Ambient pressure XPS at MAX IV.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has emerged as an important technique for investigating surface and interface chemistry under realistic conditions, overcoming the limitations of conventional XPS restricted to ultrahigh vacuum. This review highlights the capabilities and scientific impact of APXPS at the MAX IV Laboratory, the world's first fourth-generation synchrotron light source. With the APXPS beamlines SPECIES and HIPPIE, MAX IV offers state-of-the-art instrumentation for in situ and operando studies across a broad pressure range, enabling research in catalysis, corrosion, energy storage, and thin film growth. The high brilliance and small beam size of MAX IV's synchrotron light are essential for pushing the time-resolution boundaries of APXPS, especially in the soft X-ray regime. We discuss representative studies at MAX IV, including investigations of single-atom catalysts, confined catalysis, time-resolved catalysis, atomic layer deposition, and electrochemical interfaces, showcasing the role of APXPS in advancing material and surface science.