Can the Fragmentation of Hydrogen-Bonded Dimers Be Predicted: Predissociation of C2H2−HX

Abstract

HX rotational state distributions following vibrational predissociation (VP) of C(2)H(2)-HX (X = Cl, F, O) dimers are predicted by expressing the predissociation process as the joint probability of rovibrational excitation in the fragments following "internal collision" in the vibrationally excited dimer. Calculations of these joint probabilities for the T-shaped dimers of acetylene with HCl, DCl, HF, and OH using the angular momentum (AM) method reproduce experimental distributions with reasonable accuracy. In dimers of this complex, many different pathways for the disposal of initial energy and momentum exist in principle. The use of simple physical arguments based on (a) the direction of initial impulse upon excitation and (b) restricted relative geometries due to limited amplitude of relative motion of the dimer components allows the number of effective dissociation pathways to be much reduced. For these, the probability of rotational and rovibrational transfer into the fragments is calculated, a process that generally involves summing over a number of C(2)H(2) rovibrational states for each value of j(HX). In calculating relative rotational populations in the fragments, it was found essential to first calculate the threshold value of available energy for that transition and the threshold value of b(n), the effective impact parameter. Without these modifications, channels of lowest j(HX) and/or j(C(2)H(2)) dominate, which generally is not found experimentally. The need for these modifications is attributed to energy conservation in the dissociation and the limited range of relative orientations that the dimer pair can explore. The AM method is able to predict the very different fragment rotational excitations in this series of dimers fairly well using only readily available data. In addition, a number of new insights into the physical principles that control the dissociation of molecule-molecule dimers have emerged and are discussed. The results suggest that each fragment quantum state pair results from a very specific relative geometry of dissociation and that the balance between vibrational and rotational excitations is determined by the requirement to restrict the angular momentum "load" in the predissociation.