution of the molecular concentration in the molecular beam, formula (9) averages the concentration over the angle θ, formula (10) calculates the number of molecules per pulse from the long-term averaged flow of HCl molecules, and expression (11) yields the maximal possible speed of the cold molecular beam. If we assume that the translational and rotational temperatures of HCl are similar, Ttr ≈ Trot, then from the ratio Ttr/T0 = 1 − (V/Vmax)2, we obtain V = 0.93Vmax. However, the rotational quanta in HCl are normally larger than typical translational energies; therefore, the rotationaltranslational energy exchange may be inefficient and hence, it is probable that Trot > Ttr. Thus, the speed V should be between 0.93Vmax and Vmax. Finally, we assume V ≈ Vmax. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-5 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) From Equations (8)–(11), one obtains the final expression [HCl(L, θ = 0)] = (n + 1)⟨dNHCl/dt⟩/2πνL 2V t˜. The value of ⟨dNHCl/dt⟩ was measured from the decrease of the gas pressure in the calibrated volume before the nozzle. The time profile of the molecular beam was recorded by observing HCl+ ions from REMPI of HCl at different delays between the laser pulse and the valve-opening electric pulse. It was found that it has an approximately rectangular shape with duration of t˜. It is known from the literature74 that the angular distribution of molecules in the molecular beam should be proportional to cos4 θ. We confirmed this law, n = 4, using the experimental setup shown in Fig. 2. In these measurements, we carefully arranged eleven flexible plastic wires in vacuum (10−4 mbar) and took a video of the wire movement through a glass viewport. The distances between the wires and the nozzle were the same for all wires, but the θ angles were different. The frequency of the nozzle was very low (0.1 Hz); therefore, the oscillations of the wires completely finished at the start of a new nozzle pulse. It was assumed in these measurements that the deviations of the wires from their equilibrium positions after the blow by the molecular beam are proportional to the gas flow. Finally, we obtained that the typical concentration in our experiments was [H35Cl(J = 0)] = 3.4 × 1012 cm−3. F. Reconstruction of the speed distribution from 3D VMI In order to obtain speed and angular distributions of the ions produced by REMPI, one should analyze the experimental 3D distributions of the ions, over the surface of the detector and over arrival times. The analysis of these 3D distributions obtained in the HF configuration63 is simple and straightforward, because the movements of the ions along the axis of ⃗ and along the radial direction R ⃗ the mass spectrometer Z ⃗ are independent. Cylindrical coordinates are assumed here, R ⃗ ⊥ Z. When we use VMI in the IL configuration64,65 in combination with the 3D detection of the ions, the analysis is more complicated. Therefore, a reconstruction of the HFdistribution from the observed IL-distribution is necessary. In our previous paper, the solution of this problem is described in detail.61 Here, we use a simpler approach. First, the electric field in the mass spectrometer is calculated analytically.61 The basic ⃗ assumption is the independence between movements along Z ⃗ directions. This assumption was checked by numerical and R calculations and was found to be very good. Let us compare, for example, HF- and IL-distributions of the ions over TOF. The first one is narrower than the second one. In this discussion, the relative TOF for the ith ion dt i,C = t i,C − ⟨t C ⟩ is introduced, where C denotes the configuration, C=HF or C=IL, t i,C is the TOF of the ith ion, and ⟨t C ⟩ denotes the time of the center of the TOF profile. For each ion i, the difference dt i,IL is known and we want to know what would be the dt i,HF value if we change from IL to HF configuration. The time dt i,HF is equal to63 dt i,HF = Vi, Z /aHF, (12) where Vi, Z denotes the initial speed of the ion along the Z axis, and aHF is the acceleration in the homogeneous electrical field. According to our simulations, the TOFs for HF- and ILconfigurations may be related by dt i,HF = dt i,IL(a + bVi, Z + cVi,2Z ), (13) where the coefficients a, b, and c may be obtained from modeling. They depend on the voltage configuration, massspectrometer geometry, and ion mass. Thus, each ion has its own scale factor dt i,HF/dt i,IL which depends on its initial speed Vi, Z . In Equations (12) and (13), the values for dt i,IL, aHF, a, b, and c are known, and therefore, the values for dt i,HF and Vi, Z may be calculated. The same approach is used to transform IL-distributions to HF-distributions in the XY -plane. G. Absolute detection efficiency (ADE) of MCP The knowledge of the ADE of the MCP is necessary in the present study. Under our experimental conditions, it may be given by75 FIG. 2. (a) Experimental study of the angular distribution of molecules in the molecular beam. I: Nozzle, II: metallic screen, III: video camera. The angle between the axis of the nozzle and the direction of interest is θ. The wires are labeled as 1, 2, ..., 11. (b) The initial amplitudes of the wire oscillations vs angles θ. The curve is a fitting by cos4 θ, see Eq. (8). Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-6 Poretskiy et al. ADE = OAR × f (P, Et ,UMCP), Rev. Sci. Instrum. 87, 023107 (2016) (14) where OAR means Open Area Ratio, the ratio of integrated pore area to the total disc area. It is 0.65 for our MCP. This expression assumes that particles incident on the MCP between channels are not detected. f (P, Et ,UMCP) is the efficiency function which depends on the nature of the particle P, the kinetic energy of the particle Et , the voltage between front and back sides of the MCP UMCP, and many other factors, like the angle between the axis of the MCP channels and the velocity vector of the incident ion. We also include in the f function the efficiency for assigning DL pulses to ions. We estimated that in our case, f (P, Et ,UMCP) ≈ 0.9. The deviation from unity is mainly due to the analysis being unable to assign all DL pulses to ions. This may be the case, first, when the intensity of some DL pulses drops below the noise level due to fluctuations; second, when the pulses overlap in time; third, when two ions hit the same place of the MCP. H. Mass-spectrometer detection configurations Our double-arm mass spectrometer has several detection configurations, some of which are shown in Fig. 3. The main one is configuration (b), where positive and negative ions are detected simultaneously. Configuration (c) of Fig. 3 is a trap for negative ions, with detection of positive ions. This configuration was used to prove that negative ions never produce positive ions in collisions with the grids. We