Electron Impact Ionization in Helium Nanodroplets:  Controlling Fragmentation by Active Cooling of Molecular Ions

Abstract

Reported here is a study of the effects of liquid helium cooling on the fragmentation of ions formed by electron impact mass ionization. The molecules of interest are picked up by the helium nanodroplets as they pass through a low pressure oven. Electron impact ionization of a helium atom in the droplet is followed by resonant charge transfer to neighboring helium atoms. When the charge is transferred to the target molecule, the difference in the ionization potentials between helium and the molecule results in the formation of a vibrationally hot ion. In isolation, the hot parent ion would undergo subsequent fragmentation. On the other hand, if the cooling due to the helium is fast enough, the parent ion will be actively cooled before fragmentation occurs. The target molecule used in the present study is triphenylmethanol (TPM), an important species in synthetic chemistry, used to sterically protect hydroxyl groups. Threshold PhotoElectron PhotoIon COincidence (TPEPICO) experiments are also reported for gas-phase TPM to help quantify the ion energetics resulting from the cooling effects of the helium droplets.

Model for the charge-transfer probability in helium nanodroplets following electron-impact ionization

“…Miller and co-workers claimed that this point was reached when the helium droplets achieved an average size of approximately 40,000 helium atoms, as evidenced by the very small dopant signal in the mass spectrum [7]. This concurs roughly with our own experimental findings, although in our laboratory we have managed to obtain mass spectra with droplets with an estimated mean size of up to 60,000 atoms [18].…”

Spectroscopy and dynamics in helium nanodroplets

“…The work refined the ion hopping probability, and demonstrated that there is a long range steering, such that the He cation ion preferentially approaches the negative parts of the molecule, leading to regio-selective ionization. Fragmentation after ionization was studied for HCN [219] and the large organic molecule, triphenyl-methanol [220].…”

Photoionization and Photofragmentation of SF6in Helium Nanodroplets

“…A similar value was estimated by Lewis et al based on another ion, triphenyl methanol. 25 Note, however, that the cooling rates calculated from the rate constant suffer from a large uncertainty because of the difficulties in estimating rate constants in these systems. Thus the "direct" method is much more robust.…”