Chemiluminescence from the Reactions of Ba[6s5d( 3D J)] with CF 3Br and CF 2Br 2 Including Branching Ratios into the Electronic States BaBr(A 2Π 1/2, A 2Π 3/2, B 2Σ) following the Pulsed Dye-Laser Excitation of Atomic Barium

Abstract

Chemiluminescence from BaBr(A 2Π 1/2,3/2, B 2Σ +-X 2Σ +) following the exothermic Br-abstraction reactions of Ba[6s5d( 3D J )], 1.151 eV above the 6s 2( 1S 0) ground state, with CF 3Br and CF 2Br 2 is investigated in the time domain. The Ba( 3D J ) is produced following the initial pulsed dye-laser excitation of atomic barium via the allowed transition at λ = 553.5 nm {Ba[6s6p( 1P 1)] ← Ba[6s 2( 1S 0)]} in excess helium buffer gas at 900 K. This optically metastable state is generated subsequently in the long-time domain by a combination of radiative and collisional processes. It is then monitored by the spectroscopic atomic emission marker transition at λ = 791.1 nm {Ba[6s6p( 3P 1 → Ba[6s 2( 1S 0)]} where Ba( 3P 1) results from collisional activation of Ba( 3D J ). Molecular chemiluminescence, which is weak, is monitored via the long wavelength transitions: BaBr(A 2Π 1/2 → X 2Σ +, λ = 1002 nm, Δv = 0), BaBr(A 2Π 3/2 → X 2Σ +, λ = 943 nm, Δv = 0) and BaBr(B 2Σ + → X 2Σ +, λ = 883 nm, Δv = 0). The equality in the first-order decay coefficients for the atomic and molecular profiles indicates that the three molecular states BaBr(A 2Π 1/2, A 3/2, B 2Σ +) are generated directly on collision of Ba( 3D J ) with CF 3Br and CF 2Br 2. A kinetic analysis employing both the integrated intensities of the atomic emission and these weak long wavelength molecular emissions, coupled with optical sensitivity calibrations, yields molecular electronic branching ratios in the BaBr(A 2Π 1/2,3/2, B 2Σ +) states from the two reactants. These are found to be as follows: CF 3Br: BaBr(A 2Π 1/2) 11.8 ± 4.3%, BaBr(A 2Π 3/2) 3.8 ± 1.4%, BaBr(B 2Σ +) 1.5 ± 0.8%; CF 2Br 2: BaBr(A 2Π 1/2) 11.1 ± 4.7%, BaBr(A 2Π 3/2) 6.0 ± 2.6%, BaBr(B 2Σ +) 1.4 ± 0.5%. These results, including the elucidation of the fundamental collisional processes leading to the chemiluminescence, may be contrasted with the observation of such emission, where feasible, in low pressure flames. Normally, this has involved recording the overall emission intensity from specific molecular states of BaBr on reaction of Ba with Br 2. The logarithmic variation of these branching ratios with the energies of the states is essentially Boltzmann in character in both cases as found hitherto for reactions of Ba( 3D J ) with other halogenated targets with which the present results are compared. The branching ratios observed here yield effective temperatures of ca. 940 and 1080 K for the collision of Ba( 3D J ) with CF 3Br and CF 2Br 2, close to the ambient temperature of the measurements. This is consistent with the absence of selection rules for the yields of these excited molecular states on collision and reflecting the role of late barriers in the potential surfaces involved. For the reaction of Ba( 3D J ) + CF 2Br 2, the production of the BaBr(C 2Π 1/2) is close to thermoneutral; however, the C-X chemiluminescence was not observed, presumably on account of the low branching ratios into this higher-lying molecular state.