attempted to detect these recharged positive ions by both mass spectrometers, but were unsuccessful. Configuration (d) of Fig. 3 is a “double trap,” which was used to study the production of negative ions and electrons in collisions of positive ions with the grids, see Sec. V. III. DOUBLE-ARM MASS SPECTROMETER EXPERIMENT A. H+ and Cl− detection In our recent preliminary study, the list of the ion-pair channels in REMPI of HCl was significantly extended.77 Now this list includes the intermediate states I S1 = V 1Σ+(v ′ = 8, 9, 11, 12, 13), E 1Σ+(v ′ = 0, 1), and g 3Σ−(0+)(v ′ = 0). This extension has become possible due to the detection of the Cl− ions, while all other investigations of the ion-pair channels in HCl before us were performed with imaging of H+. The example of our speed and angular distributions of the Cl− ions are shown in Fig. 4. All studies of the ion-pair channel yielded the anisotropy parameter76 of the H+ ion angular distributions to be β ≈ 2.4,77 This means that only excited states with the symmetry Σ are involved in the process of ion-pair photodissociation. The speeds of the ion pair were measured in several studies,4,77 in all cases, it was proven that the ion-pair channel is caused by the absorption of exactly three photons. This result is an important proof of the scheme of the ion-pair channel (5). B. Imaging and angle-speed distributions The detection of negative ions has several advantages over the detection of positive ions. First, the study of the ion-pair channel (5) by observation of H+ ions is usually hindered by the presence of the H+ ions produced in channels (2) and (4). Very often, the speed distributions of the ions for all three channels overlap, therefore identifying the ion-pair channel becomes questionable, see Fig. 4. In contrast, the study of the ion-pair channel by the detection of Cl− ions is easy because there exist no competing channels and hence, the detection sensitivity is large. FIG. 3. (a) Schematic representation of the double-arm mass spectrometer. LB and MB are laser beam and molecular beam, respectively. Grids are shown by vertical dashed lines. Mass-spectrometer detection configurations are: (b) double-arm, positive and negative ions are detected, (c) one-arm, positive ions are detected, negative ions are trapped, and (d) double trap, both positive and negative ions are trapped. ((b)–(d)) Voltages are shown by black lines. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-7 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) FIG. 4. Meridian plots3 for REMPI of HCl via transition X 1Σ+(v = 0, J = 0) → V 1Σ+(v ′ = 15, J ′ = 0). Left: H+ ions, right: Cl− ions. The relative scale is arbitrary. The speed of Cl− is 503 m/s, anisotropy parameter76 β ≈ 2. The width of the speed distribution for the case of H+ ions is much larger than the width for the case of Cl− ions because the H+ ions are produced via several pathways. Second, while in the REMPI of HCl, the Cl− ions are produced via only one potential energy surface with a well defined speed, the H+ speed distribution is broad, see Fig. 4, because in channels (2) and (4), the kinetic energy of the emitted electron may vary in a broad range. In addition, even the ionization step also slightly contributes to the speed of H+ in these last two channels due to electron recoil,78 while it has no influence on the ion-pair channel. Therefore, the speed distribution of the Cl− ions produced in the REMPI of HCl is very narrow and hence may be used as benchmark for testing imaging techniques. In the present study, a slightly different approach is used where the rate is defined as W (2) = σ (2)Φ2(r, z,t), where σ (2) is a lineshape-dependent “cross section” in cm4 s/photon2. In our approach, time averaging of the photon flux is not used; therefore, the statistical factor G(2) disappears, and the lineshape function g(ω) is included in the definition of the “cross section” σ (2). The photon flux may then be described as Φ(r, z,t) = Nhν IV. PROBABILITIES OF 2- AND 3-PHOTON TRANSITIONS A. Definition of 2-photon “cross sections” The rate for a one-photon transition between two states is usually defined as W (1) = σΦ(r, z,t), (15) where σ is a one-photon cross section (in cm2/photons), and Φ(r, z,t) is a photon flux (in photons/cm2 s). Hereafter, new cylindrical coordinates (r,z) are introduced with the reference point in the focus of the lens and the axis z along the laser beam (axis X in Figs. 1 and 3). The notations and approach introduced by Saxon and Eichler79 are usually used to describe the two-photon absorption “cross sections.” In their notation, the rate for a twophoton transition in s−1 is described by the expression W (2) = σ0(2)g(ω)G(2)Φ2(r, z,t), (16) where σ0(2) is a lineshape-independent “cross section” in cm4/photon2, the area-normalized lineshape function g(ω) expressed in seconds ( gdω = 1), and the photon statistical factor G(2) is dimensionless. The factor G(2) is the secondorder intensity autocorrelation function of the laser,80 G(2) ≡ ⟨ f 2(t)⟩/⟨ f (t)⟩2, where the angular brackets denote time averaging. (17) f (t) U(r, z) , ∆t πr z2(z) (18) where Nhν = I/hν, I, and ∆t represent the number of photons, the energy, and the effective duration of the laser pulse, respectively, and r z (z) is the effective radius of the laser beam. The dimensionless functions f (t) and U(r, z) are normalized to unity +∞ [ f (t)/∆t] dt = 1, (19) −∞ +∞ U(r, z) 2πr dr = 1, (20) πr z2 0 where the second equation implies a constant radiation flux along the z-axis. We have measured the functions f (t), [HCl(z)], and g(ω) experimentally. The laser pulse function f (t) was recorded by a quick photomultiplier, transferred to the LeCroy oscilloscope, and analyzed by Origin program. We tested different fitting functions and finally found that the best one is the extreme function, see Fig. 5, f (t) = e− exp(−t /∆τ) e−t /∆τ , (21) where ∆τ = 1.58 ± 0.02 ns. This function obeys condition (19), it has its maximum at t = 0, f (0) = e−1, and it has a full width at half maximum of 2.44∆τ. We prefer the extreme function over the Gaussian because the former is very easy to integrate, and because it reflects the reality: the time shape of a Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-8 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) FIG. 5. Comparison of fittings of the experimental time profile of the laser pulse by extreme and Gaussian functions. Nd-Yag laser pulse is known not to be symmetric, but to have a “quick” rise, a “slow” fall, and a “long” tail. In the simplified approach to the theory of laser beams, the spatial distribution U(r, z) of the radiation intensity is usually assumed to be Gaussian along the radius r, U(r, z) = 2 exp(−2r 2/r z2) (22) and Lorentzian along the symmetry axis z, r z2 = r 02 [1 + (z/z0)2], (23) where r 0 is the diffraction limited effective radius of the laser beam in the focus plane and z0 is the confocal parameter of the beam (Raleigh range), z0 = 2 f r 0/d, f = 20 cm is the focus length, and d is the diameter of the laser beam just after the aperture placed close to the lens. The radius r 0 may be predicted theoretically from the Airy pattern theory to be r 0 = λ f /d = 0.031 mm for an aperture of d = 1.5 mm. Also, the radius was measured experimentally. As shown in Sec. IV B, the dependence [HCl+(z)], the ion density along the laser beam, is given by [HCl+(z)] ∼ 1/r z2 ∼ (1 + z 2/z02)−1. (24) The experimental dependence [HCl+(z)] was measured by REMPI and fitted by this expression, and thus, the values of z0 and r 0 were determined. The intensity of the Fraunhofer diffraction pattern of a circular aperture, the Airy pattern, is given by U(r, z) = π 2(J1(x)/x)2, has Lorentzian shape with full width at half maximum of 1.08 cm−1; hence, g(ω) = 1.96 × 10−11 s. Note that the Doppler broadened spectrum of H(n = 2) atoms produced in channel (4) is only slightly narrower than the spectrum of the laser radiation; hence, a correction factor should be used. Mathematically, this factor is a convolution of the radiation spectrum and the absorption spectrum of the atoms. In the present work, we neglected this correction, because it was small. (25) where x ≡ πr/r z (z). In our experiment, we prefer the Gaussian shape (22), because Airy theory does not work far away from the focus plane.81 However, the Airy theory yields the correct value of r 0. The Doppler broadening in the molecular beam is negligibly small; the decay time of the upper state IS1 of the REMPI transition, spontaneous as well as radiation-induced, is ≈3 ns;82 hence, for our case, the lineshape function g(ω) almost coincides with the spectrum of laser radiation. This spectrum is determined mainly by the cavity of the dye laser and B. Determination of the 2- and 3-photon “cross sections” When discussing 2- and 3-photon REMPI processes, the major steps in the kinetic scheme are W (2) W (1) X(r, z,t) → B(r, z,t) → C(r, z,t) → D P , (26) where X, B, C, and D P denote ground, excited, superexcited, and ionic states of HCl. The states D P lead to ionic products P, which may be either H+ or Cl+ or HCl+ or Cl−. In the case of HCl, there are five ionic channels, see Eqs. (1)–(5), but in the present study, we normally do not distinguish between H+ ions produced via different channels. Under the assumption that the last step is quick, it is taken into account by branching ratio coefficients γ P only, [P] = γ P [C], [P] = γ P [HCl(C)] = [P] [HCl(C)]. [H+] + [Cl+] + [HCl+] (27) Hereafter, [S] denote [HCl(S)], with S = X, B, and C. As it follows from Eqs. (1)–(5), the REMPI of HCl produces exactly one positive ion; hence, γH+ + γCl+ + γHCl+ = 1 and γCl− is the relative probability of the ion-pair channel (5). The rate equations for the above reaction scheme are d/dt [X] = −k 2[X], d/dt [B] = k 2[X] − k 3[B], d/dt [C] = k 3[B], where concentrations [X], [B], and [C] and rate constants k2 and k3 depend on r, z,t variables. The initial conditions are Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-9 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) [X(r, z,t = 0)] = [X0(r, z)] and [B(r, z,t = 0)] = [C(r, z,t = 0)] = 0. According to definitions (15) and (17), the rate constants in these equations may be expressed as k2 = σ (2)Φ2(r, z,t), (28) k3 = σΦ(r, z,t). (29) Under the assumption that the decrease of the number of molecules in the ground state X is negligible, [X] ≈ [X0], and assuming the time shape of the laser pulse to be of extreme form (21), the solution of the equations system at the t → ∞ limit is [B(r, z, +∞)] + [C(r, z, +∞)] ( )2 σ (2) 2 U(r, z) Nhν , (30) ≈ [X0] k2 dt = [X0] 4∆τ πr z2(z) where expressions (17)–(21) and (28) have been used. If the probability of the one-photon step is assumed to be small, k3∆τ ≪ 1, then we obtain )3 ( 5σ (2)σ 3 U(r, z) . (31) Nhν [C(r, z, +∞)] = [X0] 36∆τ πr z2(z) To calculate NB, Nc , and NP , the numbers of molecules in the states B, C, and D P , respectively, their concentrations should be integrated over the volume. For example, NC = [C(r, z, +∞)] 2πr dr dz. After the integration, the final formulae are σ (2) 2 N , ∆τλ hν σ (2)σ 3 N , NP = γ P NC = γ P [X0] C3 ∆τλr 02 hν NP = γ P (NB + NC ) = γ P [X0] C2 (32) (33) where C2 and C3 are coefficients, which depend on the choice of the U(r, z) function: C2 = 1/4 = 0.250 and C3 = (5/36)(2/ 3π) = 0.0295 for the Gaussian pattern (22) and C2 = Υ2π 2/4 = 0.284 and C3 = (5/36)Υ3π 4/2 = 0.0423 for ∞the Airy pattern (25), where Υ2 and Υ3 are integrals Υn ≡ 0 2π 2−n (J1(x)/ x)2n dx, with n = 2,3. Whereas we have used the coefficients for the Gaussian pattern, the difference between the two patterns yields an estimate of the diffraction modeling uncertainty. Note that the first expression is more accurate, because there it does not contain geometrical properties of the laser beam. We did not detect all produced ions, thus the number of detected ions ÑP is related to NP by correction factor, ÑP ≡ ADE(1 − p)2 NP , where ADE is the absolute detection efficiency from Eq. (14), and the parameter p describes the ion loss in collisions with grids, see Subsection V D. This correction is used when experimental data are fitted by Eqs. (32) and (33). Let us illustrate our logic by two examples. First, let us suppose that the dependence of NP on I 2 is linear for all observed ions P, like in Fig. 6 for small laser pulse energies I. Then, the 2-photon step is a “bottle neck,” a limiting step, and all other transitions are very quick, so that they have no influence on the dependence of NP on I. Using expression (32), from the initial slopes of the curves in Fig. 6, we obtained σ (2) = 3.2 × 10−49 cm4 s/photon2 and four branching ratios γ P . For example, the relative probability of the ion-pair channel (5) is γCl− = (5% ± 1%). Applying Eq. (16) gives the lineshape-independent cross section, σ0(2) = σ (2)/g(ω) = 1.6 × 10−38 cm4/photon2. Second, let us suppose that the ionic signals are proportional to the third power of the laser energy, NP ∼ I 3, like at the top of Fig. 6. It means, that in addition to the two-photon step, there is also a one-photon transition occurring on a time scale comparable with ∆τ. In the cases of P = Cl− and P = HCl+, it is evident that this is the transition from the excited state IS1 to the superexcited state IS2 in HCl. Then, expression (33) should be used to obtain the values γ P σ (2)σ from the initial slopes of the experimental curves for each ion P. If the values γ P and σ (2) have already been determined from the NP ∼ I 2 dependencies, the absolute values of the cross sections σ may be determined. In our case, it is σ = 1.2 × 10−16 cm2/photons. This cross section is very large, but seems to be possible, because it corresponds to transitions to several superexcited states of HCl.3 FIG. 6. REMPI of HCl via the excited state V 1Σ+(v ′ = 12) at 84 745.60 cm−1.69 Displayed is the average number of detected ions vs the square of the laser energy. Inset at the top: The average number of detected Cl− ions vs third power of the laser energy. