J . Phys. Chem. 1988,92, 1397-1399
Vibrational Overtone Spectroscopy of H,O (4v,,) Impact Ionization
1397
Using Energy-Selective Electron
C. C. Hayden, S. M. Penn, K. J. Carlson, and F. F. Crim* Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706 (Received: January 26, 1988)
We describe a new method for obtaining vibrational overtone spectra of polyatomic molecules in supersonic expansions that uses low-energy electrons to ionize the vibrationally excited molecules. Measuring the excitation spectrum of water in the region of the third overtone of the OH stretching vibration (4uOH) demonstrates the technique. The ionization process is probably not direct but may occur by electron impact excitation to vibrationally and electronically excited states from which the neutral molecule subsequently ionizes.
Introduction The wealth of data obtained by photoacoustic techniques during the past 15 years1 has supported the local mode model2 as a convenient description of vibrational overtone excitation. The central feature of this model, which successfully predicts the location of vibrational overtone bands, is the treatment of stretching motions involving light atoms as localized oscillators carrying the transition strength. However, it is the coupling of these localized motions to other vibrations in the molecule that determines the derailed spectral structure and intramolecular dynamics of highly vibrationally excited molecules. For example, Fermi resonance interactions between the light-atom stretching motion and other low-frequency motions, in particular the CCH bends and wags, determine much of the structure in alkane and alkene vibrational overtone spectra.3 A primary goal of vibrational overtone spectroscopic studies is to extract the most important couplings from an analysis of the spectra, but congestion arising from thermally excited molecules complicates the interpretation of data from room-temperature gases and often prevents the assignment of individual lines in bands showing structure. Measurement of vibrational overtone spectra for cold, gas-phase samples removes many of these ambiguities, but only a few such measurements have been made. Taking advantage of the relatively high vapor pressure of methane at 77 K, Scherer et al. used a liquid-nitrogen-cooled photoacoustic cell to obtain the vibrational overtone spectra of CHI and CD3H in the region of the fifth CH overtone (6vcH).4 Supersonic expansions also provide gas-phase samples of cold molecules, but the small cross sections for vibrational overtone excitation have precluded direct absorption measurements in these environments. Indirect techniques, however, are beginning to produce data for expansion-cooled molecules. McGinley and Crim obtained vibrational overtone spectra of tetramethyldioxetane (4vCH and SUCH) by detecting the chemiluminescence from acetone produced in its unimolecular decomposition: and Butler et al. observed the 4vOHand 6 ~ 0 Htransitions of HOOH by monitoring its decomposition into O H with laser(1) For example: (a) Henry, B. R. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1977; Vol. 10, p 269. (b) Reddy, K. V.; Heller, D. F.; Berry, M. J. J. Chem. Phys. 1982, 76, 2814. (c) Wong, J. S.; Green,W. H.; Cheng, C.-K.; Moore, C. B.J. Chem. Phys. 1987,86,5994. (d) Perry, J. W.; Moll, D. J.; Kuppermann, A.; &wail, A. H. J. Chem. Phys. 1985,82, 1195. (e) Fang, H. L.; Swofford, R. L.; Compton, D. A. C. Chem. Phys. Lett. 1984,108, 539. (0 Baggot, J. E.; Law, D. W.; Lightfoot, P. D.; Mills, I. M.J. Chem. Phys. 1986,85, 5414. (g) Baggot, J. E.; Chuang, M.-C.; Zare, R. N.; Diibal, H.-R.; Quack, M. J . Chem. Phys. 1985,82, 1186. (h) Segall, J.; Zare, R. N.; Diibal, H.-R.; Lewerenz, M.; Quack, M. J . Chem. Phys. 1987,86, 634. (2) (a) Ti", B.; Mecke, R. Z. Phys. 1936, 98, 363. (b) Mecke, R. Z. Elektrochem. 1950,54,38. (c) Henry, B. R. Acc. Chem. Res. 1977, I O , 207. (d) Child, M. S. Acc. Chem. Res. 1985, 18, 45. (3) (a) Sibert, E. L. 111; Reinhardt, W. P.; Hynes, J. T. J . Chem. Phys. 1984,81,1115. (b) DUbal, H.-R.; Quack, M. J . Chem. Phys. 1984,81,3779. (c) Hutchinson, J. S.; Hynes, J. T.; Reinhardt, W. P. J . Phys. Chem. 1986, 90,3528. (4) Scherer, G. J.; Lehman, K. K.; Klemperer, W. J. Chem. Phys. 1984, 81, 5319. (5) McGinley, E. S.; Crim, F. F. J . Chem. Phys. 1986, 85, 5741.
