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

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 =1/2,3/2) and O(3PJ) fragment energies and yields resulting from electron- (6 350 eV) stimulated dissociation of chemisorbed NO2 as a function of preadsorbed atomic O coverage. The most dramatic effect associated with O coverage (up to O=0.75 monolayer) is a large (a factor of 26) enhancement in the specific NO2 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 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 ( =3/2) over the lower ( =1/2) level of the spin-orbit-split NO(2 ) ground state, whereas the spin-orbit population of the O(3PJ) fragment is statistical (2J+1) within experimental error. The O(3PJ) yield derives from dissociation of nitro-bound NO2 (N end down); no O(3PJ) yield can be associated with side-bonded NO2.