Femtosecond Laser-Induced Recombinative O + O = O-2 Reaction on Single Crystal Pd(100) Surface Requires Thermal Assistance

摘要

The process of recombinative desorption of molecular oxygen (O-adsorbed + O-adsorbed = O-2,O-gas) from the Pd(100) single crystal surface, under femtosecond laser irradiation, has been investigated with the help of pre- and postradiation temperature-programmed desorption (TPD) measurements. This femtosecond optical pulse-induced surface chemistry is found to depend strongly on the initial surface temperature. The threshold temperature is observed to be 400 K, above which this reaction remains active for the absorbed fluence of 2.86 mJ/cm(2). Furthermore, the desorption-yield is observed to be linear with respect to the absorbed fluence. We explain our observations with the help of combined two-temperature model simulation and density functional theory-based computations. A two-step mechanism for the femtosecond optical pulse-induced recombinative desorption of molecular oxygen is evident: the first step involves the hot electron-mediated activation of the oxygen atoms and the second step involves the thermal activation (phonon-mediated) of the oxygen atoms leading to the recombination of oxygen atoms to form molecular oxygen which immediately desorbs from the Pd(100) surface. This is the first report on the femtosecond optical pulse-induced recombinative surface chemistry of adsorbed oxygen atoms on the Pd single crystal surface.