Electron impact ionization of water-doped superfluid helium nanodroplets: Observation of He(H2O)n+ clusters

Abstract

Electron impact mass spectra have been recorded for helium nanodroplets containing water clusters. In addition to identification of both H + ͑H 2 O͒ n and ͑H 2 O͒ n + ions in the gas phase, additional peaks are observed which are assigned to He͑H 2 O͒ n + clusters for up to n = 27. No clusters are detected with more than one helium atom attached. The interpretation of these findings is that quenching of ͑H 2 O͒ n + by the surrounding helium can cool the cluster to the point where not only is fragmentation to H + ͑H 2 O͒ m ͑where m ഛ n −1͒ avoided, but also, in some cases, a helium atom can remain attached to the cluster ion as it escapes into the gas phase. Ab initio calculations suggest that the first step after ionization is the rapid formation of distinct H 3 O + and OH units within the ͑H 2 O͒ n + cluster. To explain the formation and survival of He͑H 2 O͒ n + clusters through to detection, the H 3 O + is assumed to be located at the surface of the cluster with a dangling O-H bond to which a single helium atom can attach via a charge-induced dipole interaction. This study suggests that, like H + ͑H 2 O͒ n ions, the preferential location for the positive charge in large ͑H 2 O͒ n + clusters is on the surface rather than as a solvated ion in the interior of the cluster.

Ion–Molecule Reactions in Helium Nanodroplets Doped with C60 and Water Clusters

“…[8][9][10] We first summarize results obtained by ionization of helium droplets doped with water (either H 2 O or D 2 O) but no C 60 . In agreement with a previous report, [11] electron impact ionization results in a prominent series of protonated water cluster ions. Unprotonated water cluster ions are observed with a 10 % abundance relative to the protonated cluster ions.…”

Ion–Molecule Reactions in Helium Nanodroplets Doped with C60 and Water Clusters

“…We first summarize results obtained by ionization of helium droplets doped with water (either H 2 O or D 2 O) but no C 60 . In agreement with a previous report,11 electron impact ionization results in a prominent series of protonated water cluster ions. Unprotonated water cluster ions are observed with a 10 % abundance relative to the protonated cluster ions.…”

Intermolecular proton-transfer in acetic acid clusters induced by vacuum-ultraviolet photoionization

“…This is because they are unstable species, which can be generated only with limited ionization methods such as vacuum-ultraviolet ͑VUV͒ one-photon ionization, 7-10 multiphoton ionization with a femtosecond laser, 12 and discharge of the protic molecules in He droplets. 26 Recently, several new vibrational spectroscopic techniques based on the VUV one-photon ionization detection have been developed and they enable us to apply IR spectroscopy to unstable cations such as the unprotonated cluster cations of the protic molecules. [27][28][29][30][31] We observed the infrared ͑IR͒ spectra of the unprotonated cluster cations of ammonia and methanol as the first examples of those of protic molecules.…”