Quantum resolved studies of electron-stimulated reactions on adsorbate covered Pt(111) surfaces

Abstract

Using laser resonance-enhanced ionisation spectroscopy, we have studied electron (6–350 eV) stimulated dissociation of NO 2 coadsorbed with up to 0.75 monolayer of atomic O on Pt(111). Several dramatic effects on NO 2 dissociation occur due to the presence of O. There is a large ( × 26) enhancement in the specific dissociation yield, a narrowing of the NO translational energy distributions, and a distinct propensity ( > 4:1 at low J) for populating the upper Ω = 3 2 NO spin-orbit level over the Ω = 1 2 level. The spin-orbit state distribution of the O( 3 P J) dissociation fragment is (5.0): (2.5): (1.0) for J = 2, 1 and 0, which is within experimental error of the statistical ( T → ∞) 2 J + 1 limit. The enhanced yield probably results from an increased excited state lifetime due to a reduction in substrate charge-transfer screening. We have also detected O( 3 P J = 2,1,0) and NO X 2 Π 3 2 , 1 2 (v = 5) above an electron (6–350 eV) beam irradiated Pt(111) surface containing coadsorbed O 2 and NO at 90 K. We conclude that both O( 3 P J) and NO(v = 5) are laser-induced photodissociation fragments of NO 2 desorbates. The NO 2 is probably the reaction product of a collision between an O atom (created by electron-stimulated dissociation of adsorbed O 2) and an NO(a). We correlate the 10 eV NO 2 production threshold with the dissociative ionization of the 3σ g molecular bonding orbital of O 2(a).