Dissertation

Photoreactions of molybdenum hexacarbonyl and nitric oxide on solid surfaces

So, Shu Kong

1991

Overview

This thesis describes the interaction of low energy photons (1-5 eV) with adsorbed molecules on surfaces of single crystals under ultrahigh vacuum conditions. This subject of study is important in understanding the fundamental mechanisms of photon induced reactions on surfaces. Low power (1-4000 mW), cw radiation was used to induce desorption and/or dissociation of adsorbed molecules. The surface before and after photon irradiation was characterized by thermal desorption spectroscopy and high resolution electron energy loss spectroscopy, and electronic electron energy loss spectroscopy. Molybdenum hexacarbonyl (Mo(CO) $\\sb6$ ) interacts weakly with Ag(111) and the basal plane of graphite. The electronic structure of the molecule is found to differ little from that of the gas phase. Photodissociation of Mo(CO) $\\sb6$occurs under low power UV irradiation. In contrast to the gas phase, the photodissociation is incomplete due to the presence of the substrate. The photodissociation spectra of different coverages of Mo(CO) $\\sb6$are shown to resemble that of the gas phase. The mechanism of photodissociation is identified to be the direct photoelectronic excitation of the adsorbed molecule. In the case of Ag(111), enhanced photodissociation appears near 325 nm due to enhanced surface electric field associated with the$d$ -band to the Fermi level transition. In contrast, both photodesorption and photodissociation occur for NO adsorbed on GaAs(110) under visible photon irradiation. By considering the photon power and wavelength dependences, the photoreaction mechanism is attributed to a substrate mediated mechanism involving photogenerated hot carriers interacting with the adsorbed NO. On the noble metal surfaces (Ag(111) and Cu(111)), only photodesorption of NO occurs. A more weakly adsorbed NO species is selectively photodesorbed. A photoreaction mechanism involving photogenerated hot carriers can be used to explain the photodesorption for photon energies$<$ 3 eV. For higher photon energies, additional reaction channels, which can be related to excitation of the NO adsorbate-surface complex, become available for photoreactions.

• Publisher: ProQuest Dissertations & Theses
• Subject: Chemistry / Condensation / Condensed matter physics / Optics / Physical chemistry
• ISBN: 9798496521055