He specialized in theoretical atomic spectroscopy, advancing the understanding of complex atomic structures (open p-, d- and f-shells) and highly-excited states. More specifically, he dealt with many-body calculations of atomic structure, inner-shell transitions and Auger decay processes in complex atoms and ions. He left a lasting impact on how physicists understand multiconfiguration Hartree-Fock methods and electron interactions.  
Since 1979, he was the frontman of the atomic spectroscopy group at the Zeeman Laboratory of the University of Amsterdam: a gentle, towering man with a beard and bare feet in sandals. In many respects, he was a key figure in the web of atomic physics. He put us in touch with many visiting scientists like Brian Judd, Charlotte Froese-Fischer, Bob Cowan, Michel Godefroid, Emil Biémont, Jean-François Wyart, Alan Hibbert and many others.
Throughout his tenure at the UvA's Zeeman Laboratory, Jørgen Hansen conducted extensive research on the quantum mechanical properties of atoms. Scientifically he was very productive, resulting in numerous peer-reviewed publications in renowned international journals. Jørgen contributed to scientific advisory boards of the Institute of Physics as well.
Yet when he presented a speech, he had also an emotional open eye for the situation in the world.
In 1990, the position of the atomic physics group in the faculty of science changed: all experimental components of our research group, notably the spectrographs, were moved out. Jørgen fighted strongly to preserve these, although it would not touch his own science much.
In the years that followed,  Jørgen and colleagues collaborated with Brian Judd to develop and apply orthogonal operators in atomic spectroscopy. This provided a more robust and physically meaningful mathematical framework for analyzing complex atomic energy levels and calculating oscillator strengths.
Jørgen got along well with both hardcore theoretical atomic physicists and those who performed semi-empirical calculations or conducted the experiments.
In 1999 the faculty decided to stop atomic physics spectroscopy all together.  Jørgen’s group was dispersed across various university institutes. In fact, this resulted in early retirement. Jørgen moved to Sweden in 2002 where he lived with his wife Ulla until 2026.

Ton Raassen and Peter Uylings
Former staff members Zeeman laboratory/WZI/Institute of Physics

Robert Spreeuw
Current staff member IoP