Hans Kleinpoppen

In Memory of Hans Kleinpoppen

On February 12, 2016 at the age of 87, Prof. Dr. Hans Kleinpoppen passed away after a long life full of physics. Born on September 30, 1928 he was one of those who, after 1945, experienced the reinstatement of physical science in Germany. Growing up in Duisburg he began his physics studies at the University of Gießen. He received his Diploma in 1955 and published, under the influence of his co-author and mentor Wilhelm Hanle, his first paper in 1958. Working for his Ph.D. he joined Hans Kopfermann in Heidelberg and, later on, Hubert Krüger at the University of Tübingen where he received his Ph.D. in 1961 and his Habilitation in 1967. There, he made measurements of the Lamb shift and fine structure of hydrogen and started investigations of electron-atom collisions. His measurements of the threshold polarization of atomic fluorescence radiation after electron impact excitation provided the first experimental test of the Percival-Seaton theory and revealed the influence of fine and hyperfine structure on the line polarization.

After his Habilitation, he spent several periods in the USA, in JILA, Boulder, Colorado, and later in Columbia University, New York. These interactions resulted in scattering experiments with electrons on polarized sodium and in early concepts of electron-photon coincidence experiments with polarized particles. The ideas of “complete experiment” were born to enable more detailed information of the scattering amplitudes and their relative phases as fundamental physical quantities.

In 1968 he accepted a position at the University of Stirling which, encouraged by Sir Harrie Massey, aimed to establish a centre for atomic physics. In 1972, the “Columbia” coincidence experiment provided the first results of angle and spin correlations of the singlet-P state of helium which for a long time lead the interpretation and analysis of scattering processes. In Stirling with Alan Duncan, the details of the two-photon decay of the metastable 2S-state of atomic hydrogen emerged. This experiment tested predictions related to the Einstein-Podolsky Rosen paradox as well as to the violation of Bell’s inequality.

His research also extended to the investigation of electron impact excitation of the inner shells of free atoms, in contrast to the usual solid state experiments. These interests resulted in long lasting collaboration with Hans Lutz and Rainer Hippler at the University of Bielefeld.

His inquisitive approach lead to investigations of photoionization processes in collaboration with the NSL in Daresbury and with a long standing and most fruitful collaboration with Uwe Becker at the Fritz-Haber Institute where he stayed as permanent Research Senior Guest from 1991. On becoming Emeritus in 1996 he moved residence to Berlin and continued with his aims towards the complete experiment. He applied circular dichroism in angle resolved photoelectron spectroscopy on polarized atoms for a complete analysis of a photoionization process.

Hans Kleinpoppen always showed great interest in the theoretical background of the physical processes investigated as reflected by his collaborations with Phil Burke (Belfast), John Briggs (Freiburg), Karl Blum and Bernd Lohmann.

He was a member and Fellow of several physical and scientific societies. He was appreciative of being appointed a Fellow of the Royal Society of Edinburgh in 1987. Particular delight occurred with his membership in the illustrious 50 member world-wide Scottish Malt-Whisky Society of Edinburgh. He was organizer and chairman of many international symposia and physics conferences, where the education of succeeding generations of physicists was one of his goals. He is remembered for initiating many summer schools in atomic physics.

During more than 60 years of scientific research Hans Kleinpoppen published more than 250 scientific papers, and was author and editor of many books in the field of atomic physics. Particularly, we note the new formulation of atomic physics in the “Bergmann-Schaefer” as well as the series “Physics of Atoms and Molecules”. A Laudatio of his scientific research is given in the Complete Scattering Experiments. Proc H Kleinpoppen Symposium. Klewer Press, London. 1-47 (2000). JF Williams and R Hippler.

Hans Kleinpoppen was a brilliant and dedicated researcher, who always showed a talent of asking and testing complicated scientific questions often in the context of simple physical systems. Together with his strong interest in theoretical questions, he was frequently able to win and inspire younger and older scientists with his research.

Besides his scientific interests he was always open to discuss the problems and obstacles of life with his co-workers and colleagues. His broad interests in arts and music maintained a well-balanced life.

We mourn together with his life companion Helga von Kosing for the passing of a wonderful atomic physicist, a dear colleague and friend. The physics community will remember him with honour and with his encouragement to seek the complete experiment.