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-10 Poretskiy et al. An interesting case is demonstrated in Fig. 6, where the dependence of NCl− on I k is linear at small energies for k = 3 and at moderate energies for k = 2. Therefore, the initial part of the curve (shown at the top) is described by Eq. (33) and the slope of the curve is proportional to γCl− σ (2) σ, while the rest of the curve is fitted by Eq. (32) and the slope of the curve is proportional to γCl− σ (2). Hence, the ratio of these two slopes provides the cross section σ. This procedure does not require the knowledge of the [X0] concentration and the duration ∆τ of the laser pulse and should therefore be rather accurate. Interestingly, the ratio of the relative yields of Cl− and + H ions from REMPI via the state V 1Σ+(v ′ = 12) may be determined from Fig. 1 of the study of Romanescu and Loock,4 [Cl−]/[H+] = γCl−/γH+ = 0.15 ± 0.015. Our value, as one can obtain from Fig. 6 by comparison of initial slopes, is almost the same, γCl−/γH+ = 0.18. This difference is probably due to the smaller laser power in our study, because the ratio [H+]/[Cl−] increases with I, see Fig. 6, This rise is partly due to competition between 3-photon processes (1) and (5) and 4photon processes (2)–(4), while the role of the HCl+ photodissociation rises with I. Hence, we believe that the determination of relative probabilities of different ionic channels by scanning the laser power is a more correct procedure than measurements at fixed laser power. Note that our present data about REMPI of HCl are preliminary only, further work is in progress to measure the cross sections accurately. V. ELECTRON EMISSION AND ION CHARGE EXCHANGE ON THE GRIDS For the measurement of absorption cross sections, we have to characterize quantitatively all possible processes leading to ion number changes in the mass spectrometer. Three kinds of such processes have been observed and are described Rev. Sci. Instrum. 87, 023107 (2016) below. For all of them, the detection of negative photofragments by the right-hand side mass spectrometer in detection configurations (b) and (d) shown in Fig. 3 is assumed; the laser polarization is oriented along the axis of the mass spectrometer; all formulae are obtained in assumption of free flights in a homogeneous electric field. A. Effect 1, electron emission: Cl− + grid→ e− Fig. 7 shows the TOF profiles of the negative photofragments produced in the ion-pair channel in REMPI of HCl, where the double-arm detection configuration shown in Fig. 3(b) is assumed. Large signals in the 5.20–5.30 µs region belong to Cl− ions with an anisotropy parameter β ≈ 2, while small signals at shorter time are produced by electrons emitted from grid 2 R . The speed of the ions is negligibly small relative to the speed of the electrons; therefore, the time shift between the arrival of emitted electrons and Cl− ions is essentially equal to the TOF of the ions from grid 2 R to the MCP R . The time shift may be calculated by the formula ∆s 2MCl ( Ua + ∆U − Ua ), (34) δτ = ∆U |e| where ∆s = 8 mm is the distance between grid 2 R and MCP R , and MCl is the mass of the Cl− ion. The calculated value of the time shift δτ is in good agreement with the observed value in Fig. 7 and supports our interpretation of the TOF spectra. We found that not only TOF but also the spatial distribution of the electrons produced on grid 2 R mimicks the space distribution of the Cl− ions, that is related with relatively high voltage ∆U and short distance ∆s. Therefore, under our experimental conditions, the study of the electron angular distribution was difficult. The same electron emission should also occur on grid 1 R , which separates acceleration and drift regions. In contrast to FIG. 7. TOF profiles of the Cl− ions and the electrons emitted from the grid 2 R in the REMPI of HCl via excited state V 1Σg+(v ′ = 12). Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-11 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) TABLE I. The relative efficiency of electron production by the Cl− ion with different kinetic energies eUa , electrons per one Cl− ion. eUa (eV) Efficiency 150 0.08 300 0.14 450 0.18 600 0.19 750 0.22 the situation with grid 2 R , most of the produced electrons were turned to the walls of the drift region by magnetic fields and were lost upon impact on the walls, and only a small portion of them was detected. A nontrivial fact is that the efficiency of electron production increases with the kinetic energy eUa of Cl−, impacting on grid 2 R , see Table I. We roughly assume here that this efficiency is equal to the ratio of e− and Cl− signals; this assumption neglects the difference in MCP R detection efficiency, f (e−, 0,UMCP) = f (Cl−,Ut ,UMCP), see Eq. (14). B. Effect 2, ejection of electrons from the MCP surface: H+, Cl+, HCl+ + MCP → e− In the first experiments with HCl photoionization, we found that at almost all laser energies, the number of photoelectrons was large enough to saturate the MCP and therefore make it almost blind, see Fig. 8, blue line. In this subsection, the detection configuration shown in Fig. 3(b) is assumed. The disturbing effect of the photoelectrons was eliminated by the installation of four permanent magnets (NdFeB, 40 × 20 × 5 mm), see Fig. 8, black line. These magnets produced a homogeneous field oriented perpendicular to the axis of the mass spectrometer. The magnetic field turned the electrons to the wall of the drift region. Unfortunately, high-energy electrons eject secondary electrons from the surface and therefore, their signals were not eliminated completely. One pair of magnets was installed at the beginning of the drift region of the rightside mass spectrometer, the second pair was installed at the end of the drift region. The experiment shows that the most effective orientation of the second field is perpendicular to the first one. The mass of the ion Cl− is much larger than the mass of the electron; therefore, the trajectory of the ion was not noticeably affected. The broad and the sharp peaks shown in Fig. 8 at approximately 9.7 µs mimick the TOF profile of Cl+ and HCl+ positive ions and disappear if the left-side mass spectrometer is switched off. The peaks obviously reflect the ability of the left-side mass spectrometer MCP L to emit electrons after the impact of positive Cl+ and HCl+ ions. The integrated pore area of our MCP is only 