Imaging the State-Specific Vibrational Predissociation of the Ammonia−Water Hydrogen-Bonded Dimer

Abstract

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-water dimer were studied following excitation of the bound OH stretch. Velocity-map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the bound OH stretch fundamental, ammonia fragments 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 the symmetric bend (nu2 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 nu2 and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well by using a dimer dissociation energy of D0 = 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 J(KaKc) states allowed by energy conservation. The rotational populations increase with decreasing c.m. translational energy. There is no evidence for ammonia products with significant excitation of the asymmetric bend (nu4) or water products with bend (nu2) excitation. 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.