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We studied the desorption of O2 and production of CO2 induced by intense femtosecond laser pulses incident on a Pt(111) surface prepared with coadsorbed CO and O2 at 90 K. The reaction pathway with femtosecond laser pulses is very different from that with nanosecond or contineous wave irradiation [1,2]. Our experiments address both the excitation mechanism as well as the chemical pathway leading to O2 desorption and CO2 production using femtosecond laser pulses.
Measurements of the yields of O2 and CO2 over a wide range of laser fluence reveal a transition around 10 mJ/mm2: the yields depend linearly on the laser fluence below this threshold, but become highly nonlinear above it. This 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 observed wavelength dependence challenges models that assume that the surface chemistry is due to thermalized electron distributions.
To study the chemical pathways for CO oxidation in CO/O2/Pt(111) and O2 desorption from O2/Pt(111), we measured the yields from various isotopic combinations of CO/O2/Pt(111), O2/Pt(111) and other surface preparations. First, we used isotopic labeling to determine if O2 desorbs as an intact molecule or if it dissociates and recombines prior to desorption. Our data conclusively show that the O2 desorption induced by femtosecond pulses is molecular. Next we used a series of experiments to distinguish between the formation of CO2 via an atomic pathway, where dissociation of O2 and subsequent interaction of an oxygen atom with CO yields CO2, and a molecular pathway, where an O2 molecule directly interacts with CO. The pathway most consistent with our data is reaction of CO with an intact O2 molecule in a geometry where the oxygen atoms are not equivalent.
References
[1] F.-J. Kao, D.G. Busch, D.G. da Costa, and W. Ho, Phys. Rev. Lett. 70, 4098 (1993).
[2] W.D. Mieher and W. Ho, J. Chem. Phys. 99, 9279 (1993).