Charge–transfer screening and the dynamics of electronically stimulated adsorbate dissociation and desorption

We present examples of substrate screening effects on two electronically stimulated surface processes: (1) The dissociation of NO2 on clean Pt(111); and (2) the desorption of NO from clean and O-covered Pt(111). For each set of experiments, we discuss quantum-resolved data obtained by time-of-flight laser resonance-ionization on the directly desorbed NO products. Two dominant NO2 dissociation channels have been assigned to 3b−12 and 1a−22 excitations on the basis of threshold measurements and lifetime arguments. Screening occurs through substrate charge transfer into the hybridized antibonding 6a1 molecular level. The 1a−22 channel has more NO vibrational and rotational internal energy than the 3b−12 channel due to the greater 6a1 charge density. However, screening is only partial, thus the internal excitations are a small fraction of those observed in gas phase photodissociation. The specific desorption yield from the 5σ−1 excitation of NO with coadsorbed O (≤0.75 ML) is enormously enhanced (>100×), while the vibrational excitation is reduced from that on the clean surface. Both effects are due to reduced charge transfer into the 2π antibonding molecular level. It is the 2π electron density that is utilized in the excitation decay.