Infrared chemiluminescence in the systems H+Br 2 and H+HBr

Abstract

In an earlier qualitative study of partially resolved infrared chemiluminescence from H+Br 2 , it was reported that HBr was formed in levels up to and including v =6. In the present work (despite a reduction in pressure to ca. 0.03 mm Hg, and in mean residence time in the vessel to 0.005 sec), the vibrational-rotational band structure was resolved and the stationary-state distribution N v in levels v =1–6 was measured. Relative rate constants k v for the direct formation of HBr in levels v =3–6 as a product of the reaction H+Br 2 → HBr v + Br are reported for the first time. These resemble the corresponding k v 's for H+Cl 2 qualitatively, but differ quantitatively. The difference is in the rate of decrease of the function k v (v) toward higher v : the function falls significantly less steeply for H+Br 2 than for H+Cl 2 . The same applies to the function k f (f) in which the variable v has been replaced by f , the fraction of the energy-of-reaction entering vibration. In both systems, H+Br 2 and H+HBr, infrared chemiluminescent emission was observed at ca. 2.71 μ due to the forbidden Br * ( 2 P 1/2 )→Br( 2 P 3/2 ) electronic transition. Evidence is presented which indicates that Br * is formed most efficiently in some binary process, other than the H+Br 2 reaction. This could be the reaction H+HBr → H 2 +Br * (the relative concentration of Br * is markedly greater in the H+HBr system), or it could be an energy transfer process.