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We report high resolution state-to-state time-of-flight (TOF) measurements for scattering of HCl(v=2, J=1) from a Au(111) single crystal surface for both vibrationally elastic (v=2-->2) as well as inelastic (v=2-->1) channels at seven incidence energies between 0.28 and 1.27 eV. The dependences of the TOF results on final HCl rotational state and surface temperature are also reported. The translational energy transferred to the surface depends linearly on incidence energy and is close to the single surface-atom impulse (Baule) limit over the entire range of incidence energies studied. The probability of vibrational relaxation is also large. For molecules that relax from v=2 to v=1, the fraction of vibrational energy that is transferred to the surface is approximately 74%. We discuss these observations in terms of an impulse approximation as well as the possible role of translational and vibrational excitations of electron-hole pairs in the solid.

The photochemistry of H2O in the VUV region is important in interstellar chemistry. Whereas previous studies of the photodissociation used excitation via unbound states, we have used a tunable VUV photolysis source to excite individual levels of the rotationally structured C state near 124 nm. The ensuing OH product state distributions were recorded by using the H-atom Rydberg tagging technique. Experimental results indicate a dramatic variation in the OH product state distributions and its stereodynamics for different resonant states. Photodissociation of H2O(C ) in rotational states with ka ‫؍‬ 0 occurs exclusively through a newly discovered homogeneous coupling to the à state, leading to OH products that are vibrationally hot (up to v ‫؍‬ 13), but rotationally cold. In contrast, for H2O in rotationally excited states with ka > 0, an additional pathway opens through Coriolis-type coupling to the B state surface. This yields extremely rotationally hot and vibrationally cold ground state OH(X) and electronically excited OH(A) products, through 2 different mechanisms. In the case of excitation via the 110 4 000 transition the H atoms for these 2 product channels are ejected in completely different directions. Quantum dynamical models for the C -state photodissociation clearly support this remarkable dynamical picture, providing a uniquely detailed illustration of nonadiabatic dynamics involving at least 4 electronic surfaces.photochemistry ͉ water ͉ photodissociation ͉ stereodynamics ͉ VUV photolysis W ater vapor absorbs light at all wavelengths in the vacuum UV region below 200 nm, resulting in fragmentation leading to the production of H and OH free radicals. The OH radical product of this unimolecular process is particularly important through its role in interstellar chemistry (1, 2). Extensive experimental and theoretical studies have revealed a rich structure of absorption bands in this spectral region, and have uncovered several active dissociative mechanisms. However, in most of these studies the excitation of the H 2 O parent molecule has been through dissociative continua, so that the observed dynamics relate to averages over an ensemble of molecules in a range of quantum states.In contrast, the present study uses a tuneable VUV laser to excite H 2 O through an electronic state that is sufficiently longlived to exhibit resolved rotational structure. This reveals 2 completely different dissociation pathways that produce strikingly different rovibrationally excited OH products. Furthermore, the competition between these 2 channels shows a strong dependence on the excited rotational quantum numbers and on the direction of mutual recoil of the H and OH fragments.These studies have led to an understanding of how the dissociation dynamics are controlled by the topologies of the nuclear potential energy surfaces of this model system. Absorption by H 2 O between Ϸ140 and 200 nm is to the first excited singlet state, labeled à and of symmetry 1 B 1 . The photodissociation on this state has been studied at both 19...