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 ${\mathrm{O}}_2$ on graphite. The only observed desorption product is ${\mathrm{O}}^{+},$ 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 ${\mathrm{O}}^{+}$ desorption proceeds through more than one channel and to identify the electronic transitions that may be involved in the dissociative ionization of the physisorbed ${\mathrm{O}}_2.$