Photofragment ion imaging in vibrational predissociation of the H2O+Ar complex ion

Vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied by photofragment ion imaging. The anisotropy parameters, beta, of the angular distributions of the photofragment ions were clearly dependent on the type (branch) of rotational excitation, beta > 0 for the P-branch excitations, while beta < 0 for the Q-branch excitations, which were consistent with the previous theoretical predictions for the rotationally resolved optical transition of a prolate symmetric top. The translational energy distributions had a similar form, irrespective of the excitation modes. This result suggests that the prepared excited states underwent a common relaxation pathway via the bending or bending overtone state of the H2O+ unit. In addition, the available energy was preferentially distributed into the rotational energy of the H2O+ fragment ions rather than the translational energy. The mechanism of the rotational excitations of the H2O+ fragment ions was discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.

Automated summary

The vibrational predissociation processes of the H2O+Ar complex ion following mid-infrared excitations of the OH stretching modes and bending overtone of the H2O+ unit were studied using photofragment ion imaging. The anisotropy parameters of the angular distributions of the photofragment ions were dependent on the type of rotational excitation, and the translational energy distributions were similar regardless of the excitation modes, suggesting a common relaxation pathway for the prepared excited states. Additionally, the rotational energy of the H2O+ fragment ions, rather than the translational energy, was preferentially distributed, with the rotational excitations discussed based on the steric configuration of the H2O+ and Ar units at the moment of dissociation.