Harvard–Smithsonian Center for Astrophysics
at the Norwegian University of Science and Technology in Trondheim) and at the Royal Institute of Technology (KTH), in Stockholm, Sweden. In 1975 he settled in Amsterdam at the FOM Institute for Atomic and Molecular Physics (AMOLF), where he became the theoretical physics group leader. Since 1992 he has worked as a Senior Scientist at the Institute for Theoretical Atomic, Molecular and Optical Physics (ITAMP) based at the Harvard–Smithsonian Center for Astrophysics, in Cambridge, Massachusetts.