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WDS'06 Proceedings of Contributed Papers, Part II, 96–101, 2006. ISBN 80-86732-85-1 © MATFYZPRESS H3 O+ Overtone Spectroscopy P. Hlavenka, I. Korolov, R. Plašil, J. Varju, T. Kotrı́k, and J. Glosı́k Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. O. Votava J. Heyrovský Institute of Physical Chemistry, Academy of Sciences of Czech Republic, Prague, Czech Republic. Abstract. In this report we present preliminary results in search for the H3 O+ spectra in the first OH stretching overtone region. At least 30 absorption lines of H3 O+ were found in near infrared region 6900 to 6960 cm−1 using a very sensitive time resolved continuous wave cavity ringdown spectrometer (cw-CRDS) with concentration modulation. H3 O+ ions were produced in pulsed microwave discharge in He/H2 O mixture. A technique based on time resolved absorption has been used to distinguish the absorption lines of short living ions from longer living radicals and water vapors. Introduction Hydronium ion, H3 O+ , is one of the key ionic species in the both gas and condensed phases. Besides its importance in both chemistry and astrophysics, H3 O+ also serves as an important model system for benchmark theoretical predictions. Due to its small size high level theoretical treatment is possible for this molecule. H3 O+ is isoelectronic with ammonia, NH3 , and shares number of important characteristics with this molecule. Thus the theoretical tools developed for characterization of ammonia can be put to an independent test when applied to H3 O+ . Equilibrium geometry of both NH3 and H3 O+ molecules is a shallow symmetrical pyramid. Consequently, the bending potential in the dihedral angle exhibits symmetric double well structure with a barrier at planar configuration. Due to tunneling through this barrier all the vibrational states in this inversion-vibration are split into doublets. The magnitude of this splitting is a sensitive measure of the inversion barrier height, and thus provides a stringent test on the theoretical predictions. Indeed, despite the general topological similarity between NH3 and H3 O+ , the observed tunneling splitting magnitude is dramatically different: 0.793 cm−1 and 55.35 cm−1 for NH3 and H3 O+ respectively. High resolution spectroscopy provides some of the most accurate experimental data, which can be used to test the theoretical predictions. Vibrational spectroscopy of the H3 O+ ion has therefore attracted substantial attention over past two decades. The first high resolution spectrum of H3 O+ was reported by Saykally and coworkers in 1983 for the asymmetric stretch, ν3 , fundamental band in the mid IR spectral region [Begemann et al., 1983] and this was followed by more detailed studies of the same band [Uy et al., 1997; Bunker et al., 1984; Spirko et al.,1989; Begemann et al.,1985]. The much weaker symmetric stretch, ν1 has also been detected and analyzed [Tang et al., 1999] in the same spectral range. The doubly degenerate asymmetric deformation mode ν4 was first detected by Gruebele in 1987 [Gruebele et al., 1987]. The inversion-tunneling mode has also received considerable attention. The absorption spectra of 0, 1± , and 2± were measured by several groups [Davies et al., 1986; Haese et al., 1984; Liu et al., 1985]. Accurate calculation of the large inversion splitting in H3 O+ has indeed proved a challenge for theorists. Reduced-dimensionality calculations were capable of semi-quantitative description of the vibrational-inversion dynamics [Spirko et al., 1989; Bunker et al., 1984] yet they lack the information on couplings between the different vibrational modes that lead to effective change in the barrier height as a function of vibrational excitation. Thus only the most recent full 96 HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY dimensional ab initio calculations of the potential energy surface (PES), coupled with variational evaluation of the vibrational states yielded satisfactory agreement with experiment in the fundamental and several low lying combination bands [Rajamaki et al., 2003; Huang et al., 2002]. While the agreement between theories based on the full dimensional ab-initio PES and the available data is very good, there is presently a clear lack of experimental data that would test the PES at higher energies that are crucial for detailed understanding the chemical properties of the H3 O+ ion. Molecular