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TDR XFEL workshop series Atomic, molecular and cluster physics applications edited by : Th. Möller, B. Sonntag February 20, 2001 This report collects contributions submitted before, at and after the workshop. The complete workshop program and copies of transparencies can be found in the DESY report TESLA-FEL 2000-23, available from DESY. For a list of workshop participants see next page. Affiliation of editors and workshop participants are listed in the TESLA Technical Design Report Part V. Participants of the workshop J. Bauer U. Becker G. Bertsch T. Brabec M. Brewczyk J. Castello E. Dörner W. Eberhardt J. Feldhaus R. Freeman M. Gavrila C. Gerth C.H. Keitel E. Kennedy J. Krzywinski T. Laarmann A. Maquet G. Materlik K.-H. Meiwes-Broer M. Meyer T. Möller R. Moshammer J.-P. Mosnier H.G. Muller M. Neeb S. Novikov J.H. Parks R.M. Potvliege G. Prümper M. Richter J.-M. Rost J.-E. Rubensson J.R. Schneider A. Scrinzi B. Sonntag K. Starke K. Taylor Th. Tschentscher A. Yashishita Contents 1 Multiple charged ions in intense laser fields 3 2 Two photon excitation/ionization of atomic inner shells : the 1sshell of the argon atom 4 3 Dynamics of atomic and molecular autoionization states studied by two-photon excitation 15 4 XUV nonlinear optics: pulse shaping and applications 17 5 X-ray fluorescence experiments on atoms and molecules using FEL radiation 20 6 Multi-ionization of metal clusters by strong FEL pulses 25 7 Trapped Ion X-Ray Diffraction 27 8 Absolute photoionization experiments 30 9 Coherence effects in atomic and molecular photoionization 32 10 Multiphoton Multiple-Ionization of Atoms and Molecules using Reaction-Microscopes 34 Resonant Single and Multi-Photon Excitation and Ionisation of Highly Charged Ions by FEL-Radiation 36 Velocity Map Imaging of Photoelectrons from Free OrientedMolecules 38 11 12 -2- Multiple charged ions in intense laser fields Christoph H. Keitel Fakultät Physik der Universität, Theoretische Quantendynamik, Hermann-Herder-Str.3, D-79104 Freiburg i. Br., Germany Multiple charged ions are appropriate to study bound dynamics in extremely intense laser fields, as ionization can be very small with a high charge state. The energy separation increases strongly with increasing charge state so that high frequency and high intensity laser fields are very attractive to study highfrequency resonant dynamics, possibly even in the weakly relativistic regime. Recent calculations in our group [1] have shown weakly relativistic results in ions with laser intensities comparable with those envisaged for the FEL in Hamburg under construction. The magnetic component of the laser fields induces a strong Lorentz force in particular in the vicinity of the ionic core where the velocity of the wavepacket is very high. Then a reduced likelyhood of the electronic wavepacket near the nucleus was predicted along with an enhanced angular momentum. The strong angular momentum induces an enhanced spin orbit coupling due to the intense laser fields with novel spin signatures in the wavepacket dynamics and the radiation spectrum. We propose to expand those calculations to the high frequency regime where ions with higher charge states can be considered and where novel resonant and possibly nonresonant effects are likely both with respect to fundamental aspects and as a source of high harmonic generation. [1] S.X. Hu and C. H. Keitel, Phys. Rev. Lett 83, 4709 (1999) and Eur. Phys. Lett. 47, 318 (1999). - 3- Two-photon excitation/ionization of atomic inner shells: the 1s-shell of the argon atom Sergey NOVIKOV Chair of Mathematics-1, Rostov State University of Transport Communication, Narodnogo Opolcheniya Square 2, Rostov-on-Don, 344038 , Russia E-mail: [email protected], [email protected] Abstract The absolute values and the shape of the two-photon one-electron excitation/ionization cross section of the 1s-shell of the argon atom with inclusion the effects of relaxation of the atomic residue in the field of the creating vacancies are calculated. The results of calculation have the character of prediction. Introduction Recently, the interest for experimental and theoretical investigations of interaction processes of x-ray radiation with atom has increased [1,2]. Similar investigations are very important for modern fundamental and applied physics. So, the explorations of non-linear processes of the x-ray photons interaction with atom, particularly, have direct relation to solving the problem of x-ray laser design [3]. In this paper we consider the problem of theoretical description of multiphoton excitation/ionization of inner shell of free atom. The cross sections of two- and three- photons absorption by many-electron atoms’ outer shells are calculated in refs [4-7]. The analogous investigations in the region of absorption thresholds in the x-ray range of photon energy are absent now. The aim of this work is to calculate the cross section of two-photon absorption by an atomic inner 1s-shell of a simple system with the 1 S 0 ground state term – the argon atom. It is well known, that at the theoretical description of the cross section of the onephoton absorption by inner and outer shells it is necessary to take into account various types of many-electron correlations [8]. The most important manyelectron effect in photoabsorption by inner shell is monopole rearrangement of electron shells of the atomic residue within the field of an inner shell vacancy [9,10]. In this work we present the results of theoretical study of the absolute -4- values and the shape of the two-photon one-electron excitation/ionization cross section of the 1s-shell of argon atom with inclusion the effects of relaxation of the atomic residue in the field of the creating vacancies. Theory The cross section of the two-photon absorption of atom is the probability of the process divided by the squared density of the flux of photons incident on atom (the dimension of the two-photon absorption cross section is cm 4 s ). In the case of absorption by 1s-shell of the argon atom we have: σ 1(s2) (ω ) = 8π 3 Dα 2 a 04 (Dω ) 2 S (ω ) , 2 (1) where S (ω ) 2 å A ms (ω ) å A md (ω ) = m> f + = S sL (ω ) m> f 2 2 + S dL (ω ) [2 D ω + E 2 [2 D ω + E 2 = Γ1 s / 2π 0 − E (1s − 1 ms ) Γ1 s / 2π 0 ] − E (1s − 1 md ) 2 ] 2 + Aε s = 2 D ω + E + Γ12s / 4 + Γ12s / 4 0 + Aε d = 2 D ω + E − E (1 s 0 −1 (ω ) ) − E (1 s − 1 ) 2 (ω ) + 2 (2) for linearly (L) polarized photons, and S (ω ) polarized photons. It is possible to show that In (2) Aì m ü ì s ü 2 = S dC (ω ) 2 2 S dC (ω ) = for circularly (C) 2 3 L S d (ω ) . 