J . Phys. Chem. 1989, 93, 5331-5333
5331
Controlling the Pathways In Molecular Decomposition: The Vibrationally Mediated Photodlssoclation of Water Randall L. Vander Wal and F. Fleming Crim* Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received: May 2, 1989)
Vibrationally mediated photodissociation, in which vibrational overtone excitation prepares a single rovibrational state of water that an ultraviolet photon subsequently dissociates, permits a fully state resolved study of the photodissociation of water. Dissociating water from six different initial rotational states and detecting the OH products by laser-induced fluorescence shows that the distribution of the products among their rotational states depends strongly on the state initially selected in the vibrational overtone excitation step. These measurements also demonstrate the control of the dissociation pathway by selection of different intermediate states. Dissociating water molecules from one vibrational state (104)-)produces almost no vibrationally excited OH products, but dissociating another state (I13)-) that corresponds to a different nuclear motion produces roughly comparable amounts of OH(v=l) and OH(v=O).
Introduction Fully state resolved photodissociation experiments that prepare and subsequently dissociate a specific initial state reveal qualitative features of the dissociation dynamics and provide a point of quantitative comparison with theory. In one pioneering experiment, Andresen et al.’ have determined the rotational state distribution for the O H product formed in the ultraviolet photodissociation of water molecules prepared in a single rovibrational state by infrared excitation of the asymmetric stretching vibration. Schinke and co-workers have modeled the state-resolved photodissociation dynamics of ground and vibrationally excited water (H20, D20, and HOD) quantum mechanically using an ab initio potential energy surface to obtain good agreement with measured absorption cross sections and product state distributions, including X doublet and spin-orbit state population^.^^^ Imre and coworkers4 have used wave packet propagation techniques to model these photodissociation processes as well as to investigate the role of initial vibrational excitation in calculations that produce similarly good agreement with experiments. This Letter describes a fully resolved vibrationally mediated photodissociation measurement on water in which we select a single rovibrational state by vibrational overtone excitation and then dissociate it with an ultraviolet photon. Exciting an overtone vibration in the first step allows the selection of vibrational states having similar energies but rather different nuclear motions, which lead to different amounts of vibrational excitation in the O H product of the subsequent photodissociation. This selective production of vibrationally excited OH fragments reflects qualitative differences in the intermediate vibrational states and is consistent with a local mode description of the highly vibrationally excited molecule. Vibrationally mediated p h o t o d i s s ~ c i a t i o n is ~ -a~ two-photon ~ (1) Andresen, P.; Beushausen, V.; Hausler, D.; Liilf, H. W.; Rothe, E. W. J. Chem. Phys. 1985,83, 1429. (2) Schinke, R.; Engle, V.; Andresen, P.; Hausler, D.; Baht-Kurti, G. G. Phys. Rev. Lett. 1985, 55, 1180. Schinke, R.; Engel, V.; Staemmler, V. J. Chem. Phvs. 1985.83.4522. Ennel, V.: Schinke. R.; Staemmler, V. Chem. Phys. Leti. 1986, 130, 413. H i d e r , D.; Andresen, P.; Schinke, R. J. Chem. Phys. 1987,87,3949. Engel, V.; Schinke, R.; Staemmler, V. J. Chem. Phys. 1988, 88, 129. Engel, V.; Schinke, R. J. Chem. Phys. 1988, 88, 6831. (3) Andresen, P.;Schinke, R. In Molecular Photodissociation Dynamics; Ashfold, M. N. R., Baggott, J. E., Eds.; Royal Society of Chemistry: London, 1987; p 61. (4) Zhang, J.; Imre, D. G.; Frederick, J. H. J. Phys. Chem. 1989,93, 1840. Zhang, J.; Imre, D. G. J. Chem. Phys. 1989, 90, 1666. Henriksen, N. E.; Zhang, J.; Imre, D. G. J. Chem. Phys. 1988,89, 5607. ( 5 ) Sinha, A.; Vander Wal, R. L.; Butler, L. J.; Crim, F. F. J. Phys. Chem. 1987, 91, 4645. (6) Ticich, T. M.; Likar, M. D.; Diibal, H.-R.; Butler, L. J.; Crim, F. F. J. Chem. Phys. 1987, 87, 5820. (7) Likar, M. D.; Sinha, A.; Ticich, T. M.; Vander Wal, R.; Crim, F. F. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 289.
