Kai Siegbahn – Nobel Laureate and pioneer in electron spectroscopy
- Kai Siegbahn (1918–2007) was awarded the Nobel Prize in Physics in 1981 for his contributions to the development of high-resolution electron spectroscopy.
- He studied physics, mathematics and chemistry at Uppsala University and later became a professor of physics at the university.
- His research began in the field of nuclear physics but went on to lead to an entirely new method for investigating the chemical properties of atoms and molecules.
- Together with his colleagues, he developed electron spectroscopy for chemical analysis (ESCA), which is now an established method in fields such as materials science and chemistry.
- Kai Siegbahn’s research lives on at Uppsala University, notably through a laboratory for X-ray photoelectron spectroscopy and a lecture theatre named after him.
From student in Uppsala to professor
Kai Siegbahn was born in Lund in 1918, the son of the physicist Manne Siegbahn, who himself was awarded the Nobel Prize in Physics in 1924. After completing his upper secondary education in Uppsala, Kai Siegbahn began studying physics, mathematics and chemistry at Uppsala University in 1936. He continued his studies and research in Stockholm and was awarded his doctorate in 1944 with a thesis on beta spectroscopy. Between 1942 and 1951, he worked at the Nobel Institute for Physics and, in 1951, became a professor at the Royal Institute of Technology in Stockholm. Three years later, he returned to Uppsala as a professor of physics.
It was a rapidly developing field of research. Nuclear physics was at the centre of much of the international research in physics, and Siegbahn developed instruments capable of measuring the energy of electrons with ever-greater precision. His work encompassed, amongst other things, beta and gamma spectroscopy, electron optics and various forms of nuclear physics measurements.
The quest for the perfect spectrometer
A key question for Kai Siegbahn was how to measure the energies of electrons with both high resolution and sufficiently high sensitivity. This required instruments that were considerably more advanced than those available at the time.
Together with his colleagues, he developed new electron spectrometers of very high precision. As early as 1955, Siegbahn, Carl Nordling and Evelyn Sokolowski built a spectrometer with what was, at the time, an astonishing resolution. The instrument was the result of extensive work involving, amongst other things, magnetic fields, detectors and vacuum technology.
The new technology made it possible to study electrons released when a substance is struck by X-rays. By measuring the energy of the electrons, the researchers were able to draw conclusions about the electronic structure of the atoms. The method also made it possible to discover something that would prove decisive: the energy levels of the electrons are influenced by the chemical environment in which the atoms are found.
Fixing salt revealed a new chemical phenomenon
One of the most important steps towards Siegbahn’s Nobel Prize was taken one Sunday evening in the autumn of 1963. Physicists Stig Hagström and Carl Nordling were carrying out measurements but had run out of the chemical compounds they were using in their experiments. The chemistry storeroom was locked, so Hagström went to the darkroom instead and fetched some sodium thiosulphate – ordinary photographic fixing salt.
When they ran the fixing salt through their high-resolution spectrometer, they obtained an unexpected result: instead of a single peak in the spectrum, two appeared. The explanation was that electrons from atoms of the same element had different binding energies depending on the chemical environment in which the atoms were found.
The result proved significant. The phenomenon they had discovered is known as a chemical shift. This meant that electron spectroscopy could be used to analyse chemical bonding – not just to determine the energy levels of atoms.
The discovery gave Kai Siegbahn’s research a new direction. The method was developed into Electron Spectroscopy for Chemical Analysis (ESCA) and became a powerful means of investigating the chemical environment of atoms. In the late 1960s, Siegbahn and his colleagues summarised the results in two comprehensive ESCA books, which proved highly significant for the continued development of electron spectroscopy.
The 1981 Nobel Prize
The work carried out in Uppsala quickly attracted international attention. Electron spectroscopy could be used to study atoms, molecules and solid materials, and provided researchers with a new way of investigating what happens at the atomic level. During the 1970s, its application was further expanded, partly through the use of synchrotron light as a source for photoelectron spectroscopy.
In 1981, Kai Siegbahn was awarded half of the Nobel Prize in Physics ‘for his contribution to the development of high-resolution electron spectroscopy’. The other half was shared between Nicolaas Bloembergen and Arthur Schawlow for their work in the field of lasers and laser spectroscopy. At the time of the award ceremony, Siegbahn was a professor at Uppsala University.
The Nobel Prize thus also served as recognition of the research environment that had been established in Uppsala. The Department of Physics had become an international hub for electron spectroscopy, and researchers from many countries came to Uppsala to work with the instruments and methods developed there.
Kai Siegbahn and Uppsala University today
Kai Siegbahn retired as a professor in 1984, but remained active at Uppsala University. He continued to work at the Ångström Laboratory during his final years and passed away in 2007.
His scientific legacy is very tangible. At Uppsala University, X-ray photoelectron spectroscopy (XPS) and its higher-energy variant, HAXPES, are still used to investigate surfaces and interface layers in materials. These methods are used, amongst other things, to understand materials for energy conversion and energy storage. At the current Kai Siegbahn Laboratory, for example, research is conducted into battery materials, corrosion, and the surfaces and interface layers of materials.
The name Siegbahn also lives on at the Ångström Laboratory through the Siegbahn Hall. This large lecture theatre was inaugurated under this name as a tribute to Manne and Kai Siegbahn and is used for teaching, conferences and scientific lectures.