Photoionization of helium nanodroplets doped with rare gas atoms

Photoionization of He droplets doped with rare gas atoms (Rg =Ne, Ar, Kr, and Xe) was studied by time-of-flight mass spectrometry, utilizing synchrotron radiation from the Advanced Light Source from 10to30eV. High resolution mass spectra were obtained at selected photon energies, and photoion yield curves were measured for several ion masses (or ranges of ion masses) over a wide range of photon energies. Only indirect ionization of the dopant rare gas atoms was observed, either by excitation or charge transfer from the surrounding He atoms. Significant dopant ionization from excitation transfer was seen at 21.6eV, the maximum of He 2pP1 absorption band for He droplets, and from charge transfer above 23eV, the threshold for ionization of pure He droplets. No Ne⁺ or Ar⁺ signal from droplet photoionization was observed, but peaks from He_nNe⁺ and He_nAr⁺ were seen that clearly originated from droplets. For droplets doped with Rg =Kr or Xe, both Rg⁺ and He_nRg⁺ ions were observed. For all rare gases, Rg₂⁺ and He_nRg_m⁺ (n,m⩾1) were produced by droplet photoionization. Mechanisms of dopant ionization and subsequent dynamics are discussed.