Adsorbate-Localized Excitation in Surface Photochemistry: Methane on Pt(111)

Methane on Pt(111) dissociates or desorbs under irradiation by 193-nm photons. The polarization-dependent cross sections of the photochemical processes have been measured as a function of the incident angle by using postirradiation temperature-programmed desorption. The incident-angle dependence of the cross sections obtained with $p$ polarization substantially deviates from that of the substrate absorbance. This clearly indicates that an adsorbate-localized excitation plays an important role in the photochemistry of methane.