Recombinative Desorption of Vibrationally ExcitedD2(v′′=1)from Clean Pd(100)

Abstract

Recombinative desorption of ${\mathrm{D}}_2$ from clean Pd(100) was studied with rotational-state selectivity by using resonantly enhanced two-photon ionization. The amount of vibrationally excited ${\mathrm{D}}_2$ molecules increased exponentially with the surface temperature $T_s$, associated with an activation energy, in addition to that for desorption of ${\mathrm{D}}_2(v^{\prime \prime }=0)$, of 210±60 meV, considerably less than the vibrational energy of free ${\mathrm{D}}_2$ molecules. This activation energy can be accounted for by a temporary trapping of ${\mathrm{D}}_2$ in a molecular chemisorption state with a softened D-D bond, in qualitative agreement with a recently proposed model.