2 is the amplitude of the process calculated in the second-order í ýí ý î ε þî d þ perturbation theory with the use of the methods of the theory of non-orthogonal orbitals [11,12]. The expression for the amplitude has the form: Aì m ü ì s ü (ω ) = í ýí ý î ε þ îd þ ∞ å β + ò dε ν 0 ìm üì s ü 1s −1 í ýí ý D0 β î ε þîd þ Dω + E 0 − E ( β ) + i β D0 0 + Γβ 2 ìmüì s ü ìm ü 1s −1 í ýí ý D0 ν ν D0 0 1s −1 í ýs 0 N e0 î ε þîd þ îε þ − . Γν 2 (Dω ) 2 Dω − IPν − ε ν + i 2 - 5- (3) Here IPν is the ionization potential of the ν -shell, D0 ≡ D z is z-component of the operator of interaction in the dipole approximation, Γν is the total width of the ν vacancy decay, N=18 is total number of the electrons of the argon atom, e0 = m e4 ≡ 1 a.u. = 27.21 eV . In the case of transition into d-symmetry the channel D2 of the last (contact) term in (3) is absent. In expressions (1),(2),(3): a0 is the Bohr radius; α is the constant of fine structure; Dω is the energy of photons incident on atom; E0 , E (1s −1 ), Eçç1s −1m í æ è ì s üö ý ÷÷ îd þ ø are the total Hartree-Fock energies of ground state, of single ionization state, and of excitation states of an atom, respectively; f is the Fermi level (the set of the quantum numbers of the valence shell of an atom); 0 = 1s 2 2 s 2 2 p 6 3s 2 3 p 6 (1 S 0 ) is initial state of the argon atom. (2) -52 4 σ1s , 10 cm s 25 10 linearly polarized photons 18 Ar 15 B 10 C 5 G-K 10 -5 D A 10 1 10 100 F E 1000 Photon energyω, eV Figure 1 The cross section σ 1(s2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated taking into account the effect of relaxation of the atomic residue in the field of the creating vacancies with (solid line) and without accounting of this effect (dotted line). Notation A,B,C etc. see in the text. -6- 3. Results of calculation In the fig. 1 (fig. 2) the results of the calculations of the absolute values of the cross section of the absorption of two linearly (circularly) polarized photons by 1s-shell of the argon atom are shown. The effects of radial monopole rearrangement of electron shells within the field of the creating vacancies is taken into account by the modification of the dipole matrix elements in expression (3) by methods of the theory of non-orthogonal orbitals [11,12]. The calculations of the cross section of the two-photon absorption are fulfilled using the following values of the decay widths: Γ1tots = 0.656 eV [13], −7 −9 Γ2tots = 2.40 eV [14], Γ2totp = 0.13 eV [15], Γ3rad eV [16], Γ3rad eV . p = 10 s = 1.4 ⋅ 10 (2) σ1s , 10 10 25 10 15 10 5 10 -5 -52 4 cm s circularly polarized photons 18 Ar G-K B C D 1 10 F E 100 1000 Photon energy ω, eV Figure 2 The same as in Figure 1 for circularly polarized photons. In expression (3) the following channels ìs ü ìs ü β = 1s −1np , 2s −1 np , 2 p −1n í ý , 3s −1 np , 3 p −1 í ý , îd þ îd þ n=3,4) for discrete - 7- have been used: n=4,5 (for d-symmetry intermediate states, ìsü ìs ü ν = 1s −1εp , 2 s −1εp , 2 p −1ε í ý , 3s −1εp , 3 p −1ε í ý for continuous intermediate îd þ îd þ ìmüì s ü states, and 1s −1 í ýí ý for the final states, m=4,5 (for d-symmetry – m=3,4). î ε þîd þ For configurations of intermediate and final states only the quantum numbers of the vacancy and excited electron are shown. The separate self-consistent solutions of the Hartree-Fock equations are used for the initial, intermediate, and final states. The contribution of contact interaction into the cross section of the process is non-zero only for linearly polarized photons, it is substantial at small energy of photon, and it rapidly decreases with the increase of the photon energy (fig.1, region A). In the case of the contact interaction the 1s-shell simultaneously absorbs two photons and creates s-electron, after that the virtual 1s-vacancy decays through Auger channels. The contact interaction appears because of the fact that the electron shells of the atom are rearranged in the field of the 1svacancy. Without inclusion of the effect of relaxation of the atomic residue in the field of the 1s-vacancy the amplitude of the contact interaction is equal to zero (fig.1, region A, dotted line). In the region B (D) , see fig.1 and fig.2 , the energy of one photon is approximately equal to the ionization potential of the 3p (2p)-shell of the argon atom IP3 p = 14.762 eV ( IP2 p = 248.741eV ). In this spectral region the theory predicts the existence of the structure caused by resonant [ Dω ≅ IP3 p ,( Dω ≅ IP2 p ) ] excitation of the 3p (2p)-shell by one photon into the final excitation/ionization (s,d)-states. The second photon is absorbed then by electron of the 1s-shell and this electron fills the 3p (2p)-vacancy. In the region B (D) of the spectrum the ì4, 5üì s ü ýí ý î3, 4þîd þ discrete intermediate states β = 3 p −1 í ì4, 5üì s ü ýí ý î3, 4þîd þ ( β = 2 p −1 í ) are apparent. For circularly polarized photons the transitions into the s-symmetry of intermediate states are forbidden. The resonant structure in the region B (D) (fig.1 and fig.2) exists exclusively due to the existence of the virtual 1s-vacancy. In the region C (E) , see fig.1 and fig.2, the energy of single photon is approximately equal to the ionization potential of the 3s (2s)-shell of the Ar atom IP3s = 33.416 eV ( IP2 s = 327.00 eV ). In this spectral region the theory predicts the existence of the structure caused by resonant [ Dω ≅ IP3s , ( Dω ≅ IP2 s )] excitation of the 3s (2s)-shell by the photon into the intermediate (p)-states of the discrete spectrum. The second photon transforms the excited intermediate (p)-states into final excitation/ionization states of (s,d)-symmetry, and one of the 1s-electrons fills the 3s (2s)-vacancy. In the region C (E) of the spectrum the discrete -8- ì4ü î5 þ ì4ü î5 þ intermediate states β = 3s −1 í ý p ( β = 2s −1 í ý p ) are apparent. The resonant structure in the region C (E) (fig.1 and fig2.) exists exclusively due to the existence of the virtual 1s-vacancy. In the region F, see fig.3 (fig.4), the energy of one photon is approximately equal to half of the 1s-shell ionization potential of argon atom IP1s = 3206.982 eV . Now the energy of two photons is enough to generate of structure of the cross section deals with resonant ( 2Dω ≅ IP1s ) formation of the 1s-vacancy and the (s,d)-states of final discrete spectrum. In the region F of the spectrum the discrete final ì4, 5üì s ü for linearly polarized photons (fig.3) and discrete final 1s −1 í ýí ý î3, 4þîd þ ì3ü states 1s −1 í ýd for circularly polarized photons (fig.4) are apparent. î4þ states -52 (2) σ1s , 10 10 -2 4 cm s F linearly polarized photons 18 Ar IP1s/2 1 2 10 -3 10 -4 -1 1 - |1s 4s;4d> -1 2 - |1s 5s;3d> MC 1600 1605 1610 1615 1620 Photon energy ω, eV Figure 3 The cross section σ 1s( 2) (the region F) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated taking into account the effect of relaxation of the atomic residue in the field of the creating vacancies with (solid line) and without accounting of this effect (dotted line). In the figure the discrete final states are indicated. MC – minimum of Cooper[17] (the amplitude of transition 4p εd change sign in this range). à - 9- Perhaps, just the range F is perspective region for future experiments. In the first place, the total decay widths of these states are more larger. Secondly, the shape of the spectrum in this region strongly depended on the type of polarization of incident x-ray photons. There are two type of resonances at the range 2800 eV ≤ Dω ≤ 3300 eV (fig.5,7 and fig.6,8). The first group of resonances (G: Dω = IP1s − IP2 s ; H: Dω = IP1s − IP2 p ; I: Dω = IP1s − IP3s ; J: Dω = IP1s − IP3 p ) corresponds to resonant transition of electron from 1s-shell into 2s,2p,3s,3p-shell, responsible. One of two photons knocks off an electron of the 2s,2p,3s,3p-shell into the continuous spectrum of final (s,d)states. The second photon is absorbed by an electron of the 1s-shell, after which the electron fills the 2s,2p,3s,3p-vacancy. The second group (K) corresponds to resonant ( Dω ≅ IP1s ) excitation of an electron of the 1s-shell into virtual (p)-states of the discrete spectrum by one of the photons. Absorbing the second photon, these (p)-states are transformed into the final excitation/ionization (s,d)-states. (2) σ1s , 10 10 -2 10 -3 10 -4 10 -5 10 -6 18 -52 Ar 4 cm s circularly polarized photons F IP1s/2 1 2 -1 1 - |1s 4d> -1 2 - |1s 3d> MC 1600 1610 1620 Photon energy ω, eV Figure 4 The same as in Figure 3 for circularly polarized photons. - 10 - (2) -52 σ1s , 10 10 4 cm s linearly polarized photons 30 18 10 20 10 10 10 0 Ar I J G H K 2900 3000 3100 3200 Photon energy ω, eV Figure 5 The cross section σ 1s( 2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated taking into account the effect of relaxation of the atomic residue in the field of the creating vacancies with (solid line) and without accounting of this effect (dotted line). Notation G,H,I,J,K see in the text. Account of the effects of relaxation of the atomic residue in the field of the creating vacancies (solid line in the figs) changes essentially the theoretical absolute values of cross section of the process calculated without inclusion of these effects (dotted line in the figs). It follows to note, that the problem of completeness of basis set of the intermediate virtual states in (3) is not considered in the present work. - 11- (2) σ1s , 10 10 -52 4 cm s 30 18 Ar 10 20 10 10 I J G 10 circularly polarized photons H K 0 2800 3000 3200 Photon energy ω, eV Figure 6 The same as in Figure 5 for circularly polarized photons (2) -52 σ1s , 10 10 -1 10 -4 18 Ar 4 linearly polarized photons cm s K J 12 -1 1 - |1s 4s> -1 2 - |1s 5s> -7 103180 3200 3220 Photon energy ω, eV Figure 7. The cross section σ 1s( 2) of the process of excitation/ionization of the Ar 1s-shell by two linearly polarized photons calculated taking into account the effect of relaxation of the atomic residue in the field of the creating vacancies with (solid line) and without accounting of this effect (dotted line). In the figure the intermediate states are indicated - 12 - (2) -52 σ1s , 10 18 0 10 4 cm s Ar circularly polarized photons J K 1 2 -4 10 -1 1 - |1s 4s> -1 2 - |1s 5s> -8 10 3190 3200 3210 Photon energy ω, eV Figure 8 The same as in Figure 7 for circularly polarized photons. Conclusion The fulfilled investigation has shown that the process of two-photon excitation/ionization of the inner atomic shell has essentially the many-electron character. The second photon realizes resonance structures by many-electron effects in the regions of absorption spectrum where they are absent for onephoton process. So, the regions A-E (fig.1 and fig.2) deal with the effect of quantum interference of the excitation/ionization process of the atomic 1s-shell with the process of Auger decay of the creating virtual 1s-vacancy. Moreover, the existence of the contact interaction of two photons with inner shell is additionally due to effect of relaxation of atomic residue in the field of inner vacancy. It follows to note that the inner atomic shell exposes itself already at the energies of laser beam photons significantly less than the ionization potential of this shell. This theoretical prediction can be experimentally investigated using existing lasers. - 13- References [1] Schmidt V 1992 Rep. Prog. Phys. 55 1483 [2] Pratt R H 1999 18th Int. Conf. X-ray and Inner Shell Processes (Chicago, IL,USA), Book of abstracts, T12, p 38 [3] Kato Y 1999 18th Int. Conf. X-ray and Inner Shell Processes (Chicago, IL, USA), Book of abstracts, T54, p 80 [4] McGuire E J 1981 Phys. Rev. A 24 835 [5] Moccia R, Rahman N K and Rizzo A 1983 J. Phys. B: At. Mol. Phys. 16 2737 [6] L’Huillier A, Jönsson L and Wendin G 1986 Phys. Rev. A 33 3938 [7] L’Huillier A and Wendin G 1987 J. Phys. B: At. Mol. Phys. 20 L37 [8] Amusia M Ya 1990 Atomic Photoeffect (New York, London: Plenum Press) [9] Amusia M Ya and Cherepkov N A 1975 Case Stud. At. Phys. 5 47 [10] Sukhorukov V L, Demekhin V F, Timoshevskaya V V and Lavrentiev S V 1979 Opt. Spectrosc. 47 228 [11] Jucys A P and Savukinas A J 1973 Mathematical Foundations of the Theory of Atom (Vilnyus: Mintis) [12] Åberg T and Howat G 1982 Handbuch der Physik vol 31, ed W Mehlhorn (Berlin: Springer) pp 469-619 [13] Chen M H, Crasemann B and Mark H 1980 Phys.Rev A 21 436 [14] Papp T, Campbell J L and Varga D 1997 17th Int. Conf. X-ray and Inner Shell Processes (Hamburg, Germany), AIP Conference Proceeding, No 389, p 431 (Woodbury, New York) [15] Nakamura M, Sasanuma M, Sato S, Watanabe M, Yamashita H, Iguchi Y, Ejiri A, Nakai S, Yamaguchi S, Sagawa T, Nakai Y and Oshio T 1968 Phys. Rev. Lett. 21 1303 [16] Lauer S, Liebel H, Vollweiler F, Schmoranzer H, Lagutin B M, Demekhin Ph V, Petrov I D and Sukhorukov V L 1999 J. Phys. B: At. Mol. Phys. 32 2015 [17] Cooper J W 1962 Phys. Rev. 128 681 - 14 - Dynamics of atomic and molecular autoionization states studied by two-photon excitation M. Meyera, A. Grum-Grzhimailob, a L.U.R.E., Centre Universitaire Paris-Sud, 91898 Orsay, France b Moscow State University, Moscow 119899, Russia As an extension of our pump-probe studies on highly excited states [1, 2] some experiments are proposed, which will benefit in particular from the short temporal width and the high photon flux available at the TESLA-FEL. With respect to the former investigations, the FEL will replace the VUV photons from synchrotron radiation or high-harmonic generation sources. This arrangement will open the vast field of studies on atomic and molecular autoionization states and their relaxation dynamics. Out of the huge number of possible experiments three examples are presented for illustration of the basic ideas: i) Investigation of autoionization states with same parity as the ground state, e.g. the Xe* 4d9 5s2 5p6 nd and ms resonances. Due to dipole selection rules, these states can only be excited either by a direct two-photon one-color excitation or by a two-photon two-color excitation via an intermediate resonant state, like the strong Xe* 4d9 6p resonance. The short lifetime of the 4d9 6p resonance (Γ = 0.11eV, τ = 6 fs) necessitates the combination and synchronization of fs-pulses (VUV+Visible). Resonant Auger spectroscopy will be used to study the relaxation dynamics and the influence of alignment of the intermediate state when the polarization of the two photon beams can be changed in a controlled way. The results will extend and complete the conventional studies using one-photon excitation and will provide new and important basis for the theoretical treatment of electronic interaction and correlation in atomic multi-electron systems. ii) Coupling of autoionization resonances by a strong laser field: Similar to the above example, the FEL and a second synchronized fs-laser are used in a pumpprobe arrangement to excite and to couple to autoionization resonances, like He* 2s2p 1P and 2s 3d 1D. The profile of the resonances, being strongly asymmetric for He* 2s2p 1P [3], will strongly depend on the intensity of the two laser beams and on small changes in the relative time delay of the pulses and the photon energy of the second laser (detuning). As a concrete example, a theoretical treatment [4] predicts a minimum in the resonance profile for laser intensities of 1GW/cm2, which will be accessible in the planned experimental configuration. iii) Relaxation of molecular autoionization states: Resonant photoexcitation of molecules in the VUV leads in most cases to the dissociation of the molecule on a time scale ranging from femtoseconds (i.e. at the same time as autoionization) - 15- to nanoseconds (i.e. after autoionization). Using high-resolution laser spectroscopy the rotational structure of diatomic fragments (N2+, CO+) which are produced upon resonant excitation of larger molecules (N2O, CO2, COS,), direct information on the geometry of the dissociating state can be obtained. Timeresolved spectroscopy on molecular or atomic fragments will give complementary insight, especially with respect to the dissociation dynamics, e.g. comparison between fast direct dissociation, stepwise dissociation via intermediate products or dissociation of metastable states of long lifetime. [1] M. Gisselbrecht, A. Marquette et M. Meyer, J.Phys.B31, L977 (1998) [2] M. Gisselbrecht, D.Descamps, C.Lynga, A.L'Huillier, C.G.Wahlström, M.Meyer, PRL 82, 4607 (1999) [3] M. Domke et al., Phys.Rev.Lett. 69, 1171 (1992) [4] H. Bachau, P. Lambropoulos et Shakeshaft, Phys.Rev.A34, 4785 (1986) - 16 - XUV nonlinear optics: pulse shaping and applications Thomas Brabec and Armin Scrinzi Institut für Photonik, Technische Unviersität Wien, Gusshausstraße 27/387, A-1040 Wien, Austria E-mail: [email protected]; [email protected] The high intensities of the FEL source at wavelength in the VUV and XUV range will allow to systematically study multiply excited states of inner shell electrons. If timing and a sufficient degree of temporal coherence of the pulse are secured, the decay dynamics of highly correlated multiply excited states may be observed directly for the first time by pump-probe schemes. We propose a scheme to control the pulse shape of the FEL source by a laser induced x-ray switch (LIXS) which has the potential to generate sub-10 fs pulses and we discuss the application of such pulses as pump in pump-probe experiments on doubly excited states of Helium. The idea of the LIXS is as follows: the resonant interaction of an xuv pulse with an ion gas leads to coherent population transfer between the ground and the resonant (excited state) causing absorption or amplification of the xuv pulse in dependence on the phase between electric field and medium polarization (which is determined by the population difference between the two states). Absorption is maximum when the phase difference between electric field and polarization is π/2, amplification is maximum when the phase difference between electric field and polarization is 3π/2. When the pulse parameters are chosen so that the phase difference is π/2 at the pulse peak and π at the trailing pulse edge, the xuv pulse experiences loss over its whole temporal profile, with maximum loss at the pulse peak, see Fig. 1. The basic idea of LIXS is to change absorption into amplification by using a short laser pulse copropagating with the the xuv pulse. The parameters of the laser pulse are chosen so that the laser pulse shifts the phase (as a result of the Stark effect) between xuv pulse and polarization by 2π, thus opening a gain window, see Fig. 1. The gain window cuts out s short fraction of the xuv-pulse resulting in pulse compression. The question whether LIXS can be realized experimentally depends crucially on the propagation behavior of the laser and xuv pulse in the ion gas. The xuv pulse experiences strong changes during propagation, which can significantly change the gain window and destroy the switching mechanism. To test LIXS under more realistic conditions including propagation we have solved self consistently 1D (laser and xuv) pulse propagation in combination with the microscopic atomic response (3 level system, 1s,2p,2s). The results are encouraging. The gain - 17- window is not changed during propagation leading to the generation of a 5fs xuv pulse. Calculations with modified parameters have shown that the mechanism works in a broad range of intensity parameters so that it should also work in the case of 3D pulse propagation. 0.10 Loss [arb.u.] Loss [arb.u.] 0.10 0.05 0.05 0.00 -0.05 -0.10 -10 -5 0 5 10 Time [fs] 0.00 -0.05 -0.10 -600 -300 0 Time [fs] 300 600 Figure 1 shows the imaginary part of the polarization (loss/gain) induced by a laser and an xuv pulse in Be3+ gas. The inset shows a zoom of the loss/gain curve between –10 fs and 10 fs. The parameters are as follows. The xuv wavelength λxuv = 7.6 nm is chosen to be resonant with the 1s => 2p transition of Be3+. The xuv peak intensity is chosen to be I xuv = 2x1010 W/cm2 and the (FWHM) pulse duration τxuv = 300 fs. The laser wavelength, intensity, and (FWHM) pulse duration are λ0 =800 nm, I0=2x1015 W/cm2, and τ0 =5 fs, respectively. The center of the laser pulse coincides with the center of the xuv pulse. A significant population of even angular momentum doubly excited states of Helium by a non-linear two-photon process require intensities beyond 1014 W/cm2. A Floquet calculation of the correlated two-electron system shows that the competition of single photon ionization is very strong and requires very short pulses with durations of less than 10 fs. For more highly charged He-like ions required times and intensities scale like Z2 and Z6, respectively. - 18 - Alternatively, even angular momentum states can be populated by combining a strong XUV pulse with an optical laser pulse, where a long-lived doubly excited P state is used as a gateway state (see Fig. 2).In this case, timing requirements on the FEL source are less stringent, since the optical laser provides the crucial time mark. Floquet calculations and a solution of the time dependent Schrödinger equation show that populations of the order 10 % can be transfered into doubly excited states with pulse durations and intensities of the projected FEL source. Figure 2 XUV-optical pumping of doubly excited states by using a long-lived double excited state as gateway. With pulse durations of 300 fs and XUV and laser intensities of the order 1013 W/cm2 and 108 W/cm2, respectively, up to 10 % of the population can be transferred into a selected doubly excited state. - 19- X-ray fluorescence experiments on atoms and molecules using FEL radiation Jan-Erik Rubensson and Joseph Nordgren Physics Department Uppsala University E-mail: [email protected], [email protected] Fundamental atomic physics in few-electron systems In comparison to measurements of charged particles emitted after exposure to VUV and soft X-ray radiation, there has been relatively little interest in measurements of secondary photons. The reasons are the notorious low count rates due to the low fluorescence yield and the low probability for direct nonresonant scattering. At FEL intensities this will no longer be a limitation. In contrast to measurements of charged particles photon based experiments are not affected by plasma potentials and radiation induced changes in the work function of the analyzing instrument. These problems have to be dealt with today in high resolution photoelectron spectroscopy [1], and at FEL intensities we anticipate that they will be much more severe. Even at conventional synchrotron radiation sources there is still a lot to learn by simply measuring the integrated fluorescence yield as a function of energy of the incoming radiation. In this way we could recently demonstrate that, contrary to common notion, the radiative decay channel dominates for most of the doubly excited 2lnl' states of helium [2]. Analysis of the behavior close to threshold in terms of many body quantum defect theory unravels a fascinating dynamics where the splitting of the N=2 threshold due to relativistic and quantum electrodynamic effects is essential [3]. The potential for the outer (Rydberg) electron is influenced by the energy of the inner (N=2) electron, which is determined by the fundamental interactions. This leads to a beak-down of LScoupling already for the helium atom. These results directly suggest several experiments with FEL radiation: • Fluorescence yield measurement as a function of energy and intensity. The simplest experiment at stage 2 of the TESLA project would be to repeat the fluorescence yield experiment on the doubly excited states of helium and to explore the intensity dependence of the cross sections. The energy should be varied not only in the 60-66 eV region where one photon can excite both - 20 - electrons, but also in the 30-33 eV energy region where two- photon excitation may occur. • Pump-probe measurements. The double excitations of helium close to the N=2 threshold have a lifetime of around 0.1 ns, set by the radiative decay rate. The long lifetime makes these states ideal for pump-probe experiments. As the energy spacing between the Rydberg states corresponds to the FEL pulse duration the states are excited coherently, and electron wavepacket dynamics may be probed by a delayed FEL pulse. Several interesting pump-probe experiments using fast conventional laser pulses as probe may be considered. • Energy-resolved photon detection Measuring the emission with high-energy resolution gives complementary information on the electron dynamics. Again, the long lifetime of the doubly excited states, together with the Rydberg wavepacket dynamics makes them ideal as a test case for time and energy resolved VUV spectroscopy. Energy analysis using grazing incidence grating spectrometers constitutes the bulk of our research activities, and we are presently developing a Fourier transform interferometer, matching the photon energies reached in stage two of the TESLA project. • Doubly excited states of He-like ions. The doubly excited states of Li+, Be2+, B3+, C4+ etc can be studied in the same way as for the helium atom. The fundamental interactions will split the N=2 level in a different way for theheavier ions, and thereby influence the Rydberg series. Highly charged ions at sufficient concentrations can be produced by ECR type ion sources. One may also use the FEL beam itself to produce the highly charged ion target. This is a suggestion for the final stage of the TESLA project. Resonant inelastic scattering in molecules Experiments where the energy distribution of the secondary photons in the subkeV region is determined is the heart of the activity in our research group. Gasphase measurements can only be performed at the most powerful beamlines of third generation sources, and still they are severely restricted by the available primary intensity. Our experiments have significantly contributed to the understanding of the corestate excitation-emission scattering process [4]. Obviously, this understanding is - 21- restricted to the linear limit, which is accessible at conventional synchrotron facilities. The validity of the dipole approximation implies that the symmetry of the involved states largely determines the scattering spectrum. If, e. g., the molecule has inversion symmetry, only transitions which change parity from gerade to ungerade (or ungerade to gerade) are allowed. At conventional intensities the symmetry selection rules are strictly valid. Due to vibronic coupling the electronic symmetry may break during the scattering process, thereby relaxing the selection rules and the appearance of 'forbidden' peaks in the spectra. Using this symmetry signature we have investigated the dynamic vibronic coupling. Most interesting, it is found that the vibronic coupling is quenched when the primary energy is tuned some eV away from the resonance [5]. We have recently used this phenomenon to characterize the bonding in a complex transition metal fulleride compound, with implications for the organometallic bond in general [6]. Of course, excitation away from threshold requires even more primary intensity. Based on this experience we suggest the following soft X-ray spectroscopy experiments using FEL radiation: • Intensity dependence A first experiment at the FEL is to repeat the measurements on small homonoculear diatomics at different intensities. In the absence of vibronic coupling the appearance of 'forbidden' peaks would immediately indicate a departure from the conventional description of the scattering process. For stage two of the FEL project the L shell of the Cl2 molecule is just within reach for one-photon excitations. Sufficient density of more unusual diatomic molecules can be created in cluster sources. For two-photon resonant excitation we note that K shell excitations of the N2 molecule can be reached already with the energies of stage two of the TESLA project. In the scattering spectra two-photon excitation can easily be separated from higher-order excitation due to the complementary selection rules. • Pump-probe experiments A laser pulse can be used to pre-excite a symmetric molecule. By excitation to high vibrational and rotational states, one can appreciably increase the average bond distance in a homonuclear diatomic. Close to the dissociation limit the vibrational wavefunction has a large weight at the classical outer turning point. By performing scattering experiments with initial states which have been preexcited in this way one can examine effects beyond the dipole approximation. This approximation is expected to break down when the distance between the - 22 - atoms becomes longer than the wavelength of the radiation. Via the symmetry selection rules this can be followed experimentally. The break-down of the dipole approximation can be directly associated with the localization of the core hole to one of the atoms. • Characterization of bonding in complex molecules and clusters To take advantage of the selection rules in the scattering process it is necessary to excite a few eV below threshold. FEL intensity is required to characterize the bonding in larger molecules e. g. of biological interest, both in gas-phase or dilute liquid form. The same principle can be used to study free clusters where target density necessarily is small. • Plasma analysis At FEL intensities there is a large cross section for multielectron emission, resulting in a multitude of highly charged ions. Analysis of the soft X-rays will be a useful tool for characterization of the dynamics initiated by the FEL pulse. X-ray spectroscopy is already used for plasma diagnosis in fusion experiments. Exotic atoms and molecules In exotic atoms and molecules at least one electron is replaced by a heavier particle. The simplest system is the muonic hydrogen atom, consisting of a proton and muon. The binding energy is around 2.5 keV, and the characteristic X-rays from muonic hydrogen have already been measured [7]. Thus, the fundamental excitations are all within in reach of the X-ray FEL. The prospect of performing 'classical synchrotron radiation experiments' on such systems would open up an entirely new field of research, with the prospect of gaining new insights into the electroweak interaction and the structure of the constituent particles. We suggest that a feasibility investigation regarding such experiments be started in the near future. A key issue is to estimate the efforts needed to produce sufficient concentrations of exotic systems. Theoretical predictions and detailed formulation of the physical questions that may be addressed are also needed. For this type of studies there may also be important applications. The formation of the dµt molecule is essential for muon catalyzed fusion. This molecule is similar to molecular hydrogen, except that one proton is replaced by a d+, t+, and a µ- particle. The muon induces the d+t→α+n fusion reaction, and the rate for this process has recently been measured [8]. The feasibility to perform - 23- spectroscopy on such systems using X-ray FEL radiation may have an impact on our efforts to control the fusion process. Also regarding this question a theoretical investigation is required. References [1] G. Öhrwall, thesis, Uppsala University (1999) [2] J-E. Rubensson et al. Phys. Rev. Lett. 83, 947 (1999). [3] T. W. Gorczyca, et al. Phys. Rev. Lett., 85, 1202 (2000). [4] see e.g Appl. Phys. A65, (1997). [5] P. Skytt, et al., Phys. Rev. Lett. 77, 5035 (1996). [6] L. Qian et al, submitted for publication. [7 ] B. Lauss et al. Phys. Rev. Lett., 80, 3041 (1998). [8] M. C. Fujiwara et al., Phys. Rev. Lett 85, 1642 (2000) - 24 - Multi-ionization of metal clusters by strong FEL pulses Josef Tiggesbäumker and Karl-Heinz Meiwes-Broer Fachbereich Physik, Universität Rostock, 18051 Rostock http://www.physik.uni-rostock.de/Cluster/ The coupling of intense radiation into matter is an interesting topic in fundamental and applied sciences. In contrast to bulk surfaces, low-dimensional objects turned out to be particularly photosensitive. Rare gas, molecular as well as metal clusters exhibit giant photoabsorption cross sections when compared to the respective atomic constituents. E.g., in the case of platinum clusters we showed that the multiply charging process can be explained by the evolution of the plasmon energy of the metal cluster upon the change in electron density during the Coulomb explosion [1]. The corresponding temporal evolution of the cluster density is revealed by molecular dynamics simulation, see Fig. 1 for the example of Pt 5810+. Then, for each time step the optical response is calculated in RPA. The resulting dynamics of the absorption is in agreement with the experimentally determined charging efficiency as function of the light pulse width [2]. Figure 1 Coulomb explosion of Pt58 10+. The cluster density stays nearly unchanged during the first 100 fs. Then a rapid decrease is accompanied by an increase in radius. Similar time dependencies are found in lead clusters. Carbon clusters behave differently as they show the highest charging efficiency with the shortest light pulse [2,3]. Whereas details of the light coupling and the dynamics of the charge flow are still the matter of several theoretical investigations, the interesting - 25- question arose about the role of the wavelength of the exciting radiation. With conventional femtosecond laser systems there is not much room for a variation. FEL sources, on the other hand, will provide intense fs pulses with peak powers of 5 GW at photon energies of 60-200 eV. At these photon energies, in some systems (e.g., alkalis, semiconductors) weakly bound core levels can be ionized. Will it be possible to coherently excite a manifold of those core electrons? [1] L. Köller, M. Schumacher, J. Köhn, S. Teuber, J. Tiggesbäumker, K.H. Meiwes-Broer, Phys. Rev. Lett. 82 (1999) 3783 [2] T. Döppner, S. Teuber, M. Schumacher, J. Tiggesbäumker, K.H. MeiwesBroer, Int. J. Mass Spec. 192 (1999) 387 [3] M. Schumacher, S. Teuber, L. Köller, J. Köhn, J. Tiggesbäumker, K.H. Meiwes-Broer, Europ. Phys. J. D 9 (1999) 411 - 26 - Trapped Ion X-Ray Diffraction Joel H. Parks Rowland Institute for Science 100 Edwin H. Land Boulevard Cambridge, MA 02142 Abstract The recent application [1] of electron diffraction to trapped cluster ions has provided a measure by which structural phase changes can be observed as a function of cluster size and temperature. This technique has been applied [2] to the study of CsI clusters and has identified the appearance of bulk structure isomers at a cluster size corresponding to a completed atomic shell having rhombic dodecahedron geometry. The proposed research will investigate the feasibility of extending these diffraction techniques to utilize the X-Ray FEL device to study both elastic and inelastic scattering from trapped cluster ions. The intensity of the FEL device under consideration offers the possibility of overcoming the significantly smaller cross-section for x-ray elastic scattering, in particular at larger scattering angles as shown in Fig. 1. The right hand axis indicates the ratio of x-ray to electron total scattering assuming an incident electron beam of 2.5x1012 e-/s and an x-ray flux of 4x1018 photons/s. For these parameters, the gain in large angle scattering is substantial. However, such high x-ray fluxes require orders of magnitude reduction in background scattering compared to electrons to achieve this advantage. For this reason, design of the trapping structure placement of the detector and the x ray beamline through the UHV system needs to be relatively open and free of apertures to avoid background scattering. The possibility of strong inelastic x-ray scattering is another important issue which primarily determines the rate at which the trap has to be reloaded with the primary cluster ions. The trapped ion density is limited by electrostatic interactions. Both of these issues suggest that a linear quadrupole trap may be more appropriate than a standard rf Paul trap because it provides a more open structure and the stability of trapping ~10 times more clusters. The technical issues which were central to achieving electron diffraction of trapped clusters which will determine the feasibility of x-ray diffraction are listed in Tables 1 and 2. A current projected parameter value and its physical constraints are included for each item listed. The issues will be discussed in the presentation and more fully in a detailed proposal. - 27- Table 1 : Trapped Ion Electron Diffraction Issues δI/I0 ~2 x 10–8 1. Background Scattering - aperatures - He N ~ 2 x 104 ions 2. Cluster Number - source/trapping efficiency - space charge ~ 3 x 1012 e - /s 3. Incident Flux - detector saturation 4. Exposure Time ~ 30 s - inelastic scattering - detector saturation 5. Data Duty Cycle ~ 30 % - trap conductance 6. M/Z Range M/Z ~ 35 kD - Vrf perturbation escattering 7. Mass Selection/Resolution M/δM ~ 50 - space charge Table 2 : Trapped Ion X-Ray Diffraction Issues << 10–8 1. Background Scattering - aperatures - He N ≥ 2 x 105 ions 2. Cluster Number - source/trapping efficiency - space charge ~ 4 x 1018 hν/s 3. Incident Flux - detector saturation - 28 - 4. Exposure Time 1 min - inelastic scattering - detector saturation ≥ 50 % 5. Data Duty Cycle - inelastic scattering 6. M/Z Range M/Z ~ 50 kD - trap electronics δM/M ~ 50 7. Mass Selection/Resolution - space charge References [1] M. Maier-Borst, D. B. Cameron, M. Rokni and J. H. Parks Phys. Rev. A 59, R3162 (19 [2] J. H. Parks, presented at The Third International Symposium on Theory of Atomic and Molecular Clusters, Humboldt-Universität Berlin, Berlin, Germany, 1999 (unpublished). - 29- Absolute photoionization experiments M. Richter a, G. Ulm a, S. V. Bobashev b, A. A. Sorokin b a PTB Berlin, Germany b Ioffe-Institute St. Petersburg, Russia Experience of the team The team brings together the experience of PTB, the German national institute of science and technology, and Ioffe Physico-Technical Institute St. Petersburg in the fields of photon metrology and photoionization experiments in the spectral ranges of ultraviolet (UV) radiation, vacuum ultraviolet (VUV) radiation, and x rays. For many years, both institutes cooperate on the development of absolute photon-flux detectors as well as on absolute ionization experiments to obtain cross-section data for photon and electron impact on rare gases. The activities are embedded in fundamental experience in the physics of electronic and atomic collisions, UV, VUV, and x ray radiometry and spectroscopy, research with synchrotron radiation and laser-produced plasmas, electron, ionmass and photoabsorption spectroscopy, as well as time-of-flight and coincidence techniques. The experience is demonstrated by numerous refereed articles in international journals and contributed as well as invited papers at national and international conferences for more than twenty years up to now. Research proposal Previous research at the radiometry laboratory of PTB at the electron-storage ring BESSY I in Berlin, the two groups have developed a common total ion-yield experiment that enables to directly compare photon- and electron-impact ionization of rare gases. The current of the ionising impact particles is measured using a Faraday cup for electrons and precisely calibrated semiconductor photodiodes for photons, respectively. Based on available photoionization crosssection data obtained by precise photoabsorption measurements, resent experiments considerably improved the data base for absolute electron-impact ionization cross sections [1]. With the help of these data, the experimental setup now can be used as a