State-resolved study of coadsorption effects on the stimulated dissociation of NO₂on Pt(111)

Stimulated surface processes, which are induced by electronic transitions, are especially sensitive to the presence of coadsorbates. We present here a detailed study of electron (5–350 eV) stimulated dissociation of chemisorbed NO₂ on an O-covered Pt (111) surface, utilizing state-resolved laser resonance-ionization detection of the NO gas-phase product. Several dramatic effects on NO₂ dissociation occur with increasing O atom coverage (up to θ _{0 =} 0.75 monolayer). First there is a large (X26) enhancement in the specific dissociation yield. Second, there is a narrowing of the translational energy distributions. A third observation is a distinct propensity (> 4:1 at low J) for populating the upper (Ω = 3/2) over the lower (Ω = 1/2) level of the spin-orbit-splitNO ²II_Ω ground state. The enhanced yield can be understood in terms of increased lifetime(s) due to an O-induced reduction in charge transfer screening. Diminished charge transfer also reduces the vibrational and rotational excitation of the NO product. The spin-orbit propensity might arise from substrate reneutralization of NO⁺ following Auger decay of the primary excitation.