Effects of low-energy electron irradiation on submonolayer ammonia adsorbed on Pt(111)

The effects of electron impact on ammonia-covered Pt(111) have been studied using temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). For coverages below one monolayer, ammonia adsorbs on the surface in two distinct TPD states: the alpha-state is broad and desorbs over the temperature range 150-350 K, and the beta-state appears as a sharper peak at 150 K. The beta-state was seen to be damaged by electron-beam impact much more readily than the alpha-state, resulting in the formation of atomically adsorbed N on the surface. The mass 28 recombinative nitrogen desorption TPD peak appearing at 550 K exhibited second-order desorption kinetics, further confirming the presence of atomically adsorbed nitrogen. The ESD kinetic energy distributions (KEDs) were obtained for m/e = 1 amu, which exhibited broad peaks generally. The H+ KEDs were analyzed using empirical curve fits, with the resulting conclusion that the H+ KEDs contain contributions from at least three different hydrogen-containing surface species. We believe that these three H+ KED peaks are due to ESD from adsorbed NH3, NH2 and H. The ESD cross-section for NH3 removal was measured in three different ways, all of which were found to be in general agreement, and which gave an averaged cross-section value of Q(tot) = 4 x 10(-17) cm(2). (C) 1998 John Wiley & Sons, Ltd.