PHOTOCHEMISTRY OF ADSORBED MOLECULES. 19. PHOTODISSOCIATION OF CH3I ON LIF(001) AND NACL(001) AT 248 NM: REMPI PROBING OF CH3

The photodissociation of CH3I on LiF(001) and NaCl(001) at 248 nm has been studied by probing the CH3 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 CH3 photofragments were determined for both CH3I/LiF and CH3I/NaCl. The translational-energy distributions for the fast component of the CH3 fragments resembled those for the gas phase, indicative of collision-free recoil from the substrate. The I* quantum yields obtained from adsorbed CH3I were, however, substantially lower than for the gas. At multilayer coverage the I* quantum yield from adsorbed CH3I was found to vary as a function of vibronic band of the CH3 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 ...