Siegbahn, who was born at Lund in Sweden, was the professor of physics at the Royal Institute of Technology, Stockholm, from 1951 to 1954. He taught at the University of Uppsala from 1954 to 1984. Here he worked on the emission of electrons from substances irradiated with x-rays. Siegbahn's technique was to subject a specimen to a narrow beam of x-rays with a single wavelength (i.e., energy) and measure the energy spectrum of the ejected electrons by magnetic or electrostatic deflection. The spectrum shows characteristic peaks formed by electrons ejected from different inner energy levels of atoms. Moreover, the positions of these peaks depend to a slight extent on the way in which the atom is linked to other atoms in the molecule. These ‘chemical shifts’ allow the technique to be used as an analytical tool – Siegbahn has named it ESCA (electron spectroscopy for chemical analysis). He has also worked on the related technique of ultraviolet photoelectron spectroscopy developed by David Turner.

Siegbahn is the son of Karl Manne Siegbahn, who won the Nobel Prize for physics in 1924. Kai Siegbahn was awarded the Nobel Prize in 1981.