Effects of coadsorbed atomic oxygen on the electron-stimulated desorption of neutral NO from Pt(111)

We examine the effects of an electronegative coadsorbate on the electron-stimulated-desorption (ESD) yield and desorbate energies (translational and internal) of a chemisorbed molecule. Specifically, we use laser resonance-ionization spectroscopy to characterize the ESD of neutral NO from a Pt(111) surface precovered with atomic oxygen. With increasing oxygen coverage (up to 0.75 monolayer), we observe the following for the NO desorbate: (1) an exponential increase in specific yield, (2) increased translational energy, (3) decreased vibrational energy, (4) decreased rotational energy, and (5) a growing propensity to produce the upper spin-orbit level of the spin-orbit-split electronic ground state. The first three observations are understood in terms of an O-induced reduction in charge transfer from the substrate into the adsorbate 2{π} molecular level to screen the electronic excitation (5σ^-1). This has the dual effect of reducing the Auger decay rate 5σ¹2π²(NO){→}5σ²2π⁰(NO⁺), and of lowering the NO vibrational excitation. The consequences of a reduced Auger decay rate are a larger ESD yield and more desorbate translational energy. We argue that the spin-orbit propensity arises from an O-induced rotational hindering of the NO excited state. A hindered NO⁺ rotor, ionized after Auger decay, is reneutralized by a strongly spin-orbit-split Pt(111) substrate at a greater rate into the upper level than into the lower level.