CO (a(3)Pi) quenching at a metal surface: Evidence of an electron transfer mediated mechanism.

We observe a strong influence of molecular vibration and surface temperature on electron emission promoted by the de-excitation of metastable CO(a(3)Pi) on a clean Au(111) surface using a molecular beam surface scattering apparatus. The de-excitation is independent of incidence translational energy. These observations appear incompatible with existing theories of metastable particle de-excitation on metal surfaces, which are based on the Auger effect. Instead, they strongly suggest a mechanism involving formation of a transient anion whose lifetime is similar to the vibrational period of the CO molecule