0022-3654/88/2092-1397$01.50/0
induced fluorescence.6 Using a different approach, Douketis et al. observed the 4uOHtransitions of water by detecting the vibrationally excited molecules with a bolometer? and, most recently Page et al. used a multiphoton ionization depletion scheme to obtain spectra in the 2VCH and 3VCH regions of benzene.* This Letter describes a new, general method for obtaining vibrational overtone spectra of polyatomic molecules cooled in a supersonic expansion: energy-selective electron impact ionization of vibrationally excited molecules. Figure 1 illustrates our application of this method to water molecules excited in the region of the third vibrational overtone transition ( 4 ~ 0 ~We ) . adjust the nominal electron energy to just below the ionization potential of unexcited water molecules and scan the laser wavelength over the known features of the 4uOHabsorption band to obtain the vibrational overtone excitation spectrum. Because molecules excited to high vibrational levels possess enough energy to be ionized by the low-energy electrons, excitation of the vibrational overtone increases the ion yield. Thus, the ion signal observed as a function of laser wavelength is a spectrum that convolutes the absorption probability for a particular vibrational overtone transition with the probability of ionization from the excited vibrational state involved in the transition.
Experimental Approach The molecular beam apparatus, which is described in detail elsewhere: consists of a separately pumped, pulsed molecular beam source and time-of-flight mass spectrometer. Figure 2 schematically illustrates the essential elements of the apparatus. The ionization region of the time-of-flight mass spectrometer employs a Wiley-McLaren two-stage ion extractor lo and can accommodate several ionization techniques including laser multiphoton ionization, single-photon vacuum-ultraviolet photoionization, and electron impact ionization. To implement the latter technique, we have developed a pulsed, low-energy electron gun that produces space-charge-limited current densities in the energy range of 10-200 eV. Fitting our measurements of the ionization threshold of xenon to a thermal distribution of electron energies yields T = 1300 K (2kT==0.2 eV)? This relatively narrow electron energy distribution is essential for minimizing the background ion signal from ground-state water molecules. As Figure 2 illustrates, a pulsed, skimmed molecular beam of water vapor seeded in helium passes through the ionization region of the mass spectrometer where a laser beam from a Nd:YAG laser excited, pulsed dye laser (Quanta-Ray DCR-2A/PDL- 1) intersects it at right angles. Expanding the gas mixture of approximately 5% water vapor in 360 Torr of helium through a 1-"-diameter nozzle produces a molecular beam without sig~~
~~
(6) Butler, L. J.; Ticich, T. M.; Likar, M. D.; Crim, F. F. J . Chem. Phys. 1986, 85, 2331. (7) Douketis, C.; Anex, D.; Ewing, G.; Reilly, J. P.J. Phys. Chem. 1985, 89, 4173. ( 8 ) Page, R. H.; Shen, Y. R.; Lee, Y. T. Phys. Rev. Len. 1987,59, 1293. (9) (a) Penn, S. M. Ph.D. Thesis, University of Wisconsin, 1987. (b) Hayden, C. C.; Penn, S. M.; Carlson, K. J.; Crim, F. F., in preparation. (10) Wiley, W. C.; McLaren, I. H. Reu. Sci. Instrum. 1955, 26, 1150.
0 1988 American Chemical Society
Letters
1398 The Journal of Physical Chemistry, Vol. 92, No. 6, 1988
- 12 - II
- IO
12
-9
-8
>e-
-J
-
U
E
v)
z '3
T
-8
2
-7
E 0
I-lCO
z
jl
0.025 cm-1
0
-6 0 \
6 -4 Y -5
[r
E5
J
U
z
-3
'3
-2
v)
- - ,
0 I-
v)
3
0 0
a
Figure 1. Energy level diagram for energy-selective electron impact ionization of vibrationally excited water molecules. Note that the nominal electron energy is insufficient to ionize ground-state water molecules. The levels shown above the ionization limit roughly correspond to vibrational states of the ion.
