Thermo- and photo-induced reactions of NO on Si(111)7×7. I. Adsorption and chemical reactions

Abstract

The adsorption of NO on Si(111)7×7 at 90 K and the subsequent reactions induced by thermal heating and photon irradiation have been investigated in detail, using high-resolution electron energy loss spectroscopy and mass spectrometry. It is found that, in addition to molecular and dissociative adsorption of NO, N2O is produced from NO on Si(111)7×7 at 90 K. The product N2O physisorbs on the surface and, at 110 K, partially desorbs and partially dissociates into N2 and O. Molecular adsorption of NO exists in two independent, presumable bridge and atop configurations. There is no observable conversion between the two forms of molecularly adsorbed NO. At 147 K, the bridge NO is thermally activated, which leads to simultaneous NO desorption and, to a much larger extent, N2O synthesis. Dissociation of bridge NO also occurs at about 147 K, at least at low NO exposures. The atop NO is not involved in the thermal reactions at 147 K, but dissociates between 200 and 300 K. Photon irradiation in the UV, visible, and IR induces desorption and dissociation of NO on Si(111)7×7 at 90 K via nonthermal mechanisms. Furthermore, evidence for photosynthesis of N2O on the surface is observed. It is suggested that the N2O synthesis during thermal heating and photon irradiation proceeds via reaction between an adsorbed NO molecule and a hot N atom produced from NO dissociation.