Direct Observation of the Crossover from Single to Multiple Excitations in Femtosecond Surface Photochemistry

Laser pulses of 110 fs duration at 310 nm are used to desorb ${\mathrm{O}}_2$ from a Pt(111) surface over a range of laser fluences which encompasses the single electronic excitation regime at low fluences and the multiple excitation regime at high fluences. The crossover between regimes is apparent in both the desorption yield and the translational energies of the desorbing molecules. Similar results are obtained for ${\mathrm{O}}_2$ coadsorbed with CO, where irradiation also produces C${\mathrm{O}}_2$, with a ${\mathrm{O}}_2$/C${\mathrm{O}}_2$ branching ratio which changes by 2 orders of magnitude between regimes. The results are compared to those contained using 8 ns, 355 nm and 90 fs, 620 nm laser pulses.