PHOTOCHEMISTRY OF ADSORBED MOLECULES .13. LOCALIZED ATOMIC SCATTERINGIN 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 run photolysis of HI adsorbed on L iF(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 e nergetic H was designated H(I); the energetics indicated that in this channel HI(ad) photodissociated to give ground-state I(P-2(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(P-2(1/2 )). (c) The third component was slow H observed down to <0.5 eV; it wa s interpreted as being inelastically scattered and was designated H(In el). 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 componen t 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 s cattered H at each wavelength are the same as those reported for H rec oiling from photolysed gaseous HI; it appears therefore that HI(ad) gi ves 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 f rom its gas-phase value at 193 nm. Taken together with the enhanced H( Inel) at 193 nm, this suggested markedly increased inelastic energy lo ss in collisions of the 3.4 eV H-atoms with the substrate and/or co-ad sorbate. 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-degrees m ore steeply than for HBr(ad), but pointing at F- on LiF(001) as report ed 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 com plex 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 bet ween the photorecoiling H and adjacent HI ad.