Coverage dependent desorption kinetics of CO from Rh(111) using time-resolved specular helium scattering

Abstract

Linearized measurements of the coverage dependent desorption rates of CO from Rh(111) were made with a novel three molecular beam apparatus. To measure these isothermal and essentialy isosteric rates we introduce a new kinetic response amplifier, namely time-resolved specular helium scattering, which makes use of the large attenuation cross section that CO has for specular helium scattering. The measurements are made by using one intense and continuous CO beam to establish a specific adsorbate coverage while another low intensity and chopped CO beam is used to weakly modulate the adsorbate density around the selected steady state. The transient helium reflectivity waveforms we measure during the modulated CO scattering contain the desired kinetic information, and are typically one to almost two orders of magnitude more sensitive to the desorption kinetics than are the signals arising from direct detection of desorbing CO. Desorption rates are reported for 0≤θCO≤0.22 and 440 K ≤TS≤555 K. He diffraction measurements revealed that the CO overlayer was disordered for all conditions for which kinetics were measured, and that the sticking coefficient varied with coverage as S0(1−3θ). At least a second order expansion of the chemical potential in terms of CO coverage was needed to explain these rates. The experimentally determined desorption rates can be fit equally well by placing the coverage dependence in either the pre-exponential factor or in the activation energy: k(θ,TS) =1.33×1014 exp[0.344θ+48.8θ2 −(32.3 kcal/mol)/(kbTS)]s−1, k(θ,TS) =1.33×1014 exp[−(32.3−1.62θ −38.3θ2 kcal/mol)/(kbTS)] s−1. The measured He diffraction data, sticking coefficient, specular He scattering attenuation vs CO coverage, and increase in CO desorption rate with increasing coverage imply nearest-neighbor repulsive interactions. The use of time-resolved specular He scattering for studying coverage dependent reactions is also discussed.