Femtosecond laser-induced processes: ultrafast dynamics and reaction pathways for O2/Pd(111)

Abstract

The processes of desorption and dissociation for O2 on Pd(111) under femtosecond laser irradiation have been investigated. Desorption is characterize by a high yield, a nonlinear fluence dependence, and a dominant subpicosecond feature in two-pulse correlation measurements. These observations are consistent with a process driven by the high substrate electronic temperature produced by the femtosecond laser pulse. The correlation measurements also reveal the existence of a weaker feature persisting ≳10 ps which is attributed to an enhancement of the desorption rate by adsorbate vibrational excitation. Under the same conditions where efficient desorption is occurring, an upper limit of 5% is found for the dissociation of molecular oxygen. This is in contrast to the high branching ratio for dissociation found in thermal activation and conventional photoactivation for the same system. Explanations for the anomalous branching ratio in the femtosecond surface chemistry for O2/Pd(111) within a model involving multiple cycles of electronic excitation are examined.