0022-3654/89/2093-5331$01.50/0
technique in which one photon excites an overtone vibration and a second photon promotes the vibrationally energized molecule to a dissociative electronic state. Because the excited and ground electronic state surfaces in water approach each other along the dissociation coordinate, they lie closer to each other for extended 0-H bonds, as illustrated in Figure la. Consequently, a second photon having too little energy to photodissociate the ground vibrational state of H 2 0 can dissociate the vibrationally excited molecule. Figure l b illustrates this with qualitative sketches to the lowest two potential energy surfaces of water. The broken arrow illustrates excitation from near the equilibrium geometry, and the solid arrow shows excitation from an extended 0-H bond in a vibrationally excited molecule.
Experimental Approach The experimental apparatus is similar to that used in our previous studies of vibrationally mediated photodi~sociation.~-~~ A Nd:YAG laser pumped dye laser provides a vibrational overtone excitation pulse of approximately 40 mJ near A, = 700 nm for excitation of a low-pressure (50-300-mTorr) sample of water. A second Nd:YAG/dye laser system provides a 0.1-5-mJ photolysis pulse 30 ns after the vibrational overtone excitation pulse. The photolysis wavelengths are X2 = 266 nm, the fourth harmonic of the Nd:YAG laser, 239.5 nm, or 218.5 nm. These latter wavelengths come either from Raman shifting the fourth harmonic of the NdYAG laser light in H2or from mixing frequency-doubled dye laser light with the fundamental frequency of a Nd:YAG laser. Frequency-doubled light around X3 = 308 nm from a nitrogen laser pumped dye laser probes the OH(’II3/2) fragments from the dissociation by laser-induced fluorescence on the A X transition. Gated integrators capture the fluorescence signal from a photomultiplier, which views the OH fluorescence through an f/2 optical system, as well as the signals from photodiodes that measure the laser powers. A 75-cm lens focuses the vibrational overtone excitation beam in the observation region, and different lenses, depending on the source of the light, focus the photolysis pulse. A 50-cm lens focuses the light obtained by frequency mixing, and a 2:l telescope and 1000-cm lens focus that from Raman shifting. We obtain laser-induced fluorescence excitation spectra of the OH product, from which we extract rovibrational state populations, by fixing the wavelengths of both the vibrational overtone exci-
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(8) Likar, M. D.; Baggott, J. E.; Sinha, A.; Vander Wal, R. L.; Crim, F.
F.J . Chem. Sor., Faraday Trans. 2 1988, 84, 1483.
(9) Likar, M. D.; Baggott, J. E.; Crim, F. F. J. Chem. Phys., in press. (10) Sinha, A,; Vander Wal, R. L.; Crim, F. F. J. Chem. Phys., in press. (11) Brouard, M.; Martinez, M. T.; OMahony, J.; Simons, J. P. Chem. Phys. Lett. 1988, 150, 6; J. Chem. Soc., Faraday Trans. 2, in press. (12) Conaway, W. E.; Stephens, C. To be submitted for publication.
0 1989 American Chemical Society
Letters
5332 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989
h
HI
.c.
I
= 239.5 nm
Typical Uncertainty
a
U -
ROH
W
1
ROH
Figure 1. Qualitative potential energy surfaces for the ground and first electronically excited states of water. (a) A cut through the surfaces along the O-H stretching coordinate. The first photon (A,) excites the overtone vibration, the second photon (A,) dissociates the highly vibrationally excited molecule, and the third photon (A,) probes the OH fragment by laser-induced fluorescence. (b) Energy contours for the two potential energy surfaces for a fixed bending angle. The broken arrow illustrates excitation from the equilibrium geometry, and the solid arrow shows excitation from a vibrationally excited state having an extended O-H bond.
tation laser (A,) and the photolysis laser (A,) and varying the wavelength of the probe laser (A3). The other variant of the experiment we use here is to fix the wavelengths of the photolysis and probe lasers and vary the wavelength (A,) of the vibrational overtone excitation laser to obtain excitation spectra of water molecules that decompose to produce an O H fragment in the probed quantum state.
