Communication: Electron impact ionization of binary H2O∕X clusters in helium nanodroplets: an ab initio perspective.

In a recent experiment (H2O)n/Xm binary clusters (where X = Ar, N2, CO, CO2, and several other molecules) were formed in superfluid helium nanodroplets and investigated by electron impact mass spectrometry [Liu et al., Phys. Chem. Chem. Phys. 13, 13920 (2011)10.1039/c1cp20653b]. The addition of dopant X was found to affect the branching ratio between H3O+(H2O)n and (H2O)+n+2 formation. Specifically, the addition of CO increased the proportion of protonated water cluster ions, whereas dopants such as Ar, N2, and CO2, had the opposite effect. In this work ab initio calculations have been performed on [X(H2O)2]+ ions, where X = Ar, N2, CO, and CO2, to try and explain this distinct behavior. CO is found to be unique in that it forms a HOCO-H3O+ unit in the most stable cationic complexes where the binding between HO and CO is stronger than that between H3O+ and OH. Thus, on purely energetic grounds, loss of HOCO rather than CO should be the preferred fragmentation process. No comparable chemistry occurs when X = Ar, N2, or CO2 and so the co-dopant requires less energy to depart than OH. The calculations therefore account for the experimental observations and provide evidence that HOCO formation is induced in helium droplets containing (H2O)n clusters and co-doped with CO when subject to electron impact ionization.