Recombination of Br atoms by flash photolysis over a wide temperature range

Abstract

The recombination rate constant of Br atoms in argon has been measured up to 1273°K using a flash photolysis technique. At room temperature and up to 480°K, results agree with the literature, but at 1150°K the recombination rate constant is (2·7±0·3)×10⁸ l.²/mole² sec, which is about one third of the rate constant predicted from shock-wave studies of Br₂ dissociation. The discrepancy can be explained by considering a model for coupling of vibrational relaxation and dissociation. The main assumptions of the model are that the vibrational relaxation in dissociating gas at reasonably low temperatures can be described by a single relaxation time and that diatomic molecules dissociate from the uppermost vibrational levels. The model reconciles the shock wave and flash photolysis data. In this model, the reduced vibrational relaxation time for Br₂ in argon at 1200°K was taken to be equal to 0.95 µsec atm.

An alternate model, which considers a coupling of electronic excitation and dissociation processes, is also discussed. This model suggests that the discrepancy observed may be explained if it is assumed that the activation energy of the transition between the ground and excited electronic states of Br₂ is between 25 and 35 kcal/mole. There are no known electronic states in this energy region. The temperature dependence of the recombination rate constant between 300 and 2300°K can be expressed by the equation log k_r= 13·35–1·6 log T, where k_r is expressed in l.²/mole² sec.