Collision-induced dissociation of laser-excited Br2[ B 3Π(0+ u ); v ′, J ′]: Formation of Br*(2 P 1/2)+Br(2 P 3/2) at energies 1–5 k T below dissociation

Absolute rate constants are measured for the dissociation of initially excited Br2[B 3Π(0+u);v′,J′] into Br*(2P1/2)+Br(2P3/2) fragments by thermal energy collisions with Br2(X 1Σ+g), Xe, and Ar at 297 K. A 0.04 cm−1 étalon-narrowed pulsed dye laser populates specific rovibrational levels of isotopic Br2 which are 1–5 kT below the B-state dissociation limit; in addition the laser directly photodissociates molecules which are in thermally excited vibrational levels. The method used to determine the absolute rate constants combines four sets of experimental determinations, which include infrared detection of Br*, visible Br2(B) fluorescence lifetimes, absorption spectroscopy of Br2 (B←X), and transient gain-vs-absorption spectroscopy on Br/Br*. At 1 kT below the dissociation limit, the absolute rate constants for collisional dissociation to Br*+Br are 2.9×10−10, 1.2×10−10, and 5.4×10−11 cm3 molecule−1 s−1 for Br2, Xe, and Ar, respectively. The results represent an average of some rotational and vibrational states grouped around a particular energy since rotational and vibrational energy transfer occur on a competitive time scale. The other major process that contributes to the total quenching rate is collision-induced predissociation to Br+Br. At 1 kT below the B-state dissociation limit, ∼75% of the quenching proceeds upward in energy to form Br*+Br, while at 5 kT this fraction is ∼8%. The efficiency of upward collisional dissociation correlates with the relative translational energy available.