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 (2.7k0.3) x lo8 1.2/mo1e2 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 Brz in argon at 1200°K was taken to be equal to 0-95 psec 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 kr = 13-35-1.6 log T, where kr is expressed in 1.2/mole2 sec.

Transient Phenomena in Dissociation of Diatomic Molecules

“…Specifically the Br2-Ar system seemed t o be of particular interest. It was shown previously (7,13) that the u.p.d. model reconciles, adequately, various experimental data on bromine dissociation-recombination reaction.…”

“…model were described previously (7,13). While in the previous investigation the back reaction was neglected, in the present work both dissociation and recombination are considered.…”

“…The functional form of this dependence is obtained (7,13,15) from the two assumptions, used above in eq. 1 concerning the existence of a unique Tv and of "reactive" states X2* very close to the dissociation limit.…”

“…1 concerning the existence of a unique Tv and of "reactive" states X2* very close to the dissociation limit. The fractional population of such states is given by , with Q,(T)/Q,(T,,) replaced by the high-temperature limit (TIT,), is the form given previously (7,13,15). Evaluation of the rate coefficient kd thus requires knowledge of k,; this problem has been discussed previously (16).…”

Thermal effects in flash photolysis with special reference to Br atom recombination

“…[5] and [7] gives [12] formly illuminated, both initial and recombination heating effects reduce to zero, because in that case, S = 1, even though there may be an appreciable change in the temperature of the photolyzed gas.…”