Surface photochemistry induced by ultrafast pulses of vacuum ultraviolet light: Physisorbed oxygen on graphite

We report a surface photochemistry study using ultrafast pulses of vacuum ultraviolet (vuv) light for excitation. Tunable vuv radiation (10--38 eV) is produced via high harmonic generation. We have studied the model system of physisorbed O₂ on graphite. The only observed desorption product is O⁺, the yield of which is measured as a function of the excitation photon energy. The kinetic energies of the desorbed ions are obtained by time-of-flight measurements with the pulsed source. These measurements enable us to suggest that the O⁺ desorption proceeds through more than one channel and to identify the electronic transitions that may be involved in the dissociative ionization of the physisorbed O₂.