Fragment imaging in the infrared photodissociation of the Ar-tagged protonated water clusters H 3 O + -Ar and H + (H 2 O) 2 -Ar.

Infrared photodissociation of protonated water clusters with an Ar atom, namely H 3 O + -Ar and H + (H 2 O) 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 (ν b ) and free (ν f ) OH stretching vibrations. The energy fractions in the translational motion were compared between ν b I and ν f I in H 3 O + -Ar or between ν b II and ν f II in H + (H 2 O) 2 -Ar, where the labels "I" and "II" represent H 3 O + -Ar and H + (H 2 O) 2 -Ar, respectively. In H 3 O + -Ar, the ν f I excitation exhibited a smaller translational energy than ν b I . This result can be explained by the higher vibrational energy of ν f I , which enabled it to produce bending (ν 4 ) excited H 3 O + fragments that should be favored according to the energy-gap model. In contrast to H 3 O + -Ar, the ν b II excitation of an Ar-tagged H 2 O subunit and the ν f II excitation of an untagged H 2 O subunit resulted in very similar translational energy distributions in H + (H 2 O) 2 -Ar. The similar energy fractions independent of the excited H 2 O subunits suggested that the ν b II and ν f II excited states relaxed into a common intermediate state, in which the vibrational energy was delocalized within the H 2 O-H + -H 2 O moiety. However, the translational energy distributions for H + (H 2 O) 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 + (H 2 O) 2 -Ar cluster.