Dissociation and Two-Body Emission in Shock-Heated Bromine. I. Br2 in Argon

Abstract

The dissociation of Br2 in the presence of an excess of argon was studied in shock waves using the two-body emission (2Br → Br2(1Σg+) + hν) to monitor the Br atom concentration. At 5980 Å, this emission was found to be due to 1Πu−1Σg+ transition which is also the most intense transition observed in the visible absorption spectrum of Br2. This new method of dissociation rate constant measurements is particularly insensitive to the state of internal excitation of Br2. Moreover, the method is advantageous since it involves direct determination of [Br] as a function of time; in all previous studies of Br2 dissociation, the concentration of Br atoms was calculated as a small difference between two large concentrations of Br2. The dissociation rate constants, kd's, were calculated from the initial slopes of the square-root intensity versus time plots. These constants, when related to the kr's, recombination rate constants, using the equilibrium constant Kc, may be expressed by an equation log10[kr(liter2mole−2·sec−1)] = − 104.027 + 71.507 logT − 11.378 (logT)2, which is valid between 1350° and 2400°K. Above 1500°K, these results agree with six independent published measurements of kd obtained from the optical absorption versus time plots. The agreement is gratifying, and the analysis of our experimental procedure suggests that it is not accidental. However, below 1500°K, the “emission” rate constants are smaller than the “absorption” rate constants. On the other hand, the “emission” rate constants at low shock temperatures are in agreement with kr's obtained recently by flash photolysis between 1000° and 1273°K [J. K. K. Ip and G. Burns, Discussions Faraday Soc. 44, 241 (1967)]. The discrepancy between “emission” and “absorption” rates appears to be real, and cannot be explained by the presence of impurities, boundary layer effects, or other experimental difficulties. It may be explained, however, if it is assumed that the internal degrees of freedom of Br2 were not completely relaxed in shock-wave experiments below 1500°K. Although this effect has not been observed with other diatomic molecules, it is suggested that this is possible, and that indirect evidence for the existence of this phenomenon exists for chlorine and iodine. A computer program was developed to describe the whole reaction profile behind the shock front for emission experiments. Using this program, kd,Br, the dissociation rate constant in the presence of Br atom as a collision partner, has been calculated. The value of kd,Br was found to be extremely sensitive to even minute traces of impurities, but the most reliable data based on measurements yield kd,Br / Kc(liter2mole−2·sec−1) = 8.8 × 1032T−7.3. This equation is valid between 1600° and 2240°K.