Velocity map imaging of the state-specific vibrational predissociation of water-containing hydrogen-bonded complexes

The state-to-state vibrational predissociation (VP) dynamics of several water- containing hydrogen bonded dimers were studied following excitation of a vibrational mode of each dimer. Velocity-map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product translational energy distributions. Following vibrational excitation of the dimer, fragments were detected by 2 + 1 REMPI. ❧ Following vibrational excitation of the bound OH stretch fundamental, ammonia fragments from the NH₃-H₂O dimer were detected by 2 + 1 REMPI via the B¹E' ← X¹A₁' transition. The REMPI spectra show that NH₃ is produced with one and two quanta of thesymmetric bend (v₂ umbrella mode) excitation, as well as in the ground vibrational state. Each band is quite congested, indicating population in a large number of rotational states. The fragments’ center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with zero, one, or two quanta in v₂ and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well by using a dimer bond dissociation energy of Dₒ = 1538 ± 10 cm⁻¹. The rotational state distributions in the water cofragment pair-correlated with specific rovibrational states of ammonia are broad and include all the JKaKc states allowed by energy conservation. The rotational populations increase with decreasing c.m. translational energy. There is no significant excitation of ammonia products with one quantum of asymmetric bend (v₄) or water products with bend (v₂) vibration. The results show that only restricted pathways lead to predissociation, and these do not always give rise to the smallest possible translational energy release, as favored by momentum gap models. ❧ Following vibrational excitation of the HCl stretch of the HCl-H₂O dimer, HCl fragments were detected by 2 + 1 REMPI via the f³∆₂(v' = 0) ← X¹∑⁺(v"" = 0) and V¹∑⁺(v' = 11 and 12) ← X¹∑⁺(v"" = 0) transitions. REMPI spectra clearly show fragment HCl 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, but the data show that the water rotational populations increase with decreasing translational energy. ❧ H₂O fragments of this dimer were detected by 2 + 1 REMPI via the C¹B₁(000) ← X¹A₁(000) transition. REMPI spectra clearly show that H₂O is 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 H2O and were converted to rotational state distributions of the HCl cofragment. The distributions gave Dₒ = 1334 ± 10 cm⁻¹ and showed 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. ❧ Dₒ of the water dimer is determined by using state-to-state VP measurements following excitation of the bound OH stretch fundamental of the donor unit of the dimer. H₂O fragments are detected in the ground vibrational (000) and the first excited bending (010) states by 2 + 1 REMPI via the C¹B₁(000) ← X¹A₁(000 and 010) transitions. The fragments’ velocity and center-of-mass translational energy distributions are determined from images of selected rovibrational levels of H₂O. An accurate value for Dₒ is obtained by fitting both the structure in the images and the maximum velocity of the fragments. This value, Dₒ = 1105 ± 10 cm⁻¹ (13.2 ± 0.12 kJ/mol), is in excellent agreement with the recent theoretical value of Dₒ = 1103 ± 4 cm⁻¹ (13.2 ± 0.05 kJ/mol) suggested as a benchmark by Shank et al. [J. Chem. Phys. 130, 144314 (2009)].