LOW-ENERGY ELECTRON INDUCED SURFACE PROCESSES - DESORPTION AND BOND SCISSION OF HCN(A) AND CN(A) ON PD(111) AS STUDIED BY THERMAL-DESORPTION SPECTROSCOPY

The influence of electron bombardment on the HCN/Pd(111) system at 300 K has been studied by temperature programmed desorption for a HCN exposure of 20 x 10(14) molecules cm-2 and electron energies between 5 and 400 eV. For this level of HCN exposure there is a coexistence of HCN(a) and CN(a) in the adlayer. It was found that electron bombardment leads to HCN(a) depopulation through H-CN(a) stimulated bond scission, resulting in direct hydrogen desorption and increase in the CN(a) population and probably through HCN(a) stimulated desorption. It was found that CN(a) stimulated desorption also takes place. The influence of the induced change in population on the kinetics of desorption is addressed. The cross-section for HCN(a) and CN(a) total electron stimulated depopulation was calculated on the basis of a first-order process and found to be 4.7 x 10(-18) cm2 and 0.75 x 10(-18) cm2, respectively, for an electron energy of 200 eV. A threshold at about 15-20 eV electron energy was found for the cross-section of HCN(a) electron stimulated depopulation.