WDS'6 Proceedings of Contributed Papers, Part II, 96 11, 26. ISBN 8-86732-85-1 MATFYZPRESS Overtone Spectroscopy P. Hlavenka, I. Korolov, R. Plašil, J. Varju, T. Kotrík, and J. Glosík 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 spectra in the first OH stretching overtone region. At least 3 absorption lines of were found in near infrared region 69 to 696 cm 1 using a very sensitive time resolved continuous wave cavity ringdown spectrometer (cw-crds) with concentration modulation. ions were produced in pulsed microwave discharge in He/H 2 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,, is one of the key ionic species in the both gas and condensed phases. Besides its importance in both chemistry and astrophysics, 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. is isoelectronic with ammonia, N, 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. Equilibrium geometry of both N and 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 N and, the observed tunneling splitting magnitude is dramatically different:.793 cm 1 and 55.35 cm 1 for N and 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 ion has therefore attracted substantial attention over past two decades. The first high resolution spectrum of was reported by Saykally and coworkers in 1983 for the asymmetric stretch, ν 3, 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, ν 1 has also been detected and analyzed [Tang et al., 1999] in the same spectral range. The doubly degenerate asymmetric deformation mode ν 4 was first detected by Gruebele in 1987 [Gruebele et al., 1987]. The inversion-tunneling mode has also received considerable attention. The absorption spectra of, 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 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.: 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., 23; Huang et al., 22]. 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 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 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 C 3 v molecular symmetry 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 ν 3 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 E n = ω e (.5 + n) ω e x e (.5 + n) 2. (1) Thus the absorption energy for the first overtone transition (ν = 2 ) is given by E 2 = 2( E 1 ω e x e ). (2) Where E 1 is the fundamental band origin. Estimate of the anharmonicity factor ω e x e is made by analogy with N overtone bands, scaling the factor by the ratio of fundamental vibrational energies of N and respectively. As those fundamental energies are rather close for those two species (3443 cm 1 and 3555 cm 1 for N and respectively [Rajamaki et al., 23; Lundsberg-Nielsen et al., 1993]), we assume that the error introduced by this 97
HLAVENKA ET AL.: 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 ν 3 asymmetric stretch is 48 cm 1 and therefore for the fundamental origin energy of 3555 cm 1, the expected 2ν 3 overtone origin should be near 714 cm 1. It is rather likely that the (ν 1 + ν 3 ) 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 69 cm 1 to 696 cm 1 spectral range. Experiment The ions are formed in microwave discharge in He/H 2 O mixture. At pressure of 5 mbar (5 Pa) the flow of gases was: He 4 sccm (standard cubic centimeter per minute) and H 2 O.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 3 and chamber was evacuated to remove the air. The plasma is ignited by pulses of microwave (4 ms discharge, 1 Hz repetition, 6-12 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 the OH radical and of course H 2 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., 24; Hlavenka et al., 26a; Hlavenka et al., 26b; Plašil et al., 25]. 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., 24; Hlavenka et al., 26a; Hlavenka et al., 26b], 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 5). 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
HLAVENKA ET AL.: OVERTONE SPECTROSCOPY Absorbance [dimensionless] 3x1-8 2x1-8 1x1-8 2x1-8 1x1-8 OH H 2 O Discharge.7-3.3 ms Background 4.5-7 ms 6911.5 6912. 6912.5 6913. Wavenumber [cm -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 H 2 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., 26a; Hlavenka et al., 26b]. 