Extremely High Vibrational Excitation of CO Molecules Desorbed from Transition Metal Surfaces by Electron Impact

Internal and translational energy distributions of CO molecules desorbed by 150 eV electrons from CO monolayers chemisorbed on Ru(001) and Pt(111) were measured by resonance-enhanced multiphoton ionization via the $A{}^1\Pi$, $B{}^1{\Sigma }^{+}$, and $D^{\prime }{}^1{\Sigma }^{+}$ intermediate states. Clearly, most of the molecules desorb in the electronic ground state, but with surprisingly high vibrational excitations. Occupation of vibrational levels up to $v=31\mathrm{}$ could be directly monitored. Closer analysis indicates an inverted vibrational distribution ranging up to the dissociation limit. We explain this by desorption via multiple excitations, in agreement with other data.