Introduction

Many surface techniques involve some damage, or destruction of the surface being investigated but, with the exception of the SIMS technique described in chapter 4, this is an incidental side-effect rather than a primary feature of the technique. In the case of SIMS, the destruction of the surface is by the rather brutal method of sputtering, and the analysis of the sputtered, charged fragments is carried out primarily with the object of determining the surface composition. In this chapter, we discuss two very different types of desorption spectroscopy, in which adsorbed species specifically, are desorbed from the surface in an attempt to learn about the nature of the adsorbate—substrate bonding. Information on surface composition (or more often surface coverage of an adsorbed species) may also be obtained, but this is usually incidental.

The two general methods of desorption are by thermal and by electronic stimulation. Any species adsorbed on a suface must be bound to the surface with some specific amount of energy and will desorb at a rate determined by a Boltzmann factor. Heating the surface will increase this desorption rate, and the desorbing species may be detected in the gas phase by conventional mass spectrometers. Evidently, a study of the temperature dependence of the desorption rate can lead to information on the binding energy states of the adsorbate (or, more strictly, on the desorption energies).