Measurements in the time-domain of molecular chemiluminescence from CaCl(A2Π1/2, A2Π2/3, B2Σ+ →X2Σ+)and atomic resonance fluorescence from Ca(43P1→ 41S0) have
been made in order to determine electronic branching ratios to yield CaCl(A1/2, A2/3, B)
following the Cl-atom abstraction reaction of electronically excited calcium atoms,
Ca[4s4p(3PJ)]
, 1.888 eV above the 4s2(1S0) ground state, with CH3Cl. Ca[4s4p(3P1)] was
generated by the pulsed dye-laser excitation ofground state calcium atoms at λ = 657.3 nm
21, {Ca[4s4p(n3P1)] ←Ca[4s2(1S0)]} at elevated temigeratures in the presence of CH3Cl and
excess helium buffer gas in a slow flow system, kinetically equivalent to a static system.
Atomic and molecular emissions were recorded following rapid Boltzmann equilibration
within the Ca(43P0,1.2) spin-orbit manifold, atomic emission being restricted to
the 3P1 state as the 3P0 and 3P2 states are long-lived, being so called ‘reservoir states’. Atomic fluorescence emission profiles at the resonance wavelength together with the molecular chemiluminescence emissions CaCl (A2Π1/2→X2Σ+, Δν=0, λ=621nm),CaClA2Π3/2→X2Σ+, Δν=0, λ=618nm and CaCl(B2Σ+→X2Σ+, Δν=0, λ=593nm)
were recorded. The atomic and molecular emissions demonstrated exponential
decays characterised by decay coefficients which were equal under identical chemical
conditions thus demonstrating the production of CaCl(A2Π1/2,3/2, B2Σ+) by direct
reaction of Ca(3pJ) with CH3Cl. The combination of the time-dependences of the atomic and molecular profiles as a function of the concentration of CH3Cl 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 A2Π1/2, A2Π3/2 and B2Σ+ states of CaCl. These were found to be as follows: A1/
(2.6±1.1)2×10−3; A2/3(1.9±0.8)×10−3 and B(3.6±2.0)×10−4. To the best of our
knowledge, these data represent the first measurements of electronic branching ratios of
calcium chlorides into specific molecular states for reactions of Ca(3pJ) determined in the time-domain. 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 CH3Cl using molecular beams. These are also compared with branching ratios of
SrCl(A2Π1/2, A2Π2/3, B2Σ+)
following the reaction of Sr[5s5p(3pJ)]+ CH2Cl2 investigated following pulsed dye-laser excitation where the yields of SrCl(A1/2, A3/2, B)were of similar low magnitudes.
