 About 1956 Kai Siegbahn developed X-ray photoelectron spectroscopy (XPS) for surface chemical analysis.This method uses x-ray sources to study energy levels of atomic core electrons, and at the time had an energy resolution of about 1 eV (electronvolt).The ultraviolet method (UPS) was pioneered by Feodor I. Vilesov, a physicist at St. Petersburg (Leningrad) State University in Russia (USSR) in 1961 to study the photoelectron spectra of free molecules in the gas phase.