Vibrational predissociation of the phenol-water dimer: a view from the water.

The vibrational predissociation (VP) dynamics of the phenol-water (PhOH-H2O) dimer were studied by detecting H2O fragments and using velocity map imaging (VMI) to infer the internal energy distributions of PhOH cofragments, pair-correlated with selected rotational levels of the H2O fragments. Following infrared (IR) laser excitation of the hydrogen-bonded OH stretch fundamental of PhOH (Pathway 1) or the asymmetric OH stretch localized on H2O (Pathway 2), dissociation to H2O + PhOH was observed. H2O fragments were monitored state-selectively by using 2+1 Resonance-Enhanced Multiphoton Ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). VMI of H2O in selected rotational levels was used to derive center-of-mass (c.m.) translational energy (ET) distributions. The pair-correlated internal energy distributions of the PhOH cofragments derived via Pathway 1 were well described by a statistical prior distribution. On the other hand, the corresponding distributions obtained via Pathway 2 show a propensity to populate higher-energy rovibrational levels of PhOH than expected from a statistical distribution and agree better with an energy-gap model. The REMPI spectra of the H2O fragments from both pathways could be fit by Boltzmann plots truncated at the maximum allowed energy, with a higher temperature for Pathway 2 than that for Pathway 1. We conclude that the VP dynamics depends on the OH stretch level initially excited.

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.

Vibrational Predissociation of the HCl-(H2O)3 Tetramer.

The vibrational predissociation of the HCl-(H2O)3 tetramer, the largest HCl-(H2O)n cluster for which HCl is not predicted to be ionized, is reported. This work focuses on the predissociation pathway giving rise to H2O + HCl-(H2O)2 following IR laser excitation of the H-bonded OH stretch fundamental. H2O fragments are monitored state selectively by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) combined with time-of-flight mass spectrometry (TOF-MS). Velocity map images of H2O in selected rotational levels are used to determine translational energy distributions from which the internal energy distributions in the pair-correlated cofragments are derived. From the maximum translational energy release, the bond dissociation energy, D0 = 2400 ± 100 cm-1, is determined for the investigated channel. The energy distributions in the fragments are broad, encompassing the entire range of allowed states. The importance of cooperative (nonpairwise) interactions is discussed.