Femtosecond surface photochemistry: what is the role of the substraste electrons?

We studied the femtosecond laser-induced desorption of O2 and production of CO2 from a CO/O2/Pt(111) surface at 90K. The reaction pathway with 0.3-ps laser pulses is very different from that with nanosecond or continuous wave irradiation.[1,2] Our experiments address both the excitation excitation mechanism and the chemical pathway leading to O2 desorption and CO2 production using femtosecond laser pulses. When the fluence is below 10 µJ/mm2 the yields of O2 and CO2 vary linearly with laser fluence. Above 10 µJ/mm2 the yield is highly nonlinear in the fluence. The transition indicates a change in the excitation mechanism. Above the transition fluence, the nonlinearity and the relative yields of O2 and CO2 depend on wavelength. The wavelength dependence in the yields challenges models which assume the chemistry is due to thermalized electron distributions. We used isotopic labeling to study the chemical pathways to CO2 production from CO/O2/Pt(111) and O2 desorption from O2/Pt(111). The data show that the O2 desorption is molecular and not recombinative. Two pathways to CO2 production are discussed. The CO may react with molecular oxygen, or it may react with atomic oxygen produced from laser-dissociated O2. [1] F.-J. Kao, D.G. Busch, D.G. da Costa, and W. Ho, Phys.Rev.Lett. 70, 4098-4101 (1993) [2] W.D. Mieher and W. Ho, J. Chem. Phys. 99, 9279-9295 (1993)