Results and Discussion Our goal is to determine the details of the photodissociation dynamics of molecules from individual rovibrational states, which we select in the vibrational overtone excitation step. The most important aspect of dissociating these highly vibrationally excited molecules is the preparation of different initial vibrational states having similar energy but very different nuclear motions in order to control the populations of the vibrational states of the OH fragment. We use local mode notation, which is particularly suitable for highly excited H20,13J4to describe the vibrational states in our experiments. Local modes are a much better zero-order basis for highly vibrationally excited water than normal modes, but an expansion in either basis describes the same molecular eigenstate of the complete Hamilt~nian.’~*’~ Local mode notation designates a vibrational overtone state containing “a” quanta in one O-H bond and “b” quanta in the other as lab), but only the symmetric and antisymmetric linear combinations,designated lab)+ and lab); respectively, are eigenfunctions of the zero-order Hamiltonian. The two different vibrational states that we excite are 104)- and 113)-.16 (13) Child, M. S.; Lawton, R.T. Chem. Phys. Lett. 1982,87,217. Child, M. S.; Halonen, L. Ado. Chem. Phys. 1984,57, 1. (14) Sibert, E. L.; Hynes, J. T.; Reinhardt, W.P.J. Chem. Phys. 1982, 3583,3595. (15) Mills, I. M.; Robiette, A. B. Mol. Phys. 1985, 56, 743. (16) The normal mode labels for the 104)- and 113)- states are (301) and (103), respectively. The three indices (vIu2v3)are the quantum numbers for the for the symmetric stretch ( q ) ,bend (v2), and asymmetric stretch ( V J
normal modes.
2 3 4 5 OH Rotational Level (N)
6
Figure 2. Relative populations of the rotational states of the OH(’II3p) fragment from photolysis of different initial rotational states of highly vibrationally excited water. The initial rotational states of water, selected by vibrational overtone excitation of the 104)- vibrational state, are labeled JKA where J is the total angular momentum quantum number and K, and K , are the projections on the top axis in the prolate and oblate symmetric top limits. The quantum number for the OH product, N , is that for the total angular momentum except for electron spin. The photolysis wavelength is A2 = 239.5 nm, and the total product yield for each rotational state of water is adjusted to agree with the relative intensities of the rotational transitions in the photoacoustic spectrum of water.
Product Rotational State Populations. Exciting single rotational transitions to the 104)- state selects different rotational states of water, designated JKsc where J, K,, and K, are the quantum numbers for the total angular momentum and its projection on the a and c axes in the prolate and oblate symmetric top limits. Figure 2 shows the distribution of the O H products among their rotational states for photodissociation from six different initial rotational states of water. The distributions in Figure 2 are for a photolysis wavelength of A, = 239.5 nm, but they are essentially identical for A, = 266 nm. Clearly, the rotational state distribution of the OH product is very sensitive to the initial rotational state of the parent molecule, in agreement with the results of Andresen et a1.l The distributions for decomposition from a particular rotational state, whether prepared by excitation of the asymmetric stretch’ or by excitation of an overtone vibration, are similar within the uncertainties of the two experiments. In both measurements, we would expect the distributions for dissociation from all of the rotational states of H 2 0 to be the same if the electronically excited surface controlled this aspect of the dynamics, but the unique dependence of the product state rotational distribution on the initial rotational state shows that interactions on the excited surface have little influence on the rotation of the pr~duct.~ Dissociationfrom Selected VibrationalStates. The production of vibrationally excited OH fragments in the vibrationally mediated photodissociation of water depends strongly on the intermediate vibrational state prepared in the vibrational overtone excitation step. Using A, = 239.5 nm to dissociate H 2 0 molecules from the 104)- state produces very little (
