ALKYL HALIDE PHOTOCHEMISTRY ON AG(111) .1. ETHYL CHLORIDE

The chemistry, thermal and photo, of ethyl chloride (C2H5Cl) adsorbed on Ag(111) at 100 K has been studied using temperature programmed desorption (TPD), work function change (DELTA-phi), ultraviolet and X-ray photoelectron spectroscopy (UPS, XPS). Thermally, desorption of adsorbed C2H5Cl occurs molecularly at 124 K (physisorbed) and 154 K (chemisorbed) and there is no detectable decomposition. UV irradiation leads to bond breaking, interpreted by anology with well-established literature, in terms of C-Cl cleavage. While some hydrocarbon fragments, probably C2H5, desorb during irradiation, a fraction is retained and recombines to form butane (C4H10) in post-irradiation TPD. There is no evidence for C-C or C-H bond breaking. All Cl accumulates on the surface and desorbs as AgCl above 700 K in post-irradiation TPD. There is no photodesorption of molecular C2H5Cl. The cross section for the photodissociation of a chemisorbed monolayer of C2H5Cl is approximately 2.6 x 10(-20) cm2 at 254 nm (4.9 eV). It decreases with increasing wavelength and has a threshold of 350-380 nm (3.3-3.5 eV). The average photolysis cross section decreases with increasing initial C2H5Cl coverages. For physiorbed multilayers, the photodissociation has a lower rate and higher threshold (approximately 4.1 eV). The thresholds for both physisorbed and chemisorbed C2H5Cl are lower than for gas phase photodissociation. The results are discussed in terms of substrate excitation processes, quenching by the substrate and by neighboring adsorbed molecules, and the inactivity of Ag toward C-C and C-H bond breaking.