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., 26a; Hlavenka et al., 26b]. The observed water transitions served to check the calibration reliability. The error of wavelength calibration is.1 cm 1. Results The first OH stretching overtone region of ion in NIR was scanned with CRD spectrometer in the region 69-696 cm 1 with approximately 8% coverage. The remaining 2% 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.: OVERTONE SPECTROSCOPY 3x1-8 2x1-8 6948.93 cm -1 Absorbance evolution on peak center 1x1-8 Absorbance [dimensionless] 6x1-8 4x1-8 2x1-8 6x1-7 4x1-7 2x1-7 OH 6956.4 cm -1 H 2 O 6956.32 cm -1 Microwave intensity [a.u.] -.4 -.2..2.4.6 Wavenumber [cm -1 ] 2 4 6 8 1 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 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 (H 2 O), the long-lived species (such as the OH radical), and the short lived ionic species such as the. This is clearly demonstrated in Fig. 3 that shows the absorption signal time evolution for the three representative spectral lines. The 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 H 2 O absorption line. If one puts the discharge OFF detection window closer to the end of active discharge, where the concentration of 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 H 2 O lines, both signal and background spectra are identical. This approach can give us a clue, how the 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 3 lines as, more than 4 as H 2 O and more than 15 as OH radical or other neutral metastable molecule. 1
Conclusion HLAVENKA ET AL.: OVERTONE SPECTROSCOPY The position of OH stretching overtone band of ion has been estimated. The region 69-696 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 ions from metastable radicals (OH) and stable molecules (H 2 O). The recorded lines will serve after assignment as a valuable benchmark for theoretical prediction done for and N, and can be used for corrections of the multidimensional potential energy surface (PES) of at higher energies. Acknowledgments. This work is a part of the research plan MSM 2162834 that are financed by the Ministry of Education of the Czech Republic and partly was supported by Kontakt CZ-8/24, GACR (22/3/H162, 25/5/39, 22/5/P95 and 22/3/827) and GAUK-278/24 /B-FYZ/MFF. References Begemann M. H., C. S. Gudeman, J. Pfaff, and R. J. Saykally, Detection of the Hydronium Ion ( ) by High-Resolution Infrared Sepctroscopy, Phys. Rev. Lett. 51, 554., 1983. Begemann M. H., R. J. Saykally, A study of the structure and dynamics of the hydronium ion by high-resolution infrared laser spectroscopy. I. The ν 3 band of H 16 3 O+, J. Chem. Phys. 82, 357, 1985. Bunker P. R., T. Amano, and V. Spirko, A Preliminary Determination of the Equilibrium Geometry and Inversion Potential in from Experiment, J. Mol. Spectr. 17, 28, 1984. Davies P. B., S. A. Johnson, P. A. Hamilton, and T. J. Sears, Infrared diode laser spectroscopy of the ν 2 (2 + 1 ) band of, Chem. Phys. 18, 335, 1986. Gruebele M., M. Polak, and R. J. Saykally, A study of the structure and dynamics of the hydronium ion by high resolution infrared laser spectroscopy. II. The ν 4 perpendicular bending mode of H 16 3 O+, J. Chem. Phys. 87, 3347, 1987. Haese N. N. and T. Oka, Observation of the ν 2 (1 + ) inversion mode band in by high resolution infrared spectroscopy, J. Chem. Phys. 8, 572, 1984. Hlavenka P., R. Plašil, G. Bánó, I. Korolov, D. Gerlich, J. Ramanlal, J. Tennyson, J. Glosík, Near infrared Second overtone cw-cavity Ringdown spectroscopy of D 2 H + ions, Int. J. Mass Spect. 26a, in press. Hlavenka P., I. Korolov, R. Plašil, J. Varju, T. Kotrík, J. Glosík, Near infrared Second overtone cw-cavity Ringdown spectroscopy of H 2 D + ions, Czechoslovak J. of Phys. Suppl. B 56, 26b. Huang X., S. Carter, and J. M. Bowman, Ab Initio Potential Energy Surface and Vibrational Energies of and Its Isotopes, J. Phys. Chem. B 16, 8182, 22. Liu D. and T. Oka, Experimental Determination of the Ground-State Inversion Splitting in, Phys. Rev. Lett. 54, 1787, 1985. Lundsberg-Nielsen L., F. Hegelund, and F. M. Nicolaisen, Analysis of the High-Resolution Spectrum of Ammonia ( 14 N ) in the Near-Infrared Region, 64-69 cm 1, J. Mol. Sp. 162, 23, 1993. Macko P., G. Bánó, P. Hlavenka, R. Plašil, V. Poterya, A. Pysanenko, O. Votava, R. Johnsen and J. Glosík, Afterglow studies of H + 3 (ν = ) recombination using time resolved cw-diode laser cavity ring-down spectroscopy, Int. J. Mass Spect. 233, 299, 24. Plašil R., P. Hlavenka, P. Macko, G. Bánó, A. Pysanenko and J. Glosík, The recombination of spectroscopically identified H + 3 (ν = ) ions with electrons, J. Phys. Conf. Series 4 118, 25. Rajamaki T., A. Miani, and L. Halonen, Six-dimensional ab-initio potential energy surfaces for and N : Approaching the subwave number accuracy for the inversion splittings, J. Chem. Phys. 118, 1929, 23. Spirko V. and W. P. Kraemer, Anharmonic Potential Function and Rotation-Inversion Energy Levels of, J. Mol. Spectr.134, 72, 1989. Tang J. and T. Oka, Infrared Spectroscopy of : The ν 1 Fundamental band, J. Mol. Spectr. 196, 12, 1999. Uy D., E. T. White, and T. Oka, Observation of k-i = 3 Transitions in the ν 3 Band of, J. Mol. Spectr. 183, 24, 1997. 11