65% of the total MCP surface, see Subsection II G, so the other 35% of the surface emits electrons into the space between the front side of the MCP L and the grid 2 L . If the positive ion falls into one of the pores, it produces electrons which are then accelerated into the MCP L . However, if the positive ion hits the surface between the channels, the electric field accelerates the emitted electrons away from the MCP L surface towards the right-side mass spectrometer. On the way towards the negative ion detector, they are once again accelerated in the acceleration region. Because of the small mass of the electrons relative to ions, they cover the distance between the two detectors in a very short time. As a result, the time of impact of these electrons on the negative ion detector almost coincides with the time of impact of the negative chlorine ion. To guarantee the correct separation of negative photoions and secondary electrons emitted from the opposite-side MCP R one needs to install the set of magnets and have a sufficiently high TOF resolution of the imaging detector. C. Effect 3, forward/backward scattered electrons and positive ion recharge: Cl+, HCl+ + grid → e−, Cl−, HCl− The emission of negative ions and electrons under the bombardment of a surface by positive ions is known since 1931.83 We observed these emission processes also. They were studied with the “double trap” detection configuration shown in Fig. 3(d). In the right-side mass spectrometer, a trap for positive ions (H+, Cl+, and HCl+) produced after REMPI of HCl is organized. Moving periodically in the trap, the positive ions impact on grid 2 R two times per period. The first impact occurs on the way towards the right MCP R , the second impact occurs on the way back from the MCP R . In both cases, we detect negative particles emitted from grid 2 R and flying to the MCP R ; the first are forward scattered secondary particles, the second are backward scattered secondary particles. These forward and backward scattering processes are shown in Fig. 9, and the corresponding TOF profiles are shown in Fig. 10. In Figure 10, peaks 1 and 2 correspond to the forward scattered secondary electrons, produced in the processes Cl+ + grid 2 R → e− and HCl+ + grid 2 R → e−. Peaks 4 and 5 represent the backward scattered secondary electrons, produced by the ions Cl+ and HCl+. The peak 3 reflects the recharge of forward scattered HCl+ ions. This interpretation of the observed TOF profiles was confirmed by simple quantitative calculations. Thus, the time between forward and backward impacts ∆t is equal to the turn around time of positive ions in the space between grid 2 R and FIG. 8. Influence of the magnet installation in the drift region of the righthand side mass spectrometer on TOF profiles of negative ions. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-12 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) FIG. 9. Schematic representation of the processes occurring on grid 2 R : (a) forward scattered electron production, (b) HCl+ ion recharge, (c) positive ions are ⃗ (d) backward scattered electron production. reflected in opposite direction by the electric field E, MCP R . It may be calculated by the formula 4∆s MHClUa , ∆t = UMCP 2|e| D. Periodic motion of ions in the mass spectrometer (35) where ∆s is the distance between the grid 2 R and the MCP R . The calculated ∆t values are in perfect agreement with the experimental ones, as well as the times δτ between signals of the HCl− ions and electrons produced by the forward scattered HCl+ ions, where the δτ values were calculated with Eq. (34). In Subsection V C, we have described forward and backward scatterings of trapped positive ions on grid 2 R during one period of the ionic motion. We continue the discussion of the same experiments, but now we extend the time window of observation in order to include the scattering on both grids during several periods. The detection configuration shown in Fig. 3(d) is assumed. The scheme of periodic motion as well as the TOF profile is shown in Fig. 11. The period starts and finishes in the acceleration region in the point where the positive ions were produced by REMPI. During the period, the positive ions may impact on each grid two times, producing emission of electrons and negative ions. If these emitted particles fly towards MCP R , they may be detected. The time τ is equal to the TOF of the positive ion in the classical experiment and does not change for the subsequent cycles of motion. Thus, the period of motion is equal to the doubled TOF of the positive ion. The TOF profile shown in Fig. 11(b) confirms the idea that the first signature of electrons and recharged ions produced on grid 2 R takes place at time τ, τ + ∆t, the second one occurs at time 3τ + ∆t, 3τ + 2∆t, and the third one at the time 5τ + 2∆t, 5τ + 3∆t. Here, ∆t is the time between the production of forward and backward scattered electrons. Also, Fig. 11(b) shows the formation of backward scattered electrons on grid 1 R at times 1.5τ + ∆t and 3.5τ + 2∆t, produced by positive ions flying in the direction of the acceleration region. The forward scattered electrons are less probable than backward scattered electrons, see Fig. 10, which is why we have not observed the electrons on grid 1 R at times 0.5τ, 2.5τ + ∆t, 4.5τ + 2∆t. FIG. 10. TOF profile of negative particles observed in the REMPI of HCl via the excited state V 1Σ+(v ′ = 12) at Ua = 800 V. Positive ions (H+, Cl+, and HCl+) oscillate in the right-side mass spectrometer and produce negative particles in collisions with grids 1 R and 2 R . The TOF profile shows the processes occurring on grid 2 R . Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-13 Poretskiy et al. Rev. Sci. Instrum. 87, 023107 (2016) FIG. 11. Periodic motion of positive ions in the mass spectrometer at Ua = 1500 V. (a) Scheme of three cycles of motion. (b) TOF profile of negative particles produced in collisions of the positive ions with the grids 1 R and 2 R during three cycles. Fig. 10 shows a small part of such a profile in detail. The number of produced electrons decreases with each new cycle of motion. This results from the decrease of the number of positive ions owing to the impacts on the grid and to the formation of negative ions and neutral atoms on the grid. Let us introduce the flight direction independent probability p of ion loss on the grid. For N positive ions flying to the grid, only (1 − p)N positive ions continue their flight after the grid. Before the first cycle of electrons and recharged particles (time gate around τ), N ions approach grid 2 R from the drift region. Before the second cycle, the ions pass the grids four times: two times near the MCP R (τ, τ + ∆t), one time entering the acceleration region (1.5τ + ∆t) and another time