NO ELECTRONIC DESORPTION PROCESSES FROM STEP SITES ON PT(112) - A COMPARISON BETWEEN PHOTO-STIMULATED AND ELECTRON-STIMULATED DESORPTION

Using Fourier-transform infrared reflectance absorption spectroscopy a s a surface analytical method, the total photo- and electron-stimulate d desorption cross sections for specific chemisorbed NO species on the steps of Pt(112) have been measured. These species-specific cross sec tions for photo-and electron stimulated desorption are compared with e ach other. An unexpected result was obtained for photo-stimulated deso rption, using photons with an energy of 3.5 eV; bridged-NO desorbs wit h a higher cross section (1.1X10(-22) cm(2)) than terminal-NO (6.1X10( -23) cm(2)). For electron stimulated desorption, using electrons of an energy of 275 eV, the opposite is observed. Terminal-NO desorbs with a higher cross section (2.3X10(-18) cm(2)) than bridged-NO, with a cro ss section of 8.7X10(-19) cm(2). The photo-stimulated process is discu ssed 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 propagatio n of the short-lived intermediate NO-(a) toward the metal surface. Thi s leads to a closer approach to the surface of NO-(a) produced from br idged-NO than from terminal-NO, giving a higher probability of photode sorption of bridged-NO compared to terminal-NO. For the bridged-NO spe cies, this is due to a steeper slope of the NO-(a) potential energy su rface at the Franck-Condon excitation point compared to the slope for the terminal-NO which is initially located further from the surface. E lectron 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 inter action with the surface.