Photochemistry of adsorbed molecules. Part 3. - Localised atomic scattering in the photolysis of HI/LiF(001)

We have measured the translational energy distribution, P(E′_T), for atomic H coming from 248 nm and 193 nm photolysis of HI adsorbed on LiF(001) at a coverage of 0.7 ML (monolayers). At both wavelengths P(E′_T) showed evidence of three contributions as follows: (a) The most energetic H was designated H(I); the energetics indicated that in this channel HI(ad) photodissociated to give ground-state I(²P_3/2). (b) Fast H with approximately 1 eV lower peak energy was designated H(I*); in this case the energy corresponded to HI(ad), giving H + I*(²P_1/2). (c) The third component was slow H observed down to <0.5 eV; it was interpreted as being inelastically scattered and was designated H(Inel). For photolysis at 248 nm the highest energy component, H(I), had a peak translational energy (E′_T)_p = 2.0 eV, and the second component H(I*) had (E′_T)_p = 1.1 eV. For photolysis at 193 nm H(I) had (E′_T)_p = 3.4 eV and H(I*) had (E′_T)_p = 2.5 eV. These energies for the scattered H at each wavelength are the same as those reported for H recoiling from photolysed gaseous HI; it appears therefore that HI(ad) gives the contributions H(I) and H(I*) by elastic scattering. The yield ratio H(I)/H(I*) from HI/LiF(001) was comparable with that for the gas phase for 248 nm photolysis of HI/LiF(001), but was greatly reduced from its gas-phase value at 193 nm. Taken together with the enhanced H(Inel) at 193 nm, this suggested markedly increased inelastic energy loss in collisions of the 3.4 eV H-atoms with the substrate and/or co-adsorbate. Theory, also reported here for the first time, predicted at 0.7 ML that HI(ad) would be tilted with the H-end down ca. 15° more steeply than for HBr(ad), but pointing at F^- on LiF(001) as reported previously for HBr/LiF(001) [E. B. D. Bourdon et al., J. Chem. Phys., 1991, 95, 1361]. This resulted in localised atomic scattering (LAS) off F^-. Energy loss from the 3.4 eV H photorecoiling from HI(ad) and then colliding with F^- in the substrate can be due (i) to the more complex trajectories that theory predicts for the case that HX is tilted downwards more steeply, (ii) to increased 'chattering' due to the high impact energy, and (iii) to inelasticity due to strong encounters between the photorecoiling H and adjacent HI(ad).