ESCAPE DEPTH OF PHOTOELECTRONS

The sampling depth of electron spectroscopies, AES and XPS, is frequently directly associated with the value of the inelastic mean free path (IMFP). Owing to elastic scattering of signal electrons, a separate parameter, the escape depth (ED), should be used to describe the analysed volume. There is no simple relation between the ED and the IMFP. In the present work, extensive Monte Carlo calculations were performed to calculate the ED for photoelectrons emitted from gold (Au 4s, Au 4p(3/2), Au 4d(5/2) and Au 4f(7/2)). All XPS configurations within a plane normal to the surface were considered. It has been found that the anisotropy of photoelectron emission is strongly reflected in the angular dependences of the ED. Furthermore, the value of ED divided by the cosine of the detection angle, which is equivalent to the attenuation length (AL) described by one of the definitions, may considerably exceed the IMFP, in contrast with common expectations. This occurs at large detection angles and/or in experimental configurations with the direction of x-rays close to the direction towards the analyser. A simple and accurate parameterization was proposed in the present work, taking into account the anisotropy of photoemission, from which the ED can be predicted for photoelectrons considered and for any XPS configuration.