Jan Peter Toennies

Toennies was born in Philadelphia, Pennsylvania on 3 May 1930 to German immigrant parents.^[1] He is the grandson of sociologist Ferdinand Tönnies. He graduated from Lower Merion High School, outside of Philadelphia, in 1948. He went on to Amherst College, where he received a B.A. in 1952, and to Brown University, where he received a Ph.D. in chemistry in 1957. During graduate school he was a Fulbright student in Göttingen 1953–1954.^[1]

After graduation in 1957 he moved to the Physics Department of the University of Bonn where he was a postdoctoral researcher with Wolfgang Paul. In 1965 he obtained the Habilitation in experimental physics, becoming an assistant professor,^[1] as well as a guest professor in the Department of Physical Chemistry at the Gothenburg University.^[2] In 1969 he became director at the Max Planck Institute for Fluid Dynamics. From 1971 he was a professor in Göttingen and honorary professor at the University of Bonn. He retired officially in 1998, but was acting director until 2004.^[1]

Toennies measured total and inelastic collision cross sections for transitions between rotational states of various gases with quantum-state resolution.^[3] He investigated vibrational excitation of H₂ in central collisions with Lithium ions using the time-of-flight method, as well as dissociation in single collision events.^[4] In Göttingen, his group solved the Boltzmann equation while accounting for quantum effects and realistic interaction potentials in helium free-jet expansion, and proposed an improved model for van der Waals potential,^[5] referred to as Tang-Toennies model.^[6] High-resolution measurements of surface phonon dispersion for Ag,^[7] LiF, NaF, KCl,^[8] and Pt-crystals^[9] were carried out using inelastic scattering of helium atoms. The group achieved non-destructive detection of fragile He, H₂ and D₂ clusters by utilizing diffraction from nanoscopic transmission gratings.^[10] A spectroscopic study of SF₆ doped in helium nanodroplets revealed sharp spectral features of the embedded molecule. This indicated that the molecule was extremely cold and that it resides in its ground state at a temperature of 0.37 K, practically unaffected by the helium environment and could rotate freely as if in a vacuum.^[11] Subsequent spectroscopic experiments demonstrated that the free rotations were related to the microscopic superfluidity of helium droplets,^[12] and, for the first time, of a small numbers of hydrogen molecules.^[13]

• E. F. Greene and J. Peter Toennies: Chemische Reaktionen in Stoßwellen, Dr. Dietrich Steinkopff Verlag, Darmstadt, 1959
• E. F. Greene and J. Peter Toennies: Chemical Reactions in Shock Waves, Edward Arnold (Publishers) Ltd. London, 1964
• G. Benedek and J. Peter Toennies: Atomic Scale Dynamics at Surfaces: Theory and Experimental Studies with Helium Atom Scattering, Springer, Heidelberg, 2018
• A. Slenczka and J. Peter Toennies: Molecules in Superfluid Helium Nanodroplets: Spectroscopy, Structure, and Dynamics, Springer Cham, 2022