Imaging H₂O Photofragments in the Predissociation of the HCl−H₂O Hydrogen-Bonded Dimer

Abstract

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded HCl-H(2)O dimer was studied following excitation of the dimer's HCl stretch by detecting the H(2)O fragment. Velocity map imaging (VMI) 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, H(2)O fragments were detected by 2 + 1 REMPI via the C (1)B(1) (000) ← X (1)A(1) (000) transition. REMPI spectra clearly show H(2)O from dissociation produced in the ground vibrational state. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H(2)O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D(0) = 1334 ± 10 cm(-1) and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 + 1 REMPI detection of water fragments is discussed.

Communication: A new ab initio potential energy surface for HCl–H2O, diffusion Monte Carlo calculations of D and a delocalized zero-point wavefunction

“…It will be necessary to include higher-order interactions from multiple water molecules interacting with HCl. Preliminary investigation of the HCl-(H 2 O) 2 cluster indicates that 85% of the interaction energy is accounted for by the two-body interaction and the remaining 15% is from the three-body interaction. Previously generated potentials for (H 2 O) n, 53 can be used in conjunction with any future HCl-(H 2 O) n potential for complete studies of cluster prior to ionization.…”

“…The D 0 of the fully deuterated isotopologue is calculated to be 1476 ± 3 cm −1 , which we hope will stand as a benchmark for future experimental work. [1][2][3][4][5][6] and theoretical [7][8][9][10][11] studies. The high interest in these complexes is due to their impact on atmospheric chemistry, specifically their role in ozone depletion.…”

Communication: Determination of the bond dissociation energy (D) of the water dimer, (H2O)2, by velocity map imaging

“…[41][42][43][44][45][46] In this method, isolated rovibrational levels of a fragment are monitored following IR vibrational predissociation (VP) of the dimer, and photofragment images are recorded. From the structured velocity and center-of-mass (c.m.)…”

“…Fitting the structural features and maxima in the velocity distributions in several such images gave unique and accurate values of D 0 for the dimers with an accuracy of ±10 cm −1 . Our work on the H 2 O-HCl dimer 41,42 is most relevant to the present study, being a first case in which the water fragment was monitored directly by resonance-enhanced multiphoton ionization (REMPI). 42 From images obtained by monitoring selected rotational levels of either HCl or H 2 O fragments D 0 = 1334 ± 10 cm −1 was obtained.…”

“…The experimental procedures were similar to those used in our previous studies. [41][42][43][44][45][46] Dimers were formed in a pulsed supersonic molecular beam by expanding a mixture of ∼2% H 2 O in He at a stagnation pressure of ∼1.5 atm through the 0.5-mm orifice of a pulsed valve. The rotational temperature of the dimer was estimated to be 10 ± 5 K. 41,43 Focused IR laser radiation [∼2-12 mJ/pulse, lens focal length (f.l.)…”