Electronically stimulated adsorbate dissociation in the presence of an electronegative coadsorbate: (NO2+O) on Pt(111).

Abstract

The effects of an electronegative coadsorbate on the stimulated dissociation of a chemisorbed molecule are investigated using state-resolved laser-ionization spectroscopy of those dissociation fragments which leave the surface. Specifically, we examine the NO${(}^{2}$${\mathrm{\ensuremath{\Pi}}}_{\mathrm{\ensuremath{\Omega}}=1/2,3/2}$) and O${(}^{3}$${\mathit{P}}_{\mathit{J}}$) fragment energies and yields resulting from electron- (6--350 eV) stimulated dissociation of chemisorbed ${\mathrm{NO}}_{2}$ as a function of preadsorbed atomic O coverage. The most dramatic effect associated with O coverage (up to ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{O}}$=0.75 monolayer) is a large (a factor of 26) enhancement in the specific ${\mathrm{NO}}_{2}$ dissociation yield. There is also an O-induced narrowing of the translational energy distributions and a decrease in both the rotational and vibrational energy of the NO fragment. The dissociation threshold of \ensuremath{\sim}10 eV, together with lifetime arguments, suggest that the dominant excitation(s) are shallow-valence two-hole excitations. The above observations can be understood in terms of reduced substrate charge-transfer screening of these excitations. In addition, we observe a distinct propensity (>4:1 at low J) for populating the upper (\ensuremath{\Omega}=3/2) over the lower (\ensuremath{\Omega}=1/2) level of the spin-orbit-split NO${(}^{2}$${\mathrm{\ensuremath{\Pi}}}_{\mathrm{\ensuremath{\Omega}}}$) ground state, whereas the spin-orbit population of the O${(}^{3}$${\mathit{P}}_{\mathit{J}}$) fragment is statistical (2J+1) within experimental error. The O${(}^{3}$${\mathit{P}}_{\mathit{J}}$) yield derives from dissociation of nitro-bound ${\mathrm{NO}}_{2}$ (N end down); no O${(}^{3}$${\mathit{P}}_{\mathit{J}}$) yield can be associated with side-bonded ${\mathrm{NO}}_{2}$.