Infrared Spectroscopy of (NH 3 –H 2 O) + Radical Cations in Helium Nanodroplets

Radical cation clusters such as (NH 3 –H 2 O) + remain elusive because in typical molecular beam experiments they rapidly react with molecules and neutral clusters to produce protonated clusters. Ionization of neutral NH 3 –H 2 O clusters in helium droplets enables isolation of the primary ionization products, a large fraction of which is found to be (NH 3 –H 2 O) + cations. These cations were studied via laser infrared spectroscopy and their structures were determined based on comparison to results of previous quantum chemical calculations. The formation of the cations via the electron impact ionization of the corresponding neutral dimers yielded two distinct isomers, containing either an NH 3 + or NH 4 + ion core. We also found that the lowest energy isomer, H 2 NH + –OH 2, could be selectively produced upon the reaction of preformed NH 3 + embedded in helium droplets with H 2 O molecules added to the droplets downstream. This work shows that helium droplets can serve as a vehicle to study ion–molecule reactions between two well-defined reactants.