Vibrational spectroscopy of small-sized hydrogen-bonded clusters and their ions

Vibrational spectroscopies of small-sized hydrogen-bonded clusters of organic acids and related molecules, as well as their ions, are reviewed based on our recent results. OH stretching vibrations of the jet-cooled clusters generated by supersonic expansions are observed by the various size-selected and population-labelling spectroscopic methods; ionization detected infrared (IR) and/or stimulated Raman spectroscopies for the neutral clusters in the electronical ground state (S₀) and fluorescence detected IR spectroscopy for the clusters in the electronically excited state (S₁). The hydrogen-bond structures of phenol-(H₂O)_nclusters are extensively investigated on the basis of the spectral analysis combined with ab initio calculations of their stable forms and vibrations. Remarkable enhancement of the hydrogen-bond strength upon electronic excitation is demonstrated for the IR spectra of the Sj clusters of phenol. For tropolone-(H₂O)_nand - (CH₃OH)_nclusters, (phenol)₃and fluorobenzene-(CH₃OH)_nclusters, cluster-size-dependent rearrangements and transformations of their hydrogen bonds are also investigated. IR dissociation spectroscopy of the cluster ions involving an ion-trapping technique is also described. The method is used to obtain vibrational spectra of (phenol)_n⁺[phenol-(H₂O)_n]⁺and (phenol-benzene)⁺; their characteristic spectra of the OH stretching vibrations indicate that extremely large changes of the intermolecular hydrogen bonds are induced upon ionization of the clusters. Finally, a novel method for vibrational spectroscopy of bare molecular ions, for which no dissociation spectroscopic techniqueis successful, is described and its application to the IR spectrum of the phenol cation is given. The method involves autoionization process following vibrational excitation of the high-Rydberg-state molecule whose core has essentially the same vibrational structure as that of the bare ion. Future applications and directions of vibrational spectroscopy of clusters are discussed.