Figure 1: A cartoon demonstrates how XPS works. For normal XPS, incident X-ray shines on a sample with a fixed angle, a (respect to the surface normal). The traveling distance of X-ray in a thin film is about 10 nm. In order to collect information only from super-thin film, angle resolved XPS is used by changing incident X-ray angle from a to a+q.
Figure 2: Angle resolved XPS spectrums taken with different angle, q (0 to 60 degree), on the same sample. Two energy shifts (∆E1 & ∆E2) can be clearly observed by comparing two spectrums taken at q=0 degree and q=60 degree.
Figure 3: By using peak differentiation method on the angle resolved XPS spectrums (q=0 and 50 degree), individual orbital can be obtained. It can be clearly shown the results taken at q=0 degree and q=50 degree spectrums are quite different. This strongly suggests that when q<50 degree, spectrums would contain information not only from super-thin film but structures underneath. Precise chemical composition in this case can be determined when q>=50 degree.