exiting it (2.5τ + ∆t). Thus, the number of the positive ions and the number of electrons produced on grid 2 R are reduced by the factor (1 − p)4. From the intensities of the peaks in the experimental time-of-flight profile, corresponding to the electrons produced in the first and in the second cycles, one can then calculate the probability of positive ion loss on grid 2 R . The probability is measured for Cl+ and HCl+ ions with different kinetic energies. It is found to be independent on the ion nature or the kinetic energy of the ion and equal to 19.5% ± 0.05%. With the value p, it is possible to obtain the exact number of ions produced as a result of HCl photodissociation or ionization, from the number of ions detected in the classical configuration experiment. To do this, one needs to divide the number of detected ions by the factor (1 − p)2. like HCl∗ + hν → HCl∗∗, and the probability of the nonadiabatic transitions HCl∗∗ → HCl(B) → H+ + Cl−, leading to ionic pairs. All these data should be obtained from the analysis of the dependencies of the number of ions on the laser energy. Further work is in progress to measure accurately 2- and 3-photon cross sections and to analyze the mechanism of the ion-pair channel in REMPI of HCl. A full characterization of the laser beam and the knowledge of the probability for ion detection are necessary parts of the analysis. A detailed knowledge of losses of produced ions in the mass spectrometer before detection requires understanding and characterization of such processes like the electron emission from metal grids under ion bombardment and charge transfer between positive ions and the metal surface of the grids, Cl+ + grid → Cl−, e−. An additional special configuration of the mass spectrometer was proposed where only the directions of the electrical fields are different from the directions in the main configuration. This configuration organizes a trap for positive ions. Moving periodically in the trap, the ions collide with two grids of the mass spectrometer placed at both sides of the drift region. This configuration allowed us to measure the probabilities of production, branching ratios, time distribution, and space distributions of electrons and negative ions produced on the grids after the impact of positive ions. It was found, for example, that each grid eliminates approximately 20% of the positive ions. 1C. Romanescu, S. Manzhos, D. Boldovsky, J. Clarke, and H.-P. Loock, “Superexcited state reconstruction of HCl using photoelectron and photoion imaging,” J. Chem. Phys. 120, 767 (2004). The current paper presents a new symmetric double-arm 2A. I. Chichinin, P. S. Shternin, N. Gödecke, S. Kauczok, C. Maul, O. S. mass spectrometer with 3D imaging detectors at both sides Vasyutinskii, and K.-H. Gericke, “Intermediate state polarization in multiwhich allows us to measure simultaneously the 3D velocphoton ionization of HCl,” J. Chem. Phys. 125, 034310 (2006). 3A. I. Chichinin, C. Maul, and K.-H. Gericke, “Photoionization and photoity vector distributions of positive and negative photoions. dissociation of HCl(B 1Σ+, J = 0) near 236 and 239 nm using threeAlthough our spectrometer has two grids at each side, it has an dimensional imaging,” J. Chem. Phys. 124, 224324 (2006). optional one-side VMI capability and magnetic field to filter 4C. Romanescu and H.-P. Loock, “Proton formation in 2+1 resonance out unwanted electron signals. enhanced multiphoton excitation of HCl and HBr via (Ω = 0) Rydberg and As a benchmark system, the REMPI of HCl molecules ion-pair states,” J. Chem. Phys. 127, 124304 (2007). 5K. P. Lawley and R. J. Donovan, “Spectroscopy and electronic structure of via the excited states V 1Σ+(v ′ = 12, 15) is used, and the ion-pair states,” J. Chem. Soc., Faraday Trans. 89, 1885 (1993). ions H+, HCl+, Cl+, Cl−, and e− are detected. The main 6J. Berkowitz, in VUV and Soft X-Ray Photoionization (Plenum Press, New aim of our studies is a full quantitative characterization York, 1996), Chap. 8, p. 263. 7X. Liu, R. L. Gross, and A. G. Suits, “‘Heavy electron’ photoelectron of the multichannel REMPI of small molecules in cases spectroscopy: Rotationally resolved ion pair imaging of CH+3 ,” Science 294, where the ion-pair dissociation channel is important. The 2527 (2001). characterization consists of the determination of two-photon 8M. J. Simpson and R. P. Tuckett, “Vacuum-UV negative photoion spectrosabsorption cross sections for the process HCl(X) + 2hν copy of gas-phase polyatomic molecules,” Int. Rev. Phys. Chem. 30, 197 (2011). → HCl∗, one-photon absorption cross sections for processes Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun VI. CONCLUSION AND OUTLOOK 2016 05:34:49 023107-14 9D. Poretskiy et al. A. Shaw, D. M. P. Holland, and I. C. Walker, “Ion-pair formation mechanisms in chloromethane, bromomethane and dichlorodifluoromethane,” J. Phys. B 39, 3549 (2006). 10T. Ridley, J. T. Hennessy, R. J. Donovan, K. P. Lawley, S. Wang, P. Brint, and E. Lane, “Evidence for Rydberg Doorway States in Photoion Pair Formation in Bromomethane,” J. Phys. Chem. A 112, 7170 (2008). 11T. Baer, “The dissociation dynamics of energy-selected ions,” Adv. Chem. Phys. 64, 111–202 (1986). 12D. P. de Bruijn and J. Los, “Time and position-sensitive detector for dissociative processes in fast beams,” Rev. Sci. Instrum. 53, 1020 (1982). 13R. E. Continetti, D. R. Cyr, R. B. Metz, and D. M. Neumark, “Fast beam studies of N3 photodissociation,” Chem. Phys. Lett. 182, 406 (1991). 14Y. Hikosaka and J. H. D. Eland, “Time focus with velocity imaging of charged particles in coincidence: Application to photoionization of CO and N2O,” Chem. Phys. 281, 91 (2002). 15Y. Morishita, M. Kato, G. Prümper, X.-J. Liu, T. Lischke, K. Ueda, Y. Tamenori, M. Oura, H. Yamaoka, I. H. Suzuki, and N. Saito, “A new apparatus for electron-ion multiple coincidence momentum imaging spectroscopy,” Radiat. Phys. Chem. 75, 1977 (2006). 16C. Wu, C. Wu, Y. Yang, Z. Wu, X. Liu, X. Xie, H. Liu, Y. Deng, Y. Liu, H. Jiang, and Q. Gong, “Coincidence imaging of photoelectrons and photo-ions of molecules in strong laser fields,” J. Mod. Opt. 60, 1388 (2013). 17J. A. Davies, J. E. LeClaire, R. E. Continetti, and C. C. Hayden, “Femtosecond time-resolved photoelectron–photoion coincidence imaging studies of dissociation dynamics,” J. Phys. Chem. 111, 1 (1999). 18J. A. Davies, R. E. Continetti, D. W. Chandler, and C. C. Hayden, “Femtosecond time-resolved photoelectron angular distributions probed during photodissociation of NO2,” Phys. Rev. Lett. 84, 5983 (2000). 19A. M. Rijs, M. H. Janssen, E. t. H. Chrysostom, and C. C. Hayden, “Femtosecond coincidence imaging of multichannel multiphoton dynamics,” Phys. Rev. Lett. 92, 123002 (2004). 