Isotope Effect in Electron Stimulated Desorption — The Role of Internal Degrees of Freedom in CO Desorption from Pt(111)

Abstract

Electron stimulated desorption of CO⁺, O⁺, metastable neutral CO* and ground state neutral CO from CO/Pt(111) was studied, using isotopic substitution of CO. Four isotopic versions of CO were compared in their desorption behavior. Contrary to the prediction of theoretical models that suggest a decrease of the ESD yield with increasing mass of a given desorption product, the CO⁺ and CO* ESD yield were found to be larger from the (heavier) ¹²C¹⁸O than from the (lighter) ¹³C¹⁶O adsorbate. The O⁺ ESD yields followed the expected trend Y(¹⁶O⁺) > Y(¹⁸O⁺). No isotope effect was observed for ground state neutral CO desorption. Qualitative arguments explaining the anomalous isotope effect, and emphasizing the importance of internal dynamics, in particular the rotation of diatomic desorption products in the ESD process, are presented. Here it is shown that the velocity of departure of the carbon end of the rotating CO molecule may control neutralization or quenching effects for CO⁺ or CO produced by electron stimulated desorption.