Photochemistry of adsorbed molecules. XV. Localized atomic scattering in the photolysis of HI/LiF(001) and HI/NaF(001)

We have compared the "elastic" H atom angular-scattering, P_el(θ′), of HI(ad) photolyzed at 248 nm for LiF and NaF substrates, both experimentally and theoretically. The observed P_el(θ′) for elastic scattering were similar for LiF and NaF, with a peak in the range 43°-48°. The P_el(θ′) for H from HI/NaF was broader than that for H from HI/LiF. Energy distributions, P(E_T′), for H recoiling from HI/LiF and HI/NaF are also reported. A semiempirical potential-energy surface (pes) is given for HI adsorbed on NaF, based on an earlier one for LiF [J. Chem. Phys. 94, 978 (1991)]. Ab initio "density function theory" (DFT) calculations were performed to verify the suitability of this pes; the DFT points were in satisfactory agreement with the semiempirical pes. For HI/LiF the DFT calculation gave a heat of adsorption of about 9 kcal/mol, the semiempirical model gave 5 kcal/mol, and experiment gave 7 kcal/mol. We have computed H atom scattering by the classical trajectory method for two different models one involving dense and the other perforated arrangements of the HI adsorbate molecules. Comparison with the experimental results showed that the second model gave a better description of both P_el(θ′) and P(E_T′) for HI/LiF and HI/NaF. The calculations showed that the angular and energy distributions of the scattered H were sensitive to the size of the vacancies between adjacent adsorbate molecules. The study also indicates the importance of unit-cell size in relation to the size of the adsorbate molecule in determining adsorbate geometry and hence the energy and angular distributions of the scattered photofragments.