Determination of the Branching Ratios of CaBr(A2Π1/2,3/2, B2Σ+) Resulting from the Reaction of Ca[4s4p (3PJ)] with CH3Br by Time-Resolved Atomic Emission and Molecular Chemiluminescence Following Pulsed Dye-Laser Excitation of Atomic Calcium

Abstract

Molecular electronic branching ratios to yield CaBr(A 2 Π 1/2 ), CaBr(A 2 Π 3/2 ) and CaBr(B 2 Σ + ) have been determined in the time-domain following the reaction of Ca[4s4 p( 3 P J )], 1.888 eV above the 4s 2 ( 1 S 0 ) ground state, with CH 3 Br. Ca[4s4 p( 3 P 1 )] was generated by the pulsed dye-laser excitation of ground state calcium atoms at λ = 657.3 nm {Ca[4s4 p( 3 P 1 )] ← Ca[4s 2 ( 1 S 0 )]} at elevated temperatures in the presence of CH 3 Br and excess helium buffer gas. Measurements were then made of the atomic resonance fluorescence from Ca(4 3 P 1 → 4 1 S 0 ) and the molecular chemiluminescence from CaBr(A 2 Π 1/2 , A 2 Π 3/2 , B 2 Σ + → X 2 Σ + ) after Boltzmann equilibration with Ca(4 3 P 0,1,2 ). The following molecular chemiluminescence emissions were recorded: CaBr(A 2 Π 1/2 → X 2 Σ + , ∆v = 0, λ = 628 nm), CaBr(A 2 Π 3/2 → X 2 Σ + , ∆v = 0, λ = 625 nm) and CaBr(B 2 Σ + → X 2 Σ + , ∆v = 0, λ = 611 nm). The atomic and molecular emissions demonstrated exponential decays characterised by decay coefficients which were equal under identical chemical conditions thus demonstrating the production of CaBr(A 2 Π 1/2,3/2 , B 2 Σ + ) by direct reaction of Ca( 3 P J ) with CH 3 Br. The combination of the time-dependences of the atomic and molecular profiles together with the integrated atomic and molecular intensities, placed on a common intensity scale by calibration of the optical system against a spectral radiometer, yielded branching ratios into the A 2 Π 1/2 , A 2 Π 3/2 and B 2 Σ + states of CaBr. These were found to be as follows: A 1/2 (9.8 ± 3.0) · 10 −4 ; A 3/2 (8.5 ± 3.4) · 10 −4 and B, (3.4 ± 1.7) · 10 −4 . To the best of our knowledge, these data represent the first measurements of electronic branching ratios of calcium bromide into specific molecular states for reactions of Ca(4 3 P J ). The results are compared with various related data for total branching ratios into electronic states studied in the single collision condition for this atomic state with CH 3 Br using molecular beams, and with analogous measurements for Ca(4 3 P J ) + CH 3 F and CH 3 Cl. These results are also compared with branching ratios of SrBr(A 2 Π 1/2 , A 2 Π 3/2 , B 2 Σ + ) following the reaction of Sr[5s5 p( 3 P J )] + CH 2 Br 2 investigated following pulsed dye-laser generation of Sr(5 3 P J ).