overtone spectroscopy is inherently a probe into those parts of molecular potential energy surfaces and thus is capable providing this critical information on the chemically relevant energy levels. Unlike the fundamental bands, the overtone transitions are strictly forbidden in the harmonic limit of the potential, and therefore their non-zero oscillator strengths are derived from the anharmonic effects. Moreover due to the higher energy of the excitation photons a wider range of the PES is sampled. In the case of high frequency stretching modes in hydrogen contained bonds, such as O-H, C-H, and N-H, chemically significant levels of excitation on the order of 1 eV can be achieved with only few quanta of vibrational excitation. From the experimental point of view however, the orders of magnitude lower oscillator strengths in the overtone bands as compared to the fundamental bands make it often challenging to detect the weak molecular absorptions. This is especially the case for short-lived transient species such as molecular radicals or ions, because concentrations at which those species can be prepared in the laboratory are typically orders of magnitude lower than for stable molecules. As a result, the overtone spectroscopy is not well characterized for many important species. In this report we present preliminary results in search of the H3 O+ spectra in the first OH stretching overtone region. The experiments were made possible by use of very high sensitivity spectroscopic techniques based on time resolved cw cavity ringdown scheme coupled with AC microwave discharge absorption cell. Based on the time evolution of the absorption signals it is possible to distinguish the weak ionic absorptions from those of much stronger signals originating from the stable or metastable species in the gas mixture. Overtone band position predictions Due to its near planar geometry and C3 v molecular symmetry H3 O+ is an oblate symmetric top molecule. Because of the complexity of the symmetric top spectra that is further complicated by the presence of l-type doubling in the degenerate ν3 vibration and by the tunneling splitting of the vibrational bands. High number of lines of comparable intensity is observed over a wide range of spectra. The initial assignment of the spectra is further complicated by limited tunability and incomplete coverage of the spectral range scanned with the diode laser spectrometer. Consequently it is crucial to use a systematic, theory assisted approach to the initial assignment of the spectra. To guide the initial experimental search of the overtone spectra, estimates of the band positions are calculated based on fundamental band frequencies and empirical estimates for anharmonicity constants. The Morse-like anharmonic energy levels can be written as En = ωe (0.5 + n) − ωe xe (0.5 + n)2 . (1) Thus the absorption energy for the first overtone transition (ν = 2 ← 0) is given by ∆E2←0 = 2(∆E1←0 − ωe xe ). (2) Where ∆E1←0 is the fundamental band origin. Estimate of the anharmonicity factor ωe xe is made by analogy with NH3 overtone bands, scaling the factor by the ratio of fundamental vibrational energies of NH3 and H3 O+ respectively. As those fundamental energies are rather close for those two species (3443 cm−1 and 3555 cm−1 for NH3 and H3 O+ respectively [Rajamaki et al., 2003; Lundsberg-Nielsen et al., 1993]), we assume that the error introduced by this 97 HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY Figure 1. Schematic diagram of the apparatus. The pulsed microwave discharge takes place in silicate glass tube. For CRDS two highly reflective mirrors are mounted on tilt stages inside the vacuum. estimate will be only on the order of few cm−1 , and therefore will not affect the spectroscopic search. The anharmonicity estimated by this procedure for the ν3 asymmetric stretch is 48 cm−1 and therefore for the fundamental origin energy of 3555 cm−1 , the expected 2ν3 overtone origin should be near 7014 cm−1 . It is rather likely that the (ν1 + ν3 ) combination band will lie in the same spectral range, and assuming the same effective anharmonicity we can estimate its origin near 6927 cm−1 . Based on those estimates the initial spectroscopic search was performed in the 6900 cm−1 to 6960 cm−1 spectral range. Experiment The H3 O+ ions are formed in microwave discharge in He/H2 O mixture. At pressure of 5 mbar (500 Pa) the flow of gases was: He 400 sccm (standard