Infrared Spectroscopy of Water and Zundel Cations in Helium Nanodroplets

Here, we show that electron impact ionization of helium (He) droplets doped with water molecules and clusters yield water and Zundel cations embedded in the droplets consisting of a few thousand helium atoms. Infrared spectra in the OH-stretching range were obtained using the release of the cations from the droplets upon laser excitation. The spectra in He droplets appear to have about a factor of 10 narrower bands and similar matrix shifts as compared to those obtained via tagging with He and Ar atoms. The results confirm the calculated structure of the free Zundel ion, where the proton is equidistant from the two water units. The effect of the He environment on the spectra of ions is discussed. The signal shows nonlinear laser pulse energy dependence consistent with the evaporation of the entire droplet upon multiple absorptions of infrared photons. This conclusion is supported by the model calculations of the efficiency of the cations' release vs laser flux.