Branching ratios for the production of the electronic states [formula omitted] from the reaction of Ba[6s5d( 3D J)] with CH 3Br investigated by time-resolved atomic emission and molecular chemiluminescence following pulsed dye laser excitation of atomic barium

Abstract

We present measurements of the branching ratios for the production of the electronic states BaBr(A 2Π 1 2 3 2 , B 2Σ +) from the reaction of Ba[6s5d( 3D J )] with CH 3Br investigated by time-resolved atomic emission and molecular chemiluminescence following pulsed dye laser excitation of atomic barium at elevated temperature (900 K). Ba[6s5d( 3D J )], 1.151 eV above the 6s 2( 1S 0) ground state, was generated after the initial pulsed dye laser excitation of atomic barium via the allowed transition at λ = 553.5 nm ( Ba[6 s6 p( 1 P 1)] ← Ba[6 s 2( 1 S 0)]) in excess helium buffer gas at 900 K. A combination of radiative and collisional processes in excess helium then yielded the optically metastable Ba( 3D J ) state in the “long-time domain”. This may be monitored at λ = 791.1 nm ( Ba[6 s6 p( 3 P 1)] → Ba(6 s 2( 1 S 0)]) , employed as the spectroscopic atomic emission transition. The following long-wavelength molecular chemiluminescence transitions of BaBr were also monitored as a function of time (resulting from the collision of Ba[6s5d( 3D J )] with CH 3 Br): BaBr(A 2Π 1 2 → X 2Σ +, λ = 1002 nm, Δν = 0) ; BaBr(A 2Π 3 2 → X 2Σ +, λ = 943 nm, Δν=0) ; BaBr)B 2 Σ + → X 2 Σ +, λ = 833 nm, Δν = 0). These measurements in the time domain, particularly the A 2Π 1 2 - X 1Σ + transition, are feasible using a long-wavelength photomultiplier tube operating in this region. The exponential atomic molecular profiles are characterized by decay coefficients which are equal in given reactant mixtures, from which it may be concluded that the three molecule states, BaBr(A 2Π 1 2 3 2 , B 2Σ +) , are generated directly on collision of Ba( 3D J ) with CH 3Br via exothermic reactions. The determination of the integrated intensities for the atomic emission and molecular emissions, coupled with optical sensitivity calibrations, yielded branching ratios in the BaBr(A 2Π 1 2 3 2 , B 2Σ +) , states, with the following results: BaBr(A 2Π 1 2 ) , 3.80%; BaBr(A 2Π 1 2 ) , 3.80%; BaBr(A 2Π 3 2 ) , 1.69%; BaBr(B 2 Σ +), 0.49%. The absolute magnitudes of these branching ratios are comparable with analogous results for the reactions of Ba( 3D J ) with CH 3Cl and CH 3F reported previously. The relative variation of the branching ratios, as seen through their logarithmic variation with the energies of the states, is essentially Boltzmann in character, with an effective temperature close to the ambient temperature of the measurements, indicating the absence of propensity in the yields of these excited molecular states on collision and reflecting the role of late barriers in the potential surfaces involved.