ION DETECTOR
u
0 I-
O I
a
I3904
13852
WAVENUMBER, Q(cm-1)
Figure 3. Vibrational overtone spectra of water ( 4 ~ 0 ~ )(a) . Selective ionization of expansion-cooled water after correction for background drift. The ion signal as a function of laser wavelength arises from electron impact ionization of the vibrational overtone excited molecules. (b) Room-temperature photoacoustic spectrum for comparison.
rovibrational features in the 4vOHabsorption band of water. Figure 3b shows a room-temperature photoacoustic spectrum for comparison. The assignments for the transitions are from Baumann and Mecke." In this asymmetric top n ~ t a t i o n , ' the ~ , ~rotational ~ levels are labeled by J,, where J is the total angular momentum for molecules in that level and 7 is an index that distinguishes the (2J 1) sublevels for each J according to its energy, with 7 = -J having the lowest energy and 7 = J having the highest energy. Sublevels with even 7 have rotational wave functions that are symmetric with respect to exchange of the two identical nuclei and, thus, have antisymmetric nuclear wave functions in the ground vibrational and electronic state. These even 7 sublevels have nuclear spin degeneracies of 1. Sublevels with odd 7 have symmetric nuclear spin wave functions and nuclear spin degeneracies of 3. The largest features in the spectrum are the 2-2 L1and the L1 0 transitions. These two transitions originate from the L1 and the 0 rotational states, which are respectively the lowest rotational states accessible for molecules having nuclear spin wave functions that are symmetric and antisymmetric with respect to exchange of the two identical nuclei. The 20 1' and the 4-4 3-3 transitions, which originate from higher rotational states, are less intense in the ionization spectrum than in the room-temperature photoacoustic spectrum, consistent with rotational cooling in the supersonic expansion. An analysis of the differences in the relative intensities of the features in the photoacoustic spectrum and the ionization spectrum, assuming that both the ionization probability and the photoacoustic sensitivity are independent of excited state, indicates that the rotational temperature of the symmetric nuclear spin manifold (odd 7 ) is approximately 20 K.9a Douketis et al. have shown that the natural line widths for these transitions are less than 0.000 1 cm-' using their high-resolution laser bolometric t e ~ h n i q u e but , ~ the bandwidth of our dye laser
+
PULSED VALVE
Figure 2. Schematic illustration of the molecular beam apparatus (not to scale).
nificant cluster formation. About 100 ns after the 6-ns dye laser pulse, a 3-ps pulse of low-energy electrons crosses the laser and molecular beams at 4 5 O , and an extraction pulse subsequently accelerates the ions out of the ionization region and into the drift region of the time-of-flight mass spectrometer. Gated ion-counting electronics sum the signal a t the arrival time of mass 18 and transfer the results to a laboratory computer. The ion signal for the most intense vibrational overtone transition in water is approximately 0.15 ion/laser pulse compared to a background of 0.3 ion/pulse, and we typically sum 5000 laser pulses at each laser wavelength to obtain the spectrum. We use an intracavity etalon to reduce the time-averaged laser bandwidth to approximately 0.025 cm-' and pressure scan the etalon and grating with air to vary the frequency of the laser. We also gently vibrate the output coupler of the dye laser cavity to average out the effects of the longitudinal mode structure on the narrow absorption features.9a After passing twice through the ionization region, a portion of the laser beam enters a room-temperature photoacoustic cell to ensure that the laser radiation is resonant with a vibrational overtone transition. Typical laser pulse energies with LDS 750 dye are 40 mJ/pulse. Results Figure 3a shows the ion signal arising from vibrational overtone excited molecules as a function of laser wavelength for several
-
-
-
-
(11) Baumann, W.; Mecke, R. 2. Phys. 1933,81,445. (12) Herzberg, G. Molecular Spectra and Molecular Structure. II. Infrared and Raman Spectra of Polyatomic Molecules; D. Van Nostrand: New York, 1960; pp 42-55. (1 3) Randall, H. M.; Dennison, D. M.; Ginsberg, N.; Weber, L. R. Phys. Rev. 1937, 52, 160.
The Journal of Physical Chemistry, Vol. 92, No. 6, 1988 1399
Letters limits the observed transition line widths in the ionization spectrum to 0.025 cm-I.
