SoftX‐Ray Fluorescence Spectroscopy

Abstract Soft X‐ray fluorescence (SXF) spectroscopy is X‐ray fluorescence (XRF) spectroscopy for low‐ and middle‐atomic‐number elements whose X‐ray absorption edges are in the soft X‐ray (SX) region. Electron beams have been used as excitation probes for ( nonresonant or normal ) SXF spectroscopy in laboratories. In addition, synchrotron radiation (SR) beams have been utilized as excitation probes, enabling selective excitation near the X‐ray absorption threshold. Selectively excited SXF involves soft X‐ray scattering, which can be regarded as a resonant soft X‐ray emission (SXE) spectroscopy. SXF and SXE spectroscopies provide element‐, orbital‐, and symmetry‐specific information. Thus, they are powerful tools for chemical analysis and materials characterization. In this article, the principles of SXF/SXE spectroscopies and instrumentation focused on gratings are described. Examples of nonresonant ( normal ) SXF and resonant SXE spectroscopies are shown, and details of the spectral profiles are explained. Resonant SXE spectroscopy of liquid water and operando observations of the electrode reactions are also demonstrated as advanced chemical analyses.