High-resolution photoelectron spectroscopy (PES) measures the kinetic energy of electrons ejected when X-rays or ultraviolet light strike a sample, thereby determining binding energy. Siegbahn’s cylindrical mirror analyzer improved energy resolution more than tenfold and greatly increased signal-to-noise. By recording subtle chemical shifts, scientists can precisely identify oxidation states and bonding environments. The method has become essential in catalyst research, semiconductor process control, and corrosion studies. Because the probe depth is only a few nanometers, it is especially powerful for surface science. Later, synchrotron radiation further enhanced brightness and resolution. Siegbahn’s work established a standard analytical instrument used across industry and academia.

Siegbahn redesigned spectrometers by introducing double-focusing hemispherical and later cylindrical mirror analyzers, achieving sub-100 meV energy resolution at electron kinetic energies of several keV. This made it possible to separate core-level binding energies differing by less than 0.1 eV, revealing fine structure and multiplet splitting in transition-metal and rare-earth compounds. He systematically catalogued photoelectron lines across the periodic table, establishing reference databases still used today. The technique, now known as X-ray photoelectron spectroscopy (XPS or ESCA), directly measures elemental composition and oxidation states within the top 5–10 nm of a surface. Integration with ultra-high-vacuum chambers, ion etching sources, and cryogenic stages widened its applicability to thin films, catalysts, and biological specimens. When combined with angle-resolved detection, PES provides band-structure information critical for condensed-matter physics. Modern variants such as HAXPES and time-resolved PES inherit the optical layout Siegbahn pioneered. These capabilities have transformed research on energy materials, nanotechnology, and quantum devices.

Siegbahn’s high-resolution ESCA employed coaxial cylindrical mirror analyzers satisfying the 2π focusing condition, yielding ΔE/E better than 1/2000 while maintaining high transmission. Detecting chemical shifts as small as 50 meV enabled quantitative surface stoichiometry and oxidation-state analysis. His standardized Al Kα and Mg Kα cross-section tables underpin modern quantitative XPS modelling. Early demonstrations of variable-photon-energy PES anticipated HAXPES, allowing buried interface studies. Electron-optical concepts such as retarding potentials and concentric hemispherical analyzers now appear in ARPES and spin-resolved spectrometers. Integration with in-situ reaction cells pioneered operando catalyst analysis, and current sub-picosecond pump-probe PES experiments still follow Siegbahn’s instrumental heritage. These innovations have markedly enhanced analytical capabilities in surface chemistry, quantum materials, and device engineering.