Chemisorption of CO on tungsten (100): combined flash desorption and electron stimulated desorption study. I

Abstract

The chemisorption of CO on a tungsten (100) single crystal surface has been studied at adsorption temperatures of 295 K and 195 K. A combination of flash desorption and electron stimulated desorption (ESD) techniques has been used. In CO layers produced at 295 K, the well-known α-CO state has been shown to be a superposition of two states, designated α 1 and α 2. The α 1-CO state exists preferentially at higher α-CO coverages and is characterized by a predominant ion yield of CO + upon 100 eV electron bombardment. The α 2-CO state yields O +, and consists of a number of overlapping states possessing a range of desorption activation energies. During thermal desorption of α-CO, a stoichiometric state interconversion process occurs in which α 1-CO converts to more strongly-held α 2-CO as surface coverage decreases. Similar conversion behavior is observed when the surface layer is depopulated by ESD. Adsorption of CO at 195 K resembles adsorption at 295 K, except that a small additional α-CO state, designated α 0, and yielding CO + on ESD, is populated. Structural models are presented for α 1-CO and α 2-CO, and the role of surface species' structure in determining ionic fragmentation probabilities is discussed.