Infrared spectroscopy of H3+ and Hn+ ions in helium nanodroplets

Abstract

This work reports the infrared laser spectroscopic study of the H3+ and Hn+ (n = 5, 6, 7, 9) ions in helium droplets. The ions were produced upon electron impact ionization of helium droplets and charge transfer to small hydrogen clusters formed within the He droplets. The spectrum of H3+ shows a well-resolved rotational structure, indicating a small (∼8%) decrease in the rotational constants as compared to the gas-phase values. This small change is rationalized in terms of the small anisotropy of the H3+ interaction potential with He atoms. However, it was found that the interaction with the helium environment has a profound effect on the spectrum of the fluxional H5+ ions. Whereas the previously measured gas-phase spectrum has well-separated bands, the spectrum of H5+ in helium droplets shows a broad spectrum extending over 2000 cm−1 containing some maxima with frequencies close to those of the gas phase. The bands of H7+ and H9+ due to vibrations of their H2 moieties were also observed near 4000 cm−1. The structure of these ions could be treated as an H3+ core solvated in H2 molecules and He atoms. The spectrum of larger Hn+ clusters is also reported.