Universal molecule injector in liquid helium: Pulsed cryogenic doped helium droplet source

Abstract

Progress toward the construction of a universal molecule injector for doping bulk liquid helium is reported. A pulsed valve that operates at cryogenic temperatures, down to 4 K, is demonstrated within the confinement of a cryostat, operating in the vapor above a steady level of liquid He. The insulated valve can be operated at elevated temperatures with preseeded helium gas in supersonic expansion mode, as demonstrated through laser-induced fluorescence spectra of seeded NO2. At cryogenic operating temperatures, the expansion into vapor helium produces a well-collimated liquid helium droplet beam, which is then used to transfer to the liquid impurities produced by laser ablation from a cryogenic rotating target. The operation can be visualized using copper as the ablation target: the droplet beam is imaged via Rayleigh scattering, while the beam past the plasma is imaged by the fluorescence of the entrained Cu atoms. The beam drags along copper ions and electrons, the recombination of which controls the fluorescence yield downstream.