Imaging the State-Specific Vibrational Predissociation of the Hydrogen Chloride-Water

The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H2O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f 3 ∆2 r X 1 Σ + and V 1 Σ + r X 1 Σ + transitions. REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J′′ up to 11. The fragments’ center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D0 ) 1334 ( 10 cm -1 . The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy. 1. Introduction A fundamental understanding of hydrogen bonds, their nature, strength, and dynamics, remains an important goal of physical chemistry research. Experimental results on binding energies, energy transfer pathways, and predissocation in polyatomic hydrogen-bonded dimers are needed for testing the accuracy of potential energy surfaces (PES) and extending our understanding of hydrogen bonding to larger systems. Previous studies of several hydrogen-bonded complexes have revealed nonstatistical predissociation behavior due to the disparity between the frequencies of the intramolecular and intermolecular vibrational modes. 1 The interaction of HCl with H2O (gaseous, liquid, and solid) has attracted attention for over a century as a model for the dissociation of strong acids in aqueous solutions. HCl-H2 Oi s of particular interest because of the role interactions of HCl with ice in aerosols and in polar stratospheric clouds play in