20E. Gagnon, A. S. Sandhu, A. Paul, K. Hagen, A. Czasch, T. Jahnke, P. Ranitovic, C. L. Cocke, B. Walker, M. M. Murnane, and H. C. Kapteyn, “Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source,” Rev. Sci. Instrum. 79, 063102 (2008). 21J. H. D. Eland, M. Takahashi, and Y. Hikosaka, “Photoelectron-fragment ion correlations and fixed-molecule photoelectron angular distributions from velocity-imaging coincidence experiments,” Faraday Discuss. 115, 119 (2000). 22M. Takahashi, J. P. Cave, and J. H. D. Eland, “Velocity imaging photoionization coincidence apparatus for the study of angular correlations between electrons and fragment ions,” Rev. Sci. Instrum. 71, 1337 (2000). 23Y. Hikosaka and J. H. D. Eland, “Molecular frame photoelectron angular distributions in inner valence photoionisation of CO,” Phys. Chem. Chem. Phys. 2, 4663 (2000). 24Y. Hikosaka and J. Eland, “New results on photoion pair formation from application of the velocity imaging photoionisation coincidence (VIPCO) technique,” Rapid Commun. Mass Spectrom. 14, 2305 (2000). 25Y. Hikosaka and J. H. D. Eland, “Molecular-frame photoelectron angular distributions in inner-valence photoionization of N2,” J. Phys. B 33, 3137 (2000). 26T. Baer, “Ion dissociation dynamics and thermochemistry by photoelectron photoion coincidence (PEPICO) spectroscopy,” Int. J. Mass Spectrom. 200, 443 (2000). 27T. Baer and Y. Li, “Threshold photoelectron spectroscopy with velocity focusing: An ideal match for coincidence studies,” Int. J. Mass Spectrom. 219, 381 (2002). 28D. Rolles, Z. D. Pešić, M. Perri, R. C. Bilodeau, G. D. Ackerman, B. S. Rude, A. L. D. Kilcoyne, J. D. Bozek, and N. Berrah, “A velocity map imaging spectrometer for electron-ion and ion-ion coincidence experiments with synchrotron radiation,” Nucl. Instrum. Methods Phys. Res., Sect. B 261, 170 (2007). 29A. Matsuda, M. Fushitani, and A. Hishikawa, “Electron–ion coincidence momentum imaging of molecular dissociative ionization in intense laser fields: Application to CS2,” J. Electron Spectrosc. Relat. Phenom. 169, 97 (2009). 30K. Hosaka, R. Itakura, K. Yokoyama, K. Yamanouchi, and A. Yokoyama, “Photoelectron–photoion coincidence momentum imaging for dissociative ionization of ethanol in intense laser fields,” Chem. Phys. Lett. 475, 19 (2009). 31A. Bodi, P. Hemberger, T. Gerber, and B. Sztáray, “A new double imaging velocity focusing coincidence experiment: i 2 PEPICO,” Rev. Sci. Instrum. 83, 083105 (2012). Rev. Sci. Instrum. 87, 023107 (2016) 32D. Xu, J. Huang, R. J. Price, and W. M. Jackson, “Velocity imaging studies on ion-pair dissociation of CH3Br + hν VUV → CH3+ + Br− as a function of wavelength,” J. Phys. Chem. A 108, 9916 (2004). 33C. J. Koh and S. R. Leone, “Simultaneous ion-pair photodissociation and dissociative ionization of an ionic liquid: Velocity map imaging of vacuum-ultraviolet-excited 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide,” Mol. Phys. 110, 1705 (2012). 34A. Vredenborg, W. G. Roeterdink, and M. H. M. Janssen, “Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2,” J. Chem. Phys. 128, 204311 (2008). 35A. Vredenborg, W. G. Roeterdink, and M. H. M. Janssen, “A photoelectronphotoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of σ = 18 ps and energy resolution ∆E/E = 3.5%,” Rev. Sci. Instrum. 79, 063108 (2008). 36P. O’Keeffe, P. Bolognesi, M. Coreno, A. Moise, R. Richter, G. Cautero, L. Stebel, R. Sergo, L. Pravica, Y. Ovcharenko, and L. Avaldi, “A photoelectron velocity map imaging spectrometer for experiments combining synchrotron and laser radiations,” Rev. Sci. Instrum. 82, 033109 (2011). 37D. Rolles, R. Boll, M. Adolph, A. Aquila, C. Bostedt, J. D. Bozek, H. N. Chapman, R. Coffee, N. Coppola, P. Decleva, T. Delmas, S. W. Epp, B. Erk, F. Filsinger, L. Foucar, L. Gumprecht, A. Hömke, T. Gorkhover, L. Holmegaard, P. Johnsson, C. Kaiser, F. Krasniqi, K.-U. Kühnel, J. Maurer, M. Messerschmidt, R. Moshammer, W. Quevedo, I. Rajkovic, A. Rouzée, B. Rudek, I. Schlichting, C. Schmidt, S. Schorb, C. D. Schröter, J. Schulz, H. Stapelfeldt, M. Stener, S. Stern, S. Techert, J. Thogersen, M. J. J. Vrakking, A. Rudenko, J. Küpper, and J. Ullrich, “Femtosecond x-ray photoelectron diffraction on gas-phase dibromobenzene molecules,” J. Phys. B 47, 124035 (2014). 38J. Küpper, S. Stern, L. Holmegaard, F. Filsinger, A. Rouzée, A. Rudenko, P. Johnsson, A. V. Martin, M. Adolph, A. Aquila, S. Bajt, A. Barty, C. Bostedt, J. Bozek, C. Caleman, R. Coffee, N. Coppola, T. Delmas, S. Epp, B. Erk, L. Foucar, T. Gorkhover, L. Gumprecht, A. Hartmann, R. Hartmann, G. Hauser, P. Holl, A. Hömke, N. Kimmel, F. Krasniqi, K.-U. Kühnel, J. Maurer, M. Messerschmidt, R. Moshammer, C. Reich, B. Rudek, R. Santra, I. Schlichting, C. Schmidt, S. Schorb, J. Schulz, H. Soltau, J. C. H. Spence, D. Starodub, L. Strüder, J. Thøgersen, M. J. J. Vrakking, G. Weidenspointner, T. A. White, C. Wunderer, G. Meijer, J. Ullrich, H. Stapelfeldt, D. Rolles, and H. N. Chapman, “X-ray diffraction from isolated and strongly aligned gasphase molecules with a free-electron laser,” Phys. Rev. Lett. 112, 083002 (2014). 39A. G. Suits and J. W. Hepburn, “Ion pair dissociation: Spectroscopy and dynamics,” Annu. Rev. Phys. Chem. 57, 431 (2006). 40A. Terenin and B. Popov, “Photodissociation of salt molecules into ions,” Phys. Z. Sowjetunion 2, 299 (1932). 41S. Wang, K. P. Lawley, T. Ridley, and R. J. Donovan, “Field induced ion-pair formation from ICl studied by optical triple resonance,” Faraday Discuss. 115, 345 (2000). 42M. Ahmed, D. S. Peterka, P. Regan, X. H. Liu, and A. G. Suits, “Ion pair imaging spectroscopy: CH3Cl → CH+3 + Cl−,” Chem. Phys. Lett. 339, 203 (2001). 43K. Suto, Y. Sato, C. L. Reed, Y. M. V. Skorokhodov, and M. Kawasaki, “Ion Fragment Imaging of the Ion-Pair Photodissociation of CH3Cl, CH3Br, C2H5Cl, and C2H5Br at 118 nm,” J. Phys. Chem. A 101, 1222 (1997). 44A. H. Kung, R. H. Page, R. J. Larkin, Y. R. Shen, and Y. Lee, “Rydberg spectroscopy of H2 via stepwise resonant 2-photon ion-pair (H+ + H−) production,” Phys. Rev. Lett. 56, 328 (1986). 45H. Yoshida and K. Mitsuke, “Observation of doubly excited Rydberg states of N2O by positive ion-negative ion coincidence spectroscopy,” J. Chem. Phys. 100, 8817 (1994). 46K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion massspectrometric study of ion-pair formation in the vacuum ultraviolet 1. N2O → O− + N2+,” J. Chem. Phys. 92, 6556 (1990). 47Y. Hikosaka and E. Shigemasa, “Velocity imaging spectrometer for negative fragment ions: Application to dynamics of O2 and N2O ion-pair dissociation,” J. Electron Spectrosc. Relat. Phenom. 148, 5 (2005). 48W. Li, R. R. Lucchese, A. Doyuran, Z. Wu, H. Loos, G. E. Hall, and A. G. Suits, “Superexcited state dynamics probed with an extreme-ultraviolet free electron laser,” Phys. Rev. Lett. 92, 83002 (2004). 49K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion massspectrometric study of ion-pair formation in the vacuum ultraviolet. 