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Helium nanodroplets ("HNDs") are widely used for forming tailor-made clusters and molecular complexes in a cold, transparent, and weakly interacting matrix. The characterization of embedded species by mass spectrometry is often complicated by the fragmentation and trapping of ions in the HNDs. Here, we systematically study fragment ion mass spectra of HND-aggregated water and oxygen clusters following their ionization by charge transfer ionization ("CTI") and Penning ionization ("PEI"). While the efficiency of PEI of embedded clusters is lower than for CTI by about factor 10, both the mean sizes of detected water clusters and the relative yields of unprotonated cluster ions are significantly larger, making PEI a "soft ionization" scheme. However, the tendency of ions to remain bound to HNDs leads to a reduced detection efficiency for large HNDs containing > 1 0 4 helium atoms. These results are instrumental in determining optimal conditions for mass spectrometry and photoionization spectroscopy of molecular complexes and clusters aggregated in HNDs. [ABSTRACT FROM AUTHOR]

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, O2, 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 O2 and Li ions is detected for a mean size of ∼5×104 He atoms per nanodroplet. When dopants are Penning ionized via photoexcitation of the He droplets, the highest yield of O2 and Li ions is detected for ∼103 and ∼105 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 O2 and 2% for Li, whereas for Penning ionization it is 1% for O2 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., (© 2023 Author(s). Published under an exclusive license by AIP Publishing.)