cubic centimeter per minute) and H2 O 0.5 sccm. The water vapors were evaporated from a free water level in a small chamber at room temperature; we used doubly distilled deionized water. The water was initially frozen to −30 and chamber was evacuated to remove the air. The plasma is ignited by pulses of microwave (4 ms discharge, 100 Hz repetition, 60 - 120 W, 5 cm plasma column) in a silicate glass tube ( 2.3 cm inner diameter) passing through a microwave cavity, see Fig. 1. Aside from H3 O+ the OH radical and of course H2 O absorption lines have been observed. The recently build cw cavity ringdown spectrometer (cw-CRDS) had been used in past for the time resolved absorption measurements of H+ 3 ion and its deuterated analogues using weak second overtone transitions [Macko et al., 2004; Hlavenka et al., 2006a; Hlavenka et al., 2006b; Plašil et al., 2005]. These measurements demonstrated the high sensitivity of the spectrometer that allowed us to observe the weak absorption transitions of species with low concentration, such as ions in a cold plasma. As the time resolved cavity ringdown spectrometer (TR-CRDS) has been described elsewhere [Macko et al., 2004; Hlavenka et al., 2006a; Hlavenka et al., 2006b], only a short description will be given. The apparatus consists of the discharge tube with two highly reflective mirrors on each end of the tube, making up an optical cavity. Laser beam is produced by an external cavity diode laser (SacherLaser TEC 500). After spatial matching the beam is injected into the optical cavity. The incident beam is interrupted (using an acousto-optic modulator) after sufficient build-up of the intensity in the cavity is detected, and a photodiode detector then records the light intensity decay caused by losses on mirrors 98 Absorbance [dimensionless] HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY OH 3x10-8 H H -8 2x10 O+ O+ 3 3 H H O+ 3 O 2 1x10-8 0 2x10-8 H Discharge O+ 3 0.7-3.3 ms .5-7 ms Background 4 1x10-8 0 6911.5 6912.0 6912.5 W avenumber [cm 6913.0 -1 ] Figure 2. The absorption spectra obtained by CRDS using synchronous detection. Upper panel shows the absorbance (the relative loss of light intensity per pass) in discharge and background time windows. One can clearly see the elevation of baseline in case of OH line. For H2 O, the both curves are the some. Lower panel shows absorbance spectrum with subtracted baseline. and due to the absorption of species inside the optical cavity. From each exponential decay (called ringdown) the absorption can be directly determined. The typical ringdown time was ∼33 µs for a 75 cm long empty cavity. To increase the sensitivity and to simplify the assignment of the observed absorption lines, synchronous detection scheme has been applied [Hlavenka et al., 2006a; Hlavenka et al., 2006b]. Briefly, for each wavelength of the laser the time resolved absorption measurements are obtained from many ringdown events over many discharge periods. Than, two detection windows are selected from this time series; those corresponding to the discharge and those corresponding to the end of the afterglow, where the microwaves are off and the ion concentration is negligible. By subtracting the average signals in those two windows the net ion absorption is determined. The laser wavelength was monitored using a Michelson interferometer based wavemeter (absolute calibration) and Fabry-Perot etalon (relative wavelength change), for details see [Hlavenka et al., 2006a; Hlavenka et al., 2006b]. The observed water transitions served to check the calibration reliability. The error of wavelength calibration is ∼0.01 cm−1 . Results The first OH stretching overtone region of H3 O+ ion in NIR was scanned with CRD spectrometer in the region 6900 - 6960 cm−1 with approximately 80% coverage. The remaining ∼20% of this region could not be presently reached due to the gaps in the tuning characteristics of the laser. An example of obtained spectra is plotted in Fig 2. The upper line is the spectrum corresponding to the ’discharge ON’ window and the lower line (slightly shifted downwards for clarity) is the spectrum obtained during the late afterglow (’discharge OFF’), as described above. It can be seen that indeed certain spectral lines are observed with virtually the same intensity both in the active discharge and in the late afterglow, indicating that they belong to stable species, while others exhibit vastly different intensities in the active discharge and afterglow, indicating they belong to discharge generated species. It is also however shown, that there are clear differences in the observed ON/OFF characteristics for different observed lines. 