3. SF6 → F− + SF5+,” J. Chem. Phys. 93, 8717 (1990). 50K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion massspectrometric study of ion-pair formation in the vacuum ultraviolet. 2. Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49 023107-15 Poretskiy et al. OCS → S− + CO+, O− + CS+, and CO2 → O− + CO+,” J. Chem. Phys. 93, 1710 (1990). 51K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion massspectrometric study of ion-pair formation in the vacuum ultraviolet. 4. CH4 → H− + CH3+ and CD4 → D− + CD3+,” J. Chem. Phys. 94, 6003 (1991). 52K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion massspectrometric study of ion-pair formation in the vacuum ultraviolet. 5. CF4 → F− + CF3+,” J. Chem. Phys. 95, 2398 (1991). 53S. Suzuki, K. Mitsuke, T. Imamura, and I. Koyano, “Negative-ion mass spectrometric study of ion-pair formation in the vacuum ultraviolet. 6. CH3X → X− + CH3+(X = F, Cl, Br),” J. Chem. Phys. 96, 7500 (1992). 54K. Mitsuke, S. Suzuki, T. Imamura, and I. Koyano, “Negative-ion mass-spectrometric study of ion-pair formation in the vacuum ultraviolet. 7. SO2 → O− + SO+, O− + S+ + S,” Org. Mass Spectrom. 28, 225 (1993). 55R. C. Shiell, X. K. Hu, Q. J. Hu, and J. W. Hepburn, “A determination of the bond dissociation energy (D0(H − SH)): Threshold ion-pair production spectroscopy (TIPPS) of a triatomic molecule,” J. Phys. Chem. A 104, 4339 (2000). 56A. V. Baklanov, L. M. C. Janssen, D. H. Parker, L. Poisson, B. Soep, J.-M. Mestdagh, and O. Gobert, “Direct mapping of recoil in the ion-pair dissociation of molecular oxygen by a femtosecond depletion method,” J. Chem. Phys. 129, 214306 (2008). 57T. Einfeld, C. Maul, K.-H. Gericke, and A. Chichinin, “Competing dissociation channels in the photolysis of S2Cl2 at 235 nm,” J. Chem. Phys. 117, 4214 (2002). 58T. Einfeld, A. Chichinin, C. Maul, and K.-H. Gericke, “Photodissociation of CSCl2 at 235 nm: Energy distributions and branching ratios of Cl atoms and CSCl radicals,” J. Chem. Phys. 117, 1123 (2002). 59A. I. Chichinin, T. Einfeld, K.-H. Gericke, J. Grunenberg, C. Maul, and L. Schäfer, “Photodissociation dynamics of SOCl2,” Phys. Chem. Chem. Phys. 7, 301 (2005). 60A. I. Chichinin, T. Einfeld, C. Maul, and K.-H. Gericke, “Three-dimensional imaging technique for direct observation of the complete velocity distribution of state-selected photodissociation products,” Rev. Sci. Instrum. 73, 1856 (2002). 61S. Kauczok, N. Gödecke, A. I. Chichinin, M. Veckenstedt, C. Maul, and K.-H. Gericke, “Three-dimensional velocity map imaging: Setup and resolution improvement compared to three-dimensional ion imaging,” Rev. Sci. Instrum. 80, 083301 (2009). 62A. I. Chichinin, S. Kauczok, K.-H. Gericke, and C. Maul, “Imaging chemical reactions – 3D velocity mapping,” Int. Rev. Phys. Chem. 28, 607 (2009). 63C. Maul, T. Haas, and K.-H. Gericke, “Spin selectivity in the UV photodissociation of phosgene,” J. Chem. Phys. 102, 3238 (1995). 64A. T. J. B. Eppink and D. H. Parker, “Velocity map imaging of ions and electrons using electrostatic lenses: Application in photoelectron and photofragment ion imaging of molecular oxygen,” Rev. Sci. Instrum. 68, 3477 (1997). 65D. H. Parker and A. T. J. B. Eppink, “Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics,” J. Chem. Phys. 107, 2357 (1997). Rev. Sci. Instrum. 87, 023107 (2016) 66W. C. Wiley and I. H. McLaren, “Time-of-flight mass spectrometer with improved resolution,” Rev. Sci. Instrum. 26, 1150 (1955). 67I. Ali, R. Dörner, O. Jagutzki, S. Nüttgens, V. Mergel, L. Spielberger, K. Khayyat, T. Vogt, H. Bräuning, K. Ullmann, R. Moshammer, J. Ullrich, S. Hagmann, K. O. Groeneveld, C. L. Cocke, and H. Schmidt-Bocking, “Multi-hit detector system for complete momentum balance in spectroscopy in molecular fragmentation processes,” Nucl. Instrum. Methods Phys. Res., Sect. B 149, 490 (1999). 68O. Jagutzki, V. Mergel, K. Ullmann-Pfleger, L. Spielberger, U. Spillmann, R. Dörner, and H. Schmidt-Bocking, “A broad-application microchannelplate detector system for advanced particle or photon detection tasks: Large area imaging, precise multi-hit timing information and high detection rate,” Nucl. Instrum. Methods Phys. Res., Sect. B 477, 244 (2002). 69A. Kvaran, A. Logadottir, and H. Wang, “(2 + 1) REMPI spectra of v = 0 states of the hydrogen halides: Spectroscopy, perturbations and excitation mechanisms,” J. Chem. Phys. 109, 5856 (1998). 70D. S. Green, G. A. Bickel, and S. C. Wallace, “(2 + 1) resonance enhanced multiphoton ionization of hydrogen chloride in a pulsed supersonic jet: Vacuum wavenumbers of rotational lines with detailed band analysis for excited electronic states of 1H35Cl,” J. Mol. Spectrosc. 150, 388 (1991). 71D. S. Green and S. C. Wallace, “Two-photon spectroscopy, Rydberg-valence interactions, and superexcited state dissociation of HCl probed by resonance enhanced multiphoton ionization,” J. Chem. Phys. 96, 5857 (1992). 72R. G. Bray and R. M. Hochstrasser, “Two-photon absorption by rotating diatomic molecules,” Mol. Phys. 31, 1199 (1976). 73J. Xie and R. N. Zare, “Rotational line strengths for the photoionization of diatomic molecules,” J. Chem. Phys. 97, 2891 (1992). 74D. R. Miller, in Atomic and Molecular Beam Methods, edited by G. Scoles (Oxford University Press, New York, Oxford, 1988), Vol. 1, pp. 14–53. 75J. Oberheide, P. Wilhelms, and M. Zimmer, “New results on the absolute ion detection efficiencies of a microchannel plate,” Meas. Sci. Technol. 8, 351 (1997). 76G. E. Busch and K. R. Wilson, “Triatomic photofragment spectra. II. Angular distributions from NO2 photodissociation,” J. Chem. Phys. 56, 3638 (1972). 77M. Poretskiy, A. I. Chichinin, C. Maul, and K.-H. Gericke, “Simultaneous imaging of both product ions: Exploring gateway states for HCl as a benchmark molecule,” Phys. Chem. Chem. Phys. 16, 19741 (2014). 78A. I. Chichinin, T. Einfeld, K.-H. Gericke, and C. Maul, “Photoionization of NO(A2Σ1/2) at 226 nm: Ion-recoil momentum spectroscopy,” Chem. Phys. Lett. 390, 50 (2004). 79R. P. Saxon and J. Eichler, “Theoretical calculation of two-photon absorption cross sections in atomic oxygen,” Phys. Rev. A 34, 199–206 (1986). 80J. D. Buck, D. C. Robie, A. P. Hickman, D. J. Bamford, and W. Bischel, “Two-photon excitation and excited-state absorption cross-sections for H2E,F1Σg (v = 6): Measurement and calculations,” Phys. Rev. A 39, 3932 (1989). 81M. Born and E. Wolf, Principles of Optics, 4th ed. (Pergamon Press, Oxford, 1968). 82E. F. van Dishoeck, M. C. van Hemert, and A. Dalgarno, “Photodissociation processes in the HCl molecule,” J. Chem. Phys. 77, 3693 (1982). 83K. S. Woodcock, “The emission of negative ions under the bombardment of positive ions,” Phys. Rev. 38, 1696–1703 (1931). Reuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 194.85.127.219 On: Tue, 14 Jun 2016 05:34:49