Electronic Chemiluminescence in M + X2 Reactions: Dissociation Energies of the Alkaline Earth Monohalides MX (M = Ca, Sr, Ba; X = F, Cl, Br)

Abstract

Electronic chemiluminescence has been observed for nine reactions of Group IIa metal atoms (M = Ca, Sr, Ba) with dihalogen molecules (X2 = F2, Cl2, Br2).In all cases one or more electronically excited states of the monohalide MX* were observed. Continuum emission, presumably due to dihalides MX2*, was seen in all systems, and the emission intensity increased in the order F2 < Cl2 < Br2. Recombination to form MX2* appears thus to be a fairly general process in these systems.From the highest vibrational levels of MX populated in the upper electronic state(s) we have calculated the following lower limits to the ground state dissociation energies D0° (MX) (in kcal/mol):[Formula: see text]Information on the vibrational state distributions of the excited MX* is discussed from a qualitative point of view. Accordingly, the fluorides are formed with inverted vibrational distributions, which bear clear evidence of the direct nature of the interaction dynamics. The electronically excited chlorides and bromides, however, tend to be formed with monotonically decreasing, near-statistical distributions. This is consistent with the formation of long lived collision complexes. Reasons for these trends are discussed in terms of the energetics of the complete decomposition.