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

Abstract

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 (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 H₃O⁺–Ar or between ν_b^II and ν_f^II in H⁺(H₂O)₂–Ar, where the labels “I” and “II” represent H₃O⁺–Ar and H⁺(H₂O)₂–Ar, respectively. In H₃O⁺–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 (ν₄) excited H₃O⁺ fragments that should be favored according to the energy-gap model. In contrast to H₃O⁺–Ar, the ν_b^II excitation of an Ar-tagged H₂O subunit and the ν_f^II 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 ν_b^II and ν_f^II 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.