Low Energy Excitations and Desorption Dynamics from Oxide Surfaces

Abstract

The interaction of photons with solid surfaces initiates processes which may be classified into various categories [1,2]. We will be concerned mainly with photochemical processes, including desorption of participating molecules. The measurement of the distribution of energy into translational and internal degrees of freedom possibly provides us with new insights into the mechanisms underlying the desorption after photoabsorption. In order to study the simplest cases first, various groups have studied photodesorption of NO and CO from metal and metaloxide surfaces [3–17]. A whole range of photon energies has been used so far. It appears that if we exclude photoinduced thermal desorption, the cross sections for photodesorption are orders of magnitude larger on weakly oxidized metal surfaces and in particular on oxide surfaces than on metal surfaces. Qualitatively, several effects are responsible for this difference in our view.

• i) The electronic structure of the substrates is considerably different in the sense that a metal does not exhibit a band gap while an oxide often does. Energy that is dissipated into the substrate must exceed the gap energy in the case of an oxide unless there are defect states filling the gap. For a metal energy in any small quantity may be dissipated into the solid because excitation of electron-hole pairs of low energy is always possible. The probability of such excitations depends of course on the density of states at the Fermi energy. Metals with low density of states, such as Cu, Ag, Au, etc. should have a smaller probability for electron-hole-pair-creation.

• ii) The degree of localization of the electronic charge distribution is typical for an oxide, while delocalization is a prototype metal property. This leads in the case of an oxide to a longer lifetime of the excited state. Consequently, the probability to escape the surface is larger for photodesorption from oxide surfaces due to the possibility to accumulate translational energy to leave the surface [13–15].

• iii) A weak molecule-surface interaction will favour localization of the excitation on the molecule and thus increase the photodesorption probability.