Hard-X-ray photoelectron spectroscopy of atoms and molecules

Abstract

We review here the current status of atomic and molecular HAXPES, both experimentally and theoretically. After the era of the ESCA technique, atomic and molecular HAXPES experienced several decades when the emphasis in gas-phase research was in the soft-X-ray range, mainly due to the lack of suitable experimental conditions. In the last few years, at some of the newer synchrotron radiation sources, hard-X-ray beamlines have been developed which provide state-of-the-art instrumentation for the atomic and molecular community. A substantial impulse to the field has also been recently given by the new X-ray free-electron lasers (FELs). Therefore, the high-brightness and high-resolution sources needed for a thriving gas-phase HAXPES community are now available. We concentrate here on experimental results based on the use of SR and, more recently, FELs as ionizing sources. Available parallel theoretical developments are also included. We define the hard-X-ray photon-energy range as 1 keV or higher. We provide a summary of early theoretical work in atomic and molecular HAXPES, and then an overview on HAXPES performed at first- and second-generation SR sources and at current-generation facilities. Scientific topics of particular interest in the hard-X-ray range include nondipole effects, recoil due to the photoelectron’s momentum, new interference phenomena, ultrafast nuclear motion on the femto- and sub-femtosecond scale, and double-core-hole studies. We conclude with a look at future directions in these areas and a few others of potential interest for HAXPES.