Emission depth distribution function of Al 2s photoelectrons in Al2O3

The escape probability of Al 2s photoelectrons leaving an aluminum oxide sa mple (Al2O3) has been studied as a function of depth of origin. It has been found that the escape probability (the so-called emission depth distributi on function - DDF) depends strongly on the photoelectron emission direction with respect to that of the incident X-ray beam. In particular, in the emi ssion direction close to that of photon propagation, the DDF differs substa ntially from the simple Beer-Lambert law and exhibits a nonmonotonic behavi or with a maximum in the near-surface region at a depth of about 10 Angstro m. Experimental results are in good agreement with theoretical predictions based on Monte Carlo simulations of the electron transport and with analyti cal solution of the linearized Boltzmann kinetic equation with appropriate boundary conditions. Both theoretical approaches take into account multiple elastic scattering of photoelectrons on their way out of the sample. It is shown that the commonly used straight line approximation (SLA), which negl ects elastic scattering effects, fails to describe adequately experimental data at emission directions close to minima of the differential photoelectr ic cross section.