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. At a surface temperature of 77 K, the desorption lifetime of CH4(ads) is observed to be 0.4 ± 0.2 s. Trapping probabilities for the incident CH4 are measured by the King and 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 measurable 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.