Figure 1. Grazing incidence geometry illustrating the incident, the transmitted and the reflected waves.

The energy flow through the layered structure generates ionization events due to X-ray photoabsorption. The escaping fast photoelectons (light blue) ionize the surrounding gas while the low-energy secondary photoelectrons (gray) do not participate in this ionization process. While the ionizing photoelectrons may escape only from a limited depth in the top layer the integral photoelectron yield is affected by the total energy flow in the multilayer and thus is sensitive to presence of buried layers.