Discussion There is direct evidence that vibrational excitation lowers the ionization threshold for a number of molecules. Foner and Hudson have observed shifts in the ionization thresholds of vibrationally excited H2, Dz, HD, HF, Nz, NH3, and C02,14and Baiocchi et al. attribute a shift in the electron impact ionization threshold of CD4 to vibrational e ~ c i t a t i 0 n . l The ~ spectrum shown in Figure 3a clearly demonstrates that vibrational overtone excitation lowers the electron impact ionization threshold in water molecules as well. Vibrational excitation certainly reduces the energy required to reach the ionization limit, but the mechanisms that allow coupling between vibrational excitation and the electronic process of ionization are not obvious for water. In particular, approximate threshold laws suggest that the cross section for direct ionization of vibrationally excited water molecules near threshold is small. According to these laws, the electron impact cross section for direct ionization increases linearly from zero with electron energy above the ionization t h r e ~ h o l d ' ~ * ~ ~ u = k(E - Eo) where E is the electron energy and Eois the ionization threshold. The corresponding cross section Q, for direct ionization from a particular vibrational state u of the neutral molecule to produce all the energetically allowed vibrational states v' of the ion isI7
where the coefficient kd contains the Franck-Condon factor for the U'C u transition, Ed is the threshold energy for that transition, and E is the electron energy. Because the geometry of the water ion is very similar to that of the neutral m ~ l e c u l e , ~ *the J~ Franck-Condon principle predicts large kud only for transitions that have small changes in vibrational energy. Conversely, the energy-dependent factors ( E - EuuT)in the threshold law discriminate against transitions with small changes in vibrational energy because the electron energy E in our experiment is near ~
~~~
(14) (a) Foner, S.N.; Hudson, R. L. J . Chem. Phys. 1978,68,2987. (b) Chem. Phys. k i t . 1984, 104, 504. (c) J. Vacuum Sci. Technol. 1983,A I , 1261. (d) J. Chem. Phys. 1984,80,518. (e) J . Chem. Phys. 1984,80,4013. (15) Baiocchi, F. A.; Wetzel, R. C.; Freund, R. S. Phys. Rev. Lett. 1984, 53, I l l . (16) (a) Wigner, E.P. Phys. Rev. 1948, 73, 1002. (b) Wannier, G . H. Phys. Rev. 1953,90, 817. (c) Geltman, S.Phys. Reu. 1956, 102, 171. (17) Rosenstock, H.M.;Draxl, K.;Steiner, B. W.; Herron, J. T. J . Phys. Chem. Ref Dara 1977, 6 (Suppl. l), 10-28. (18) Herzberg, G. Molecular Spectra and Molecular Structure. III. Electronic Specira and Elecironic Strucrure of Polyatomic Molecules; Van Nostrand Reinhold: New York, 1966; pp 411-415. (19) Robin, M.B.Higher Excited Stares of Polyaromic Molecules; Academic: New York, 1974; Vol. 1, pp 245-254.
the threshold energy E d for these transitions. Thus, the probability for direct ionization of vibrationally excited water molecules at an electron energy below the 0 0 transition energy is likely to be small. Foner and Hudson have made similar arguments against direct ionization in their explanation of the electron impact ionization threshold shifts they observed for vibrationally excited HF and Nz.14a,dThey observed larger shifts in the ionization thresholds than can be explained by Frahck-Condon arguments with the known potentials and suggested that autoionization is responsible for the threshold shifts. Autoionization processes may be responsible for the ion signal in our experiment as well. In the ground-state water molecule, the highest filled orbital is a nonbonding 1bl molecular orbital, and all of the ns 1bl Rydberg transitions (except the first to the 3s state that is mixed with a valence antibonding configuration) produce stable molecules with geometries and vibrational frequencies similar to those in both the ground-state neutral molecule and the ground-state ion.I9 Vibrationally excited Rydberg states that are above the ionization potential are likely to autoionize, and, in fact, Page et al. have recently observed rotationally resolved, vibrationally excited autoionizing Rydberg states of water.20 Electron impact excitation to such vibrationally excited molecular Rydberg states satisfies the h = 0 transition propensity arising from the Franck-Condon factors in our experiment. In addition, the rapid rise in electron impact excitation cross sections for neutral states at t h r e ~ h o l d ' ~ * ' ~ makes it an efficient excitation mechanism for electron energies near threshold. Thus, we believe that electron impact excitation to vibrationally excited molecular Rydberg states that subsequently autoionize is the most likely ionization mechanism for vibrationally excited water molecules.
-
-
Conclusion We have used energy-selective electron impact ionization to detect highly vibrationally excited water molecules prepared by direct, single-photon-vibrational overtone excitation in a molecular beam. These first results for the 4vOH transitions in water suggest that the combination of vibrational overtone excitation and energy-selective ionization is a versatile means of studying both vibrational overtone spectroscopy and the dynamics of ionization from highly vibrationally excited molecules. The application of electron impact ionization to the vibrational overtone spectroscopy of cold molecules is particularly promising, and we plan to use this technique to obtain vibrational overtone spectra of other polyatomic molecules in the near future. Acknowledgment. S.M.P. thanks the National Science Foundation for a predoctoral fellowship. We gratefully acknowledge the support of this work by the Army Research Office. (20) Page, R. H.; Larkin, R. J.; Shen, Y. R.; Lee, Y. T. J . Chem. Phys, in press.
