Imaging study of vibrational predissociation of the HCl–acetylene dimer: pair-correlated distributions

Abstract

The state-to-state predissociation dynamics of the HCl–acetylene dimer were studied following excitation in the asymmetric C–H (asym-CH) stretch and the HCl stretch. Velocity map imaging (VMI) and resonance enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Different vibrational predissociation mechanisms were observed for the two excited vibrational levels. Following excitation in the of the asym-CH stretch fundamental, HCl fragments in υ = 0 and j = 4–7 were observed and no HCl in υ = 1 was detected. The fragments’ center-of-mass (c.m.) translational energy distributions were derived from images of HCl (j = 4–7), and were converted to rotational state distributions of the acetylene co-fragment by assuming that acetylene is generated with one quantum of C–C stretch (ν₂) excitation. The acetylene pair-correlated rotational state distributions agree with the predictions of the statistical phase space theory, restricted to acetylene fragments in 1ν₂. It is concluded that the predissociation mechanism is dominated by the initial coupling of the asym-CH vibration to a combination of C–C stretch and bending modes in the acetylene moiety. Vibrational energy redistribution (IVR) between acetylene bending and the intermolecular dimer modes leads to predissociation that preserves the C–C stretch excitation in the acetylene product while distributing the rest of the available energy statistically. The predissociation mechanism following excitation in the Q band of the dimer’s HCl stretch fundamental was quite different. HCl (υ = 0) rotational states up to j = 8 were observed. The rovibrational state distributions in the acetylene co-fragment derived from HCl (j = 6–8) images were non-statistical with one or two quanta in acetylene bending vibrational excitation. From the observation that all the HCl(j) translational energy distributions were similar, it is proposed that there exists a constraint on conversion of linear to angular momentum during predissociation. A dimer dissociation energy of D₀ = 700 ± 10 cm⁻¹ was derived.