so-called quantometer for absolute photon flux determination. In order to realise true absolute ion-mass spectroscopy from first principles that is not based on photoabsorption data, the setup is going to be completed with respect to the detection of ionic fragments on an absolute scale by the application of time-of-flight and coincidence techniques that have been developed in a previous work [2]. The experiments will be extended to electronimpact ionization of molecular targets to considerably improve the corresponding - 30 - cross-section data bases that may also be used to enable standard-free gas analysis. Research proposal Up to now, the study of non-linear dynamics and multi-photon excitation of matter is limited to the application of highly intense laser radiation in the optical region. However, the free-electron laser (FEL) that is under construction at DESY in Hamburg promises a radiant power density of focalised VUV radiation that is sufficient to extend the experiments of strong-field physics considerably to shorter wavelengths for a better understanding of many new and fundamental phenomena. Because of the suspicion that highly intense FEL radiation will saturate or even destroy common solid state detectors, it is proposed, in a first experiment, to use the rare-gas photoionization quantometer developed by Ioffe Institute and PTB for an absolute radiometric characterisation of the DESY-FEL out of the focal plane. This gives, in a second step, the opportunity for a quantitative investigation of non-linear effects upon photoionization in free raregas atoms using the same experimental setup moved into the focal plane. Gas pressure, electron gun, and ion detection of the setup will be matched to the requirements of FEL-radiation detection. The two most significant recent references [1] A. A. Sorokin, A. Shmaenok, S. V. Bobashev, B. Möbus, and G. Ulm: Measurements of electron-impact ionization cross sections of neon by comparison with photoionization. Phys. Rev. A 58, 2900-2910 ( 1998); A. A. Sorokin, S. V. Bobashev, B. Möbus, M. Richter, and G. Ulm: Measurements of electron-impast ionization cross sections of argon, krypton and xenon by comparison with photoionization. PRA 61, 022723 [2] M. Richter: Combined Electron and Ion Spectroscopy with Synchrotron Radiation of Free Metal Atoms and Ions. J. Electr. Spectrosc. Relat. Phenom. 76, 21-28 (1995); T. Luhmann, Ch. Gerth, M. Martins, M. Richter, and P. Zimmermann: Final Ion-Charge Resolving Electron Spectroscopy: Photoionization Studies on Sm and Eu. Phys. Rev. Lett. 76, 4320-4323 (1996); A. Gottwald, Ch. Gerth, and M. Richter: 4d Photoionization of Free Singly Charged Xenon Ions. Phys. Rev. Lett. 82, 2068-2070 (1999) - 31- Coherence effects in atomic and molecular photoionization Uwe Becker Fritz-Haber Institut, Faradayweg 4-6, 14195 Berlin, Germany Coherence phenomena occur whenever two or more particle waves that have a fixed phase relation interact. The typical feature of coherence is the occurrence of interference. Quantum optics is the field where coherence became the key to fundamentally new insights and a vast number of applications. The progress in this field promoted coherent atom optics. An outstanding example is the BoseEinstein-condensation. Only in the field of electron emission or more generally electron beams coherence phenomena emerge only in rare cases. The reason for this is the difficulty to produce coherent electrons and to transport them without loss of coherence. One method is to induce coherence on incoherent electron beams by self-induced coupling, an effect that is used in the free electron laser. Another method is the use of coherent electron emission from atoms or molecules. The coherent electron emission from two equivalent centres is being discussed for many years in connection with localized and delocalized core electrons in homo-nuclear molecules. It is unanimously assumed that for the emission of photoelectrons with high kinetic energy the time for the following Auger-decay is to short to erase the memory of the photo electron about its origin. The perturbation of the symmetry by the emission process stays present and causes an incoherent emission of the photoelectron. One open question is what happens to those photoelectrons that leave the molecule so slowly that the Auger-decay occurs in a time interval shorter than the time that corresponds to the symmetry-energy separation. In this case the photo electron's memory of its origin should be erased and the emission should be coherent. The following experiment could yield evidence for this behavior. The coincidence measurement of a 1 s photo electron from N2 and the N+ fragments yields the photo electron angular distribution in the molecular reference frame. As the ionic fragmentation produces N+ + N+ the detection of N+ contains no information about the origin of the photoelectron i.e. the emission should be coherent. When the coincidence is performed for the photoelectron and the N++ fragment it is much more probable that the electrons origin is the N++ fragment than its partner the N or N+ ion. So partial information about the electron's origin is preserved and the emission should be incoherent. The two angular distributions - the coherent and the incoherent one - should differ dramatically i.e. depending on the coincidence partner one should observe distinct angular distributions. Another process that should produce a pronounced coherence effect in the angular distribution of photo electrons is the double photoionization of H2 or D2. - 32 - If the two photoelectrons are measured in coincidence with the recoil protons, the two-centre emission of spherical waves should produce interesting interference effects for the electron intensity in the molecular frame, which should be contrasted with the incoherent emission from HD. - 33- Multiphoton Multiple-Ionization of Atoms and Molecules using Reaction-Microscopes R. Moshammer, J. Ullrich University Freiburg, Germany Goal of Research Due to the high photon intensity available at the upcoming FEL sources, unique mechanisms of double and multiple ionization of atoms, molecules and clusters can be realised for the first time and shall be explored in kinematically complete experiments. Proposal It is proposed to start with the most simple system, the helium atom, where two electrons might individually be photo-ionised by subsequently absorbing one photon each, within the duration of the pulse of about 300 fs. As one competing process, only one photon might be absorbed by one of the electrons and the second electron is shaken into the continuum which is the dominant contribution for single photon absorption at third generation synchrotrons. Third, one electron might be ionised, be accelerated in the laser field and "rescattered" to its parent ion causing double ionisation due to an electron-electron collision in an intermediate Volkov state. This has recently been identified to be the dominating mechanism in non-sequential double ionisation in strong conventional laser fields. Finally each of the electrons might absorb several photons in a multi-photon ionisation reaction. Presently, it is by no means clear which of the mechanism dominates, whether all of them are possible at all or what other dynamic reactions might occur. All of these reactions shall be identified by their characteristic kinematical signatures, observed in kinematically complete experiments where the vector momenta of both electrons and of the recoiling target ion are measured in coincidence. In a further step, it is proposed to perform similar investigations using first more complicated atomic targets like neon or argon and, second, simple molecules. Such measurements will for the first time give basic information on the fundamental mechanisms of energy transfer from exotic radiation fields atoms and molecules. They will serve as benchmark systems to develop theoretical models and come to detailed understanding of more complicated situations where the radiation interacts with biological matter or solid state targets. - 34 - Experience Our former group at GSI has developed so-called reaction microscopes that enable to measure the vector momenta of up to ten electrons and of several recoiling target ions produced in one single interaction with radiation or charged particles1. Within the last years we have performed several benchmark experiments on multiple ionization of helium and neon in attosecond intense fields of relativistic highly charged ions2, 3, in collisions with electrons4 and, very recently, in 30 fs intense laser fields5. Members of our group have been involved in a number of such experiments for single photon impact at photon energies ranging from a few eV above threshold6 to 100 KeV7. Researchers Dr. R. Moshammer, Dr. A. Dorn, Dr. C.D. Schröter, Dr. J. Crespo. Dr. B. Feuerstein, C. Höhr. D. Fischer, Prof. Dr. J. Ullrich References [1] R. Moshammer, et al., NIM B108 (1996)42S [2] R. Moshammer et al., PRL 79 (1997) 3621: PRL 73 (1994): PRA 56 (1997) 1351 [3] W. Schmitt et al., PRL 81 (1998) 4337: M. Schulz et al., PRL 84 (2000) 863 [4] A. Dorn et al., PRL 82 (1999) 2496 [5] R. Moshammer et al., PRL 84 (2000) 447 [6] R. Dörner et al., PRL 76 (1996) 2654 [7] L. Spielberger et al., PRL 76 (1996) 4685 - 35- Resonant Single and Multi-Photon Excitation and Ionisation of Highly Charged Ions by FEL-Radiation J. Crespo. J. Ullrich University Freiburg, Germany Goal of Research The photon flux at the upcoming FEL sources will, for the first time, allow the systematic study of the interaction between VUV-photons (X-rays in the second stage) and highly charged ions (HCI). An electron beam ion trap (EBIT) will produce and trap ions ranging from medium nuclear charges Z up to the heaviest systems like Li-like Uranium. In this way, the static structure of heavy fewelectron systems as well as the dynamics of multi-photon processes with relativistic, bound electrons can be investigated with unprecedented resolution. Proposal Up to now, few studies have been reported on the photon-ion interaction at 3rd generation synchrotrons due to the intrinsic photon flux limitations and even in those cases only low charged ions have been used. At the FEL sources a broad spectrum of such investigations will become possible, as for example the resonant excitation or single and multiple ionisation of ions by single and multiple photon absorption, experiments on high harmonic generation as well as time-resolved pump-probe experiments on the lifetime of excited states or on wave-packet dynamics in ionic systems. We propose to build a combined electron beam ion trap and source (EBIT/S) to produce, and subsequently store or extract ions up to Li-like Uranium in a first step. A later upgrade should extend the maximum charge state to hydrogen-like ions of all elements, as demonstrated at the Livermore SuperEBIT. Due to the excellent emittance (≈ 1 π mm mrad) of these type of ion source, slow ion beams can be efficiently focussed into a scattering chamber equipped with photon detectors (VUV, visible) as well as with electron and ion spectrometers. When higher ion-target densities (1011 ions/cm-3) are required, the VUV-light might be directly guided into the trap. As one of many examples, we propose to start with the precision spectroscopy of the 2s1/2 - 2p1/2 transition in Li-like HCls. Currently, the best value for Uranium (280 eV photon energy) has a resolution of ∆E/E = 3.6·10-4. With an envisaged resolution of 105, the FEL sources would allow the most sensitive test of QED in - 36 - a three electron system in strong fields, where a perturbation expansion in terms of Z·α (α: finestructure constant) is not valid and nuclear effects become large. Experience The necessary experimental techniques have been already developed in our group. We have designed and built an EBIT/S [1,2] in Freiburg to produce hydrogenic ions up to U92+, operational since 1999. Members of the group have previous experience in high-resolution photon spectroscopy with HCIs from the visible region up to the hard x-ray dornain [3,4]. Also within the last two years, we have been performing first fully differential experiments on multiple ionisation of atoms and molecules in femtosecond intense laser fields [5] of up to 1015 W/cm2; our work with relativistic HCIs at storage rings (GSI) provides us with experience in accelerator environments. Researchers Dr. J. Crespo. A. Werdich, Dr. A. Dorn, Dr. B. Feuerstein. D. Fischer, C. Höhr, Dr. R. Moshammer, Dr. C.D. Schröter, Prof. Dr. J. Ullrich References [1] J. R. Crespo López-Urrutia et al., Physica Scripta T80 (1999) 502; [2] J. R. Crespo López-Urrutia et al., Hyperfine Interactions (2000) (accepted) [3] J. R. Crespo López-Urrutia et al., PRL 77 (1996) 826; also PRA 57 ( 1998) 879. [4] P. Beiersdorfer, A. L. Osterheld, J. H. Scofield, J. R Crespo López-Urrutia and K. Widmann, PRL 80 ( 1998) 3022; P. Beiersdorfer, J. R. Crespo López-Urrutia, E. Föster, J. Mahiri, and K. Widmann, Rev. Sci. Instrum. 68 (1997) 1077. [5] R. Moshammer et al., PRL 84 (2000) 447 - 37- Velocity Map Imaging of Photoelectrons from Free Oriented-Molecules A. Yagishita1, M.Takahashiand2, T. Kasai3 1 Photon Factory, Institute of Materials Structure Science, Oho 1 - 1, Tsuba-shi 305-0801, Japan 2 Research Institute for Scientific Measurements, Tohoku University, Katahira 2- 1 - 1, Aoba-ku 980-8577, Japan 3 Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan Background One of the authors initiated the measurements on photoelectron angular distribution (PAD) from fixed-in-space-molecules by angle-resolved photoelectron-photoion coincidence at 1995 [1]. In contrast to PAD from randomly oriented molecules, PAD from the fixed-in-space molecules provides us the most detailed information on the photoionization dynamics of molecules [2]. However, PAD measurements from the fixed-in-space molecules are limited to the dissociative photoionization channels, because photoelectron-photoion coincidence technique is applied to realise them. Proposal Here we would like to propose a project on PAD from free oriented-molecules as a general extension of PAD from the fixed-inspace molecules. Because oriented molecules, which are prepared by the supersonic beam source, the electrostatic hexapole, and the guiding field [3] (see Fig.1), are very dilute, intense photons per pulse of FEL sources are inevitable to realise the measurements of PAD from the oriented molecules. And also timing structure of FEL sources is ideal for velocity map imaging of photoelectrons (see Fig.1) [4]. Triggered by FEL signals, one can obtain the velocity map image of photoelectrons, i.e., PAD (see Fig.2). We think that a FEL is the ideal photon source for the project on PAD from the oriented molecules. We believe that the perfectly new field of molecular photoionization dynamics will be opened by this project. - 38 - References [1] E. Shigemasa, J. Adachi, M. Oura, and A. Yagishita, Phys. Rev. Lett. 74, 359(1995) [2] for example, A. Yagishita, in Proceedings of the 20th ICPEAC, Viena, 1997, edited by F. Aumayer and Winter (World Scientific, Singapore, 1998) p.149. [3] T. Kasai, T. Fukawa, T. Matunami, D.-C. Che, K. Ohashi, Y. Fukunishi, H. Ohyama, and K. Kuwata, Rev. Sci. Instrum., 64, 1150(1993). [4] A.T.J. Eppink and D. H. Parker, Rev. Sci. Instrum., 68, 3477(1997). Figure 1 - 39- Figure 2 - 40 -