Title: Dopant ionization and efficiency of ion and electron ejection from helium nanodroplets

Authors: J. D. Asmussen, L. Ben Ltaief, K. Sishodia, A. R. Abid, B. Bastian, S. R. Krishnan, H. B. Pedersen, M. Mudrich

DOI: https://doi.org/10.1063/5.0160171

Abstract: Photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets rely on the ability to efficiently detect ions and/or electrons. Using a commercial quadrupole mass spectrometer and a photoelectron–photoion coincidence spectrometer, we systematically measure yields of ions and electrons created in pure and doped He nanodroplets in a wide size range and in two ionization regimes—direct ionization and secondary ionization after resonant photoexcitation of the droplets. For two different types of dopants (oxygen molecules, O_2, and lithium atoms, Li), we infer the optimal droplet size to maximize the yield of ejected ions. When dopants are ionized by charge-transfer to photoionized He nanodroplets, the highest yield of O_2 and Li ions is detected for a mean size of ∼5 × 10^4 He atoms per nanodroplet. When dopants are Penning ionized via photoexcitation of the He droplets, the highest yield of O_2 and Li ions is detected for ∼10^3 and ∼10^5 He atoms per droplet, respectively. At optimum droplet sizes, the detection efficiency of dopant ions in proportion to the number of primary photoabsorption events is up to 20% for charge-transfer ionization of O_2 and 2% for Li, whereas for Penning ionization it is 1% for O_2 and 4% for Li. Our results are instrumental in determining optimal conditions for mass spectrometric studies and photoionization spectroscopy of molecules and complexes isolated in He nanodroplets.