The vibrational spectra of protonated water clusters around the magic number at n=21 have generated a great deal of interest with respect to the nature of the proton defect and the curiously missing bands predicted for the surface bound hydronium ion. Here we report temperature dependent predissociation spectra of the H2 tagged and free clusters prepared using a cryogenic ion trap. Most importantly, the H2-tagged spectra display strong bands in the range between 1800 and 2400 wavenumbers as indicated in the figure below, addition to a band near 1200 wavenumbers. These features are consistent with aharmonic predictions for the H3O+ umbrella and OH stretching vibrations, thus signalling the location of the long-sought charge carrier in these species. Assignments of the features in the closely related H9O4+ 'Eigen' ion are verified using IR-IR double resonance. Finally, we report temperature-dependent spectra of the clusters formed with a single HDO molecule in a field of D+(D2O)n clusters for n=5 and 9, and discuss the results in context of the onset of spectral diffusion of the absorptions associated with particular sites in the water network.

Cryogenic ion vibrational spectroscopy of the H2 tagged �magic� H+(H2O)21 spectrum over the range 800 � 4000 cm-1: The long-awaited spectral signature of the proton defect?

248th National Meeting (2014)

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