High Resolution Photoelectron Spectroscopy in Hard X-ray and low hn below 11.7 eV
Graduate School of Engineering Science, Osaka University,
1-3 Machikaneyama, Toyonaka, Osaka, Japan
It is nowadays well known that bulk sensitive photoelectron spectroscopy (PES) is necessary to probe the bulk electronic structures of solids, which have much different surface electronic structures. Strongly correlated electron systems are such examples.
In order to overcome the surface sensitivity of the conventional PES with the photoelectron kinetic energy EK in the range of 15-200 eV, soft X-ray PES was extensively employed for EK in the range of 500-1000 eV, providing the clues to genuine bulk electronic structures. It is found, however, that the bulk components are still ~50-70% of the whole spectral weight in the valence band.
Then the necessity of the hard X-ray PES (HAXPES) in the hn region above 6 keV is recognized. Although the photon flux are higher in the X-ray than in the soft X-ray because of the use of the Bragg diffraction crystal monochromators, the photoionization cross sections are orders of magnitude smaller in HAXPES, though the bulk spectral weight can be >95%. In addition, the recoil effects are recognized on photoelectron emission from the electronic structures derived from light elements in some materials. The usefulness of HAXPES and the limit by recoil effects are discussed.
In the extremely low energy PES (ELEPES) by use of Xe, Kr, and Ar microwave excited electron cyclotron resonance lamp, high total-resolution (<5meV) is easily achieved. This system is briefly explained and studies of strongly correlated materials are discussed and the comment is given on the bulk sensitivity of this technique.
The materials covered in this talk range from Al, Cu to compounds containing Li, V, O, Si as well as alloys containing Sm, Yb.