Inner-shell electron spectroscopy using hard x rays

Photoelectron and Auger electron spectroscopies excited by tunable synchrotron radiation are sensitive to electronic structure, photoionization dynamics, and core-hole decay mechanisms. These topics attract the interest of theorists who seek to explain and model the observed phenomena. The intense, polarized, tunable, narrow bandwidth x rays produced by beamlines at the Advanced Photon Source (APS) are ideal for Hard X-ray Photoelectron Spectroscopy (HAXPES). We are developing a HAXPES instrument for experiments at the APS using a high-resolution, high-collection-efficiency electron analyzer. The first experiments will explore two topics. The first is to characterize inner-shell resonance and threshold effects by tuning the x-ray energy across \textit{K}-edges of atoms and small molecules. Our second goal is to characterize double-core-hole states in molecules in which hollow core shells are produced by single-photon absorption and generated by electron correlation. The electron spectra will record states with one core-ionized electron and one core-excited electron [1]. [1] D. Koulentianos \textit{et al.}, J. Chem. Phys. \bf{149} 134313 (2018).