Ultrafast Laser Excitation of CO/Pt(111) Probed by Sum Frequency Generation: Coverage Dependent Desorption Efficiency

CO photodesorption from Pt(111) induced by femtosecond laser pulses is probed by IR + visible sum frequency generation (SFG), Steady state analysis of SFG spectra at varying CO pressure and laser fluence allows one to measure a 5 orders of magnitude decrease of the photodesorption rate constant when CO coverage decreases from 0.37 to 0.07 monolayer. We ascribe this effect in the framework of the Meazel-Gomer-Redhead mechanism to electron delocalization in the CO layer. The lifetime of electronic excitation decreases when coverage decreases.