PHOTOCHEMISTRY OF ADSORBED MOLECULES .14. PHOTOEJECTION CH(3)X (X=CL, BR) FROM CH3BR/LIF(001)

At > 1 ML coverage of methyl bromide adsorbed on LiF(100), CH 3Br is efficiently "photoejected" by ultraviolet light at 193 nm, with a mean translational energy of ∼0.3 eV; this is termed PEJ(CH3Br). The system CH3Cl/LiF(100) shows no detectable PEJ(CH3Cl) at this wavelength for which CH3Cl does not efficiently absorb UV. Similarly PEJ(CH3Br) ceased to be observable with 248 nm light, at which wavelength CH3Br does not absorb. Both observations indicate that PEJ is adsorbate mediated. If at 193 nm ∼ 1 ML of CH3Br is adsorbed beneath ∼ 1 ML of CH3Cl, efficient PEJ(CH3Cl) is observed. With the sequence of layers reversed (i.e., with the actinic layer on top), PEJ(CH3Cl) is largely suppressed. These observations support the model for PEJ proposed previously [I. Harrison et al., J. Chem. Phys. 89, 1498 (1988)] in which photoexcitation of the underlayer leads, by way of electronic-to-translational (E → T) transfer, to ejection of the top layer. The angular distribution of the photoejected species is peaked along the surface normal, consistent with repulsion operating between the under- and overlayer. The mean conversion of E → T implied by this mechanism is ∼ 5%, extending to a maximum of 12%. © 1995 American Institute of Physics.