Femtosecond versus nanosecond surface photochemistry:O2+CO on Pt(111) at 80 K

Desorption of ${\mathrm{O}}_2$ and production of ${\mathrm{CO}}_2$ from ${\mathrm{O}}_2$ coadsorbed with CO on Pt(111) at 80 K were induced by laser pulses of femtosecond and nanosecond durations. The photoyields with femtosecond pulses show highly nonlinear dependence on fluence, large enhancement of cross section, and increase of the ${\mathrm{O}}_2$ desorption to ${\mathrm{CO}}_2$ production branching ratio, when compared with desorption and reaction induced by nanosecond pulses. These differences are attributed to multiple excitations occuring within the time scale of relaxation of the excited adsorbate-surface comples, which are possible with high intensity femtosecond pulse laser irradiation.