Photon-stimulated desorption and x-ray standing waves

Introduction X-ray standing waves (XSW) using electrons and fluorescence photons as secondary signals has become a standard technique to determine adsorbate structures. However, when a sample is irradiated by intense light, desorption of ions is also a frequently observed phenomenon which is known as photon-stimulated desorption (PSD). But, this signal is of a fundamentally different physical nature. The identification of this process has been a persistent topic in surface science throughout the last decades [1]. By virtue of the inherent periodic time structure of synchrotron radiation, an efficient detection of positive ions emitted from the surface is feasible using time-of-flight spectroscopy. In this review, we will survey the relevant desorption mechanisms which initiate the prevailing processes. We will show that the combination of x-ray standing waves with x-ray PSD (XPSD) is a unique tool to identify the underlying desorption processes in a site specific manner. In general, two distinct types of stimulated desorption processes were identified in the past, i. e. direct and indirect desorption processes. This classification is based on the nature of the initial excitation which sequentially leads to ion emission. For direct processes, the desorption-active photoabsorption is located at the atom which is finally ejected. For indirect processes, the primary photons are absorbed either at the bonding partner of the desorbing ion, which results in so-called nearest-neighbor PSD, or in the underlying bulk crystal, leading to the creation of secondary electrons which cause the ion desorption by valence excitation (x-ray induced electronstimulated desorption (XESD)). Therefore, three primary desorption processes can be distinguished. While the details of the desorption mechanisms are in general highly specific to the system under investigation, it has to be emphasized that the dominating desorption processes can be identified by using synchrotron radiation. This is feasible through applying a combination of XSW and XPSD (XSW-PSD), since all three types can be related to distinct Fourier components of the atom distribution function. In the course of developing this method we have applied it to several systems, e. g. H/Pd(111), K/Ge(001), CsCl/Si(111) [2] as well as Cl/Si(001) and Cl/Si(113) [3]. In the following, we will deal with the systems Si(111)-(7x7), H/Si(111)-(1x1), Ge/Si(111)-(1x1):H [2], and Cl/Si(111) [4]. This choice serves to illustrate the peculiarities of the different kinds of desorption processes and to demonstrate the general approach to the interpretation of XSW-XPSD data.