Experimental and Theoretical Investigations of Energy Transfer and Hydrogen-Bond Breaking in the Water Dimer

The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (<i>D</i>₀) of (D₂O)₂. The value obtained, 1244 ± 10 cm^–1 (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm^–1 (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H₂O)₂. In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H₂O)₂ and (D₂O)₂ were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment