ABSTRACT
For several decades, helium nanodroplets have been the ugly duckling of cluster science. As early as 1960, intense cluster beams of helium, hydrogen, and other “permanent” gases were synthesized by a group at the Karlsruhe Nuclear Research Center, motivated by attempts to produce intense beams of hydrogen clusters at high kinetic energies in order to fuel thermonuclear devices (Becker et al., 1961). In the 1980s, however, when research on isolated, atomic, or molecular clusters was booming, few experimentalists continued research in the area, which was considered by many as esoteric, poorly funded, and, from a technical standpoint, awkward because of the need for bulky vacuum pumps and large vacuum systems. Few kept working on helium, intrigued by the possible occurrence of super uidity in nite systems.
