Kai Siegbahn
Guardian staff Frank Close
<b>Obituary:</b> Nobel prizewinning physicist whose work focused on electron spectroscopy.
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the Guardian
Kai Siegbahn, the Swedish physicist and Nobel laureate, has died of a heart attack at his summer cabin at Angelholm, southern Sweden. He was 89. The chances of winning a Nobel prize are slim enough, but on six occasions the winner's parent has also received this singular honour. Kai Siegbahn won one in 1981, his father, Karl Manne George Siegbahn, having done so in 1924. On receipt of his award, Kai said: "It's a decided advantage if you start discussing physics every day at breakfast."

Clearly, Kai had learned well, as both he and his father won their prizes for work involving spectroscopy - the detection of particles and measurement of their energies. Siegbahn Sr's award had been for studies of X-ray spectroscopy; Kai's was for the development of a technique to test the composition and purity of materials by means of electron spectroscopy. Before his work, electron spectroscopy had been little more than a laboratory curiosity. He changed it into a powerful means of analysis, which has today become a common industrial process.

The story begins in 1905 with Albert Einstein and his explanation of the photoelectric effect. When a substance is irradiated with ultraviolet light or X-rays, it may be made to emit electrons. Einstein showed that UV light or X-rays consist of individual photons, which hit the electrons in the material and eject them. The energy of one of these electrons is equal to the energy of the incident photon, minus the energy with which the electron was originally bound within the atoms of the sample.

Scientists quickly realised that if they used monochromatic light - for which all photons have the same energy - it might be possible by studying the emitted electrons to get valuable information on the electron structure of the sample being examined. The problem was that when an electron is ejected from an atom deep within a material, it gets buffeted about while escaping, and the resulting random energy losses obscure the signal. If the electrons had escaped unimpeded, the graph that the scientist obtained would consist of a series of sharp peaks and dips, whose pattern, like a fingerprint, could be translated by an expert to give the critical information.

The buffeting, however, obscured the peaks, turning them into smeared, minor hills on an undulating landscape. Consequently, the technique was a curiosity but of little practical use. The decisive step came from work that Kai Siegbahn was doing in the 1940s. At that time he had been studying electrons emitted in radioactive decays of nuclei (beta-decay). In doing so, he designed a powerful spectrometer involving a mushroom-shaped magnet that focused the beams of electrons in not one but two directions, which improved the sensitivity 10-fold over previous instruments. Having been using this device to study electrons emitted by atomic nuclei, in the 1950s he developed it so as to be sensitive to electrons emitted by the photoelectric effect.

The results were remarkable. In addition to finding the broad hillocks that had been previously observed in such experiments, Siegbahn's sensitive device showed also a number of sharp peaks that had never been seen before. These turned out to be from electrons that had managed to escape from the material without losing energy en route. These were the ones that mattered. For the first time, electron spectroscopy had become a practical means of determining what the energies of the electrons were when originally trapped in the material.

As the energies of electrons within atoms are quantised with characteristic values for different atomic elements, the information that Siegbahn was able to extract revealed the identity of the atoms in the material. Upon closer study, he found that there were subtle shifts in the energies, which turned out to depend on the molecule or the crystal to which the atom was bound. Thus, he had come up with a practically useful analytical method for studying not only which atoms are included in a sample, but also in which chemical environment these atoms exist.

This has spawned many important fields of application, for instance in the study of surface chemistry processes such as catalysis and corrosion, wear rates on prostheses and contaminants on electrical circuits. Commercial electron spectrometers based on Siegbahn's innovation have been available for many years, and electron spectroscopy is now a standard technique at hundreds of laboratories throughout the world.

Siegbahn was born in Lund, Sweden, and received his bachelor's and master's degrees from the University of Uppsala and his doctorate from the University of Stockholm. He was research associate at the Nobel Institute for Physics from 1942 to 1951, professor of physics at the Royal Institute of Technology in Stockholm from 1951 to 1954 and professor and head of the physics department at the University of Uppsala from 1954. He officially retired in 1984 but continued working in his Uppsala laboratory for the rest of his life.

The winner of many prestigious awards, he was also for many years a member of the Royal Swedish Academy of Sciences, which chooses the Nobel laureates. It is reported that in 1981, his colleagues asked him not to take part in the deliberations. If this was a strong clue to him that he was under consideration, he never let on. "I had no advance indication that I would be receiving it. The winner always comes as a surprise."

Siegbahn is survived by his wife, Anna Brita, and their three sons, two of whom, Per and Hans, are physicists.