The Negligible Role of C-H Stretch Excitation in the Physisorption of CH4 on Pt(111)

We report a molecular beam study of the effect of vibrational excitation on the physisorption of methane on a Pt(111) surface. Our experiments use a continuous molecular beam of CH4, prepared in its antisymmetic C-H stretch mode ν3 by infrared laser pumping via rapid adiabatic passage. Physisorbed CH4(ads) is detected on a Pt(111) surface by reflection absorption infrared spectroscopy (RAIRS). At a surface temperature of 77 K, the desorption lifetime of CH4(ads) is observed to be 0.4±0.2 seconds. Trapping probabilities for the incident CH4 are measured by the King & Wells beam reflectivity method with and without vibrational excitation. Vibrational excitation of the incident CH4 with one quantum of ν3 vibration does not produce a measureable effect (less than 0.2% change) on the trapping probability, in sharp contrast to the dissociative chemisorption process. The effect of resonant vibrational excitation of physisorbed CH4(ads) on its dissociation rate was also investigated.