PHOTOCHEMISTRY OF ADSORBED MOLECULES .15. LOCALIZED ATOMIC SCATTERINGIN THE PHOTOLYSIS OF HI LIF(001) AND HI/NAF(001)/

We have compared the ''elastic'' H atom angular-scattering, P-el(theta '), of HI(ad) photolyzed at 248 nm for LiF and NaF substrates, both ex perimentally and theoretically. The observed P-el(theta') for elastic scattering were similar for LiF and NaF, with a peak in the range 43 d egrees-48 degrees. The P-el(theta') for H from HI/NaF was broader than that for H from HI/LiF. Energy distributions, P(E(T')), for H recoili ng 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 ear lier one for LiF [J. Chem. Phys. 94, 978 (1991)]. Ab initio ''density function theory'' (DFT) calculations were performed to verify the suit ability of this pes; the DFT points were in satisfactory agreement wit h 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/m ol, and experiment gave 7 kcal/mol. We have computed H atom scattering by the classical trajectory method for two different models one invol ving dense and the other perforated arrangements of the HI adsorbate m olecules, Comparison with tine experimental results showed that the se cond model gave a better description of both P-el(theta') and P(E(T')) for HI/LiF and HI/NaF. The calculations showed that the angular and e nergy distributions of the scattered H were sensitive to the size of t he vacancies between adjacent adsorbate molecules. The study also indi cates the importance of unit-cell size in relation to the size of the adsorbate molecule in determining adsorbate geometry and hence the ene rgy and angular distributions of the scattered photofragments. (C) 199 6 American Institute of Physics.