Paper SS1-FrM9
Subpicosecond Photodesorption and Photoreaction of CO on Pd(111)

Friday, October 19, 2007, 10:40 am, Room 608

We have examined the dynamics of the desorption and oxidation of CO on a Pd(111) surface following irradiation with ~110-fs pulses of 780-nm light. For a mixed layer of CO and atomic oxygen, both the desorption of CO and the production of CO₂ exhibit superlinear dependences on the absorbed laser fluence. The fluence dependence of CO desorption at saturation coverage is unaffected by the presence of coadsorbed atomic oxygen. For both CO/Pd(111) and (CO+O)/Pd(111) systems, the probability of CO desorption increases with the initial CO coverage due to population of lower-adsorption-energy sites. Two-pulse correlation (2PC) measurements of the photodesorption yield show rapid relaxation of the excited adsorbate-substrate complex and are well explained by an electron-mediated process. The photoinduced CO₂ production in the (CO+O) mixed layer, by contrast, involves mainly CO molecules with higher binding energies. The 2PC of the CO₂ product is five times broader than that of CO, suggesting that a different effective molecule-surface coupling is responsible for the reaction compared to the desorption. Our results are consistent with a picture where strong coupling between substrate electrons and adsorbed CO leads to high transient adsorbate temperatures, which can cause a subsequent reaction with oxygen if the barrier to direct CO desorption is sufficiently high.