Photochemistry of Adsorbed Molecules. 19. Photodissociation of CH3I on LiF(001) and NaCl(001) at 248 nm: REMPI Probing of CH3

The photodissociation of CH 3 I on LiF(001) and NaCl(001) at 248 nm has been studied by probing the CH 3 fragments, using angular-resolved resonantly enhanced multiphoton ionization and time-of-flight mass spectrometry. At submonolayer and multilayer coverages, the translational energy, vibrational state, and angular distributions of the CH 3 photofragments were determined for both CH 3 I/LiF and CH 3 I/NaCl. The translational-energy distributions for the fast component of the CH 3 fragments resembled those for the gas phase, indicative of collision-free recoil from the substrate. The I* quantum yields obtained from adsorbed CH 3 I were, however, substantially lower than for the gas. At multilayer coverage the I* quantum yield from adsorbed CH 3 I was found to vary as a function of vibronic band of the CH 3 photofragments, from 0.76 for the band to 0.34 for the band. These results were rationalized on the basis of the Landau−Zener model for potential-energy surface hopping. The measured vibrational-state distributions of the umbrella mode also exhibited a strong dependence on reaction channel, on coverage, and on substrate. The angular distributions depended on the substrate; a sharp peak at 20° was observed in the angular distribution for the CH 3 I/LiF system, whereas a broader peak characteristic of collisional scattering was obtained for CH 3 I/NaCl. These angular distributions indicated that CH 3 I(ad) was close to upright on LiF but tilted further away from the normal on NaCl. The energy distributions, which showed evidence of greater collisional deexcitation on NaCl than on LiF, appeared consistent with these differing adlayer geometries.