Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H3O+-Ar and H+(H2O)2-Ar

Infrared photodissociation of protonated water clusters with an Ar atom, namely H3O+-Ar and H+(H2O)(2)-Ar, was investigated by an imaging technique for mass-selected ions, to reveal the intra- and intermolecular vibrational dynamics. The presented system has the advantage of achieving fragment ion images with the cluster size- and mode-selective photoexcitation of each OH stretching vibration. Translational energy distributions of photofragments were obtained from the images upon the excitation of the bound (nu(b)) and free (nu(f)) OH stretching vibrations. The energy fractions in the translational motion were compared between nu(I)(b) and nu(I)(f) in H3O+-Ar or between nu(II)(b) and nu(II)(f) in H+(H2O)(2)-Ar, where the labels "I" and "II" represent H3O+-Ar and H+(H2O)(2)-Ar, respectively. In H3O+-Ar, the nu(I)(f) excitation exhibited a smaller translational energy than nu(I)(b). This result can be explained by the higher vibrational energy of nu(I)(f), which enabled it to produce bending (nu(4)) excited H3O+ fragments that should be favored according to the energy-gap model. In contrast to H3O+-Ar, the nu(II)(b) excitation of an Ar-tagged H2O subunit and the nu(II)(f) excitation of an untagged H2O subunit resulted in very similar translational energy distributions in H+(H2O)(2)-Ar. The similar energy fractions independent of the excited H2O subunits suggested that the nu(II)(b) and nu(II)(f) excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H2O-H+-H2O moiety. However, the translational energy distributions for H+(H2O)(2)-Ar did not agree with a statistical dissociation model, which implied another aspect of the process, that is, Ar dissociation via incomplete energy randomization in the whole H+(H2O)(2)-Ar cluster.