Predissociation dynamics of the HCl-(H₂O)₃ tetramer: An experimental and theoretical investigation

The cyclic HCl-(H₂O)₃ tetramer is the largest observed neutral HCl-(H₂O)_n cluster. The vibrational predissociation of HCl-(H₂O)₃ is investigated by theory, quasiclassical trajectory (QCT) calculations, and experiment, following the infrared excitation of the hydrogen-bonded OH-stretch fundamental. The energetically possible dissociation pathways are HCl + (H₂O)₃ (Pathway 1) and H₂O + HCl-(H₂O)₂ (Pathway 2). The HCl and H₂O monomer fragments are observed by 2 + 1 resonance enhanced multiphoton ionization combined with time-of-flight mass spectrometry, and their rotational energy distributions are inferred and compared to the theoretical results. Velocity map images of the monomer fragments in selected rotational levels are used for each pathway to obtain pair-correlated speed distributions. The fragment speed distributions obtained by experiment and QCT calculations are broad and structureless, encompassing the entire range of allowed speeds for each pathway. Bond dissociation energies, D₀, are estimated experimentally from the endpoints of the speed distributions: 2100 ± 300 cm^-1 and 2400 ± 100 cm^-1 for Pathway 1 and Pathway 2, respectively. These values are lower but in the same order as the corresponding calculated D₀: 2426 ± 23 cm^-1 and 2826 ± 19 cm^-1. The differences are attributed to contributions from vibrational hot bands of the clusters that appear in the high-speed tails of the experimental pair-correlated distributions. Satisfactory agreement between theory and experiment is achieved when comparing the monomer fragments' rotational energies, the shapes of the speed distributions, and the average fragment speeds and center-of-mass translational energies. Insights into the dissociation mechanism and lifetime are gained from QCT calculations performed on a previously reported many-body potential energy surface. It is concluded that the dissociation lifetime is on the order of 10 ps and that the final trimer products are in their lowest energy cyclic forms.