NO electronic desorption processes from step sites on Pt(112): A comparison between photo- and electron-stimulated desorption

Abstract

Using Fourier-transform infrared reflectance absorption spectroscopy as a surface analytical method, the total photo- and electron-stimulated desorption cross sections for specific chemisorbed NO species on the steps of Pt(112) have been measured. These species-specific cross sections for photo- and electron stimulated desorption are compared with each other. An unexpected result was obtained for photo-stimulated desorption, using photons with an energy of 3.5 eV; bridged-NO desorbs with a higher cross section (1.1×10−22 cm2) than terminal-NO (6.1×10−23 cm2). For electron stimulated desorption, using electrons of an energy of 275 eV, the opposite is observed. Terminal-NO desorbs with a higher cross section (2.3×10−18 cm2) than bridged-NO, with a cross section of 8.7×10−19 cm2. The photo-stimulated process is discussed within the ‘‘hot electron’’ model, and it is postulated that the higher expected quenching rate of the NO−(a) species produced from the more strongly-bound bridged-NO is more than compensated by propagation of the short-lived intermediate NO−(a) toward the metal surface. This leads to a closer approach to the surface of NO−(a) produced from bridged-NO than from terminal-NO, giving a higher probability of photodesorption of bridged-NO compared to terminal-NO. For the bridged-NO species, this is due to a steeper slope of the NO−(a) potential energy surface at the Franck–Condon excitation point compared to the slope for the terminal-NO which is initially located further from the surface. Electron stimulated desorption yields the expected relationship between the total desorption cross section for bridged- and terminal-NO, with the more strongly-bound bridged-NO having the smaller cross section. Here it is postulated that bridged- and terminal-NO connect to highly-excited NO states which do not differ so much in their dynamical interaction with the surface.