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Jean-Paul Desclaux

5 February 2021
(02 April 1939 - 28 August 2020)

The physicist was “one of the pioneers and leading experts on the relativistic theory of heavy and exotic atoms.”

Jean-Paul Desclaux, who was born on 2 April 1939, died in Grenoble, France, on 28 August 2020. He was one of the pioneers and leading experts on the relativistic theory of heavy and exotic atoms, and he was a fellow of the American Physical Society.

Jean-Paul Desclaux (1939-2020)

Jean-Paul attended Lycée Saint Louis in Paris and the École Nationale des Télécommunications. In 1965 he worked for a year as an engineer at an electrical company before being hired by the CEA (Commissariat à l’Énergie Nucléaire) to work in its Limeil center. He then went back to study physics and obtained his Thèse d’État (PhD and habilitation degree) in physics from the University of Paris in 1971. He then was sent to develop new projects at Northwestern University in Evanston, Illinois, and at Argonne National Laboratory, where he met Burkhard Fricke and Yong-Ki Kim, with whom he started fruitful collaborations and long-lasting friendships. In 1974 he moved for 18 months to the Institut Laue Langevin (ILL) in Grenoble to study relativistic effects in solids. He then spent the rest of his career at the Grenoble CEA center in the fundamental research branch. After his retirement at age 61, which was a rule in the CEA at the time, he continued working from home, with numerous collaborations, and continued developing his multiconfiguration Dirac–Fock code (MCDF) with Paul Indelicato until a few weeks before his death, leading to a last joint publication in March 2020.

The CEA in Limeil originally wanted him to work on state equations and opacities for plasmas, which meant understanding atoms. He then developed a passion for this subject, and within a few years, he rose to be one of the world-leading experts on the topic. Two early achievements were the “Desclaux tables” of atomic ground-state properties at the Dirac–Fock level for elements 1–120 in 1973 and the MCDF “Desclaux program,” first published in 1975. For a decade, that program dominated among users. Much of the early development was done in contact with Ian P. Grant and David Mayers of Oxford University, who were developing another program, based on Grant’s formulation of the basic theory. This possibility for cross-checking between two independent codes was invaluable for the credibility of both.

Other applications covered x-ray physics, lanthanide and actinide ions, and the predicted p1 ground state of the lawrencium atom, published with Fricke in 1980 and observed by T. K. Sato et al. in 2015. Another achievement was to show that the differences between silver and gold, or in an even broader sense, the differences between periods 5 and 6 on the Mendeleev table, arose largely from relativistic effects. That molecular work was done with Pekka Pyykkö using a one-center Dirac–Fock code. This was one of the earliest ways to introduce relativistic effects into quantum chemistry. He also worked on muonic atoms, first by giving his code the possibility of taking into account the full interaction between the electrons and the muon, going beyond contributing understanding the large hyperfine anomaly observed in palladium and rhodium. This possibility is still unique today.

Jean-Paul also developed strong collaborations with experimentalists both in Grenoble and in Paris, in particular with Jean-Pierre Briand on highly charged ions. He also studied hollow atoms in the context of the ion surface interaction.

Later versions of the atomic MCDF code appeared, co-developed with Indelicato, after 1984, enabling the calculation of many different operators (Auger, radiative transition probabilities, hyperfine structure, g-factors), with advanced evaluation of quantum-electrodynamics contributions. The code was extended to antiprotonic atoms and to spin 0 particles like the pion and the kaon.

Although research would remain his main focus and passion, Jean-Paul would not shy away from taking responsibilities at the CEA. In 1984 he became deputy director of the physics laboratory in Grenoble. He then directed the Atomic Physics Laboratory from 1989 until he became deputy director of the Département de recherche fondamentale (now Institut de nanosciences et cryogénie) in 1992. In 1994 he became deputy director of the Direction des Sciences de la Matière, which was covering all physics research of CEA.

His scientific achievements led him to become an APS fellow in 1985, “In recognition of his pioneering work on relativistic effects in atoms, molecules, and solids.” He was awarded the Servant Prize of the French Academy of Science in 1993 and the Palmes Académiques by the French Ministry of Research. In 2004 he received the Légion d’Honneur, the highest French government medal, for both his scientific and administrative career.

Jean-Paul was a modest and quiet person, with a hidden sense of humor, who lived and died surrounded by his family, which did not prevent him from having numerous and long-lasting domestic and foreign collaborations, involving worldwide travel. Typical for him were the cases when he insisted on alternative forms of acknowledgment, even when he would easily have qualified for coauthorship.

The authors are longtime coworkers of Jean-Paul Desclaux.

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