A convenient measure of surface sensitivity in Auger-electron spectroscopy
(AES) and X-ray photoelectron spectroscopy (XPS) is the mean escape depth (
MED). If the effects of elastic-electron scattering are neglected, the MED
is equal to the electron inelastic mean foe path (IMFP) multiplied by the c
osine of the emission angle with respect to the surface normal, and depends
on the material and electron energy of interest. An overview is given hen
of recent calculations of IMFPs for 50-2000 eV electrons in a range of mate
rials. This work has led to the development of a predictive formula based o
n the Bethe equation for inelastic electron scattering in matter from which
IMFPs can be determined. Estimates show, however, that elastic-electron sc
attering can significantly modify the: MED. Thus, for AES, the MED will be
reduced by up to about 35%. For XPS, however, the MED can be changed by up
to +/- 30% for common measurement conditions although it can be much larger
(by up to a factor of 2) for near-grazing emission angles. Ratios of MED v
alues, calculated with elastic scattering considered and neglected for XPS
from the 3s, 3p, and 3d subshells of silver with Mg K alpha X-rays are appr
oximately constant (to about 10%) over a range of emission angles that vari
es from 40 degrees to 60 degrees depending on the subshell and the angle of
X-ray incidence. Recommendations are given on how to determine the optimum
range of emission angles for satisfactory analysis of angle-resolved XPS (
ARXPS) data. Definitions are included of three terms often used for describ
ing surface sensitivity (IMFP, MED, and effective attenuation length (EAL))
, and examples are given of the varying magnitudes of these quantities for
different analytical conditions. (C) 1999 Elsevier Science B.V. All rights
reserved.