99 HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY 3x10-8 -8 2x10 O+ H 3 6948.93 cm-1 Absorbance evolution on peak center Absorbance [dimensionless] 1x10-8 0 6x10-8 OH 6956.40 cm-1 -8 x10 4 2x10-8 0 6x10-7 -7 H O 2 6956.32 cm-1 x10 4 Microwave intensity [a.u.] 2x10-7 0 -0.04 -0.02 0.00 0.02 0.04 0.06 W avenumber [cm -1 0 2 4 6 8 10 12 Time [ms] ] Figure 3. Demonstration of the synchronous detection used to distinguish between different species. Left panels show the absorbance spectrum during 1-4 ms time window in active discharge (full line) and spectrum during 6-9 ms time window in the far afterglow (dashed line). Right panels show time evolution of absorbance at central wavelength of the peak. The microwave intensity evolution is depicted in lower panel. The electronics of the microwave source was not able to switch off the power rapidly, so at the falling edge both MW and H3 O+ signal falls slower compared to the edge of the switching pulse. Thus while this two-window-subtraction synchronous detection (as described above) is very useful for the initial data characterization, the details of the time evolution obtained at each laser wavelengths must be used to distinguish in more detail between the stable (H2 O), the long-lived species (such as the OH radical), and the short lived ionic species such as the H3 O+ . This is clearly demonstrated in Fig. 3 that shows the absorption signal time evolution for the three representative spectral lines. The H3 O+ average lifetime after switching off the discharge is primarily limited by the recombination with electrons and is much shorter compared to the lifetime of neutral species generated in plasma, such as the OH radical (that are removed mainly on the walls). Still the concentration of those species is rather dramatically affected by the discharge. This is in clear contrast to the stable species, as demonstrated in the third panel of Fig. 3 for representative H2 O absorption line. If one puts the ’discharge OFF’ detection window closer to the end of active discharge, where the concentration of H3 O+ is already negligible, but the OH radical concentration is still observable, one can see the background (baseline) elevation in the scan in case of a OH radical spectral line, as demonstrated on the left panel of Fig. 3. In case of H2 O lines, both signal and background spectra are identical. This approach can give us a clue, how the H3 O+ transitions can be distinguished from the transitions of other species. However, only a proper line analysis and line assignment can validate, weather a line belongs to the H3O+ band or to an unidentified short living excited molecule or ion. Although the full analysis and assignment haven’t been completed yet, the preliminary analysis identified more than 30 lines as H3 O+ , more than 40 as H2 O and more than 15 as OH radical or other neutral metastable molecule. 100 HLAVENKA ET AL.: H3 O+ OVERTONE SPECTROSCOPY Conclusion The position of OH stretching overtone band of H3 O+ ion has been estimated. The region 6900 - 6960 cm−1 have been scanned with high sensitivity and resolution using CRDS spectrometer. For higher sensitivity the concentration modulation technique have been used together with time resolved measurement. It was demonstrated, that this technique is essential in this kind of experiments to distinguish the short-lived H3 O+ ions from metastable radicals (OH) and stable molecules (H2 O). The recorded H3 O+ lines will serve after assignment as a valuable benchmark for theoretical prediction done for H3 O+ and NH3 , and can be used for corrections of the multidimensional potential energy surface (PES) of H3 O+ at higher energies. Acknowledgments. This work is a part of the research plan MSM 0021620834 that are financed by the Ministry of Education of the Czech Republic and partly was supported by Kontakt CZ-8/2004, GACR (202/03/H162, 205/05/0390, 202/05/P095 and 202/03/0827) and GAUK-278/2004 /B-FYZ/MFF. References Begemann M. H., C. S. Gudeman, J. Pfaff, and R. J. Saykally, Detection of the Hydronium Ion (H3 O+ ) by High-Resolution Infrared Sepctroscopy, Phys. Rev. Lett. 51, 554., 1983. Begemann M. H., R. J. 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