KINETIC INVESTIGATION OF THE TIME-RESOLVED ATOMIC FLUORESCENCE CA(43P1-]41S0) AND MOLECULAR CHEMILUMINESCENCE CABR(A2-PI, B2-SIGMA+-]X2-SIGMA+) FOLLOWING THE PULSE-DYE LASER GENERATION OF CA(43PJ) IN THE PRESENCE OF CH3BR

The collisional behaviour of the optically metastable, electronically excited state Ca[4s4p(3P(J))], 1.888 eV above its 4s2(1S0) electronic ground state, with the molecule CH3Br, has been studied in the time-domain in order to investigate reactive channels into specific electronic states Ca[4s4p(3P1)] was generated by pulsed dye-laser excitation of calcium vapour at elevated temperatures in the presence of CH3Br and excess helium buffer gas, and the resulting atomic fluorescence at the resonance wavelength {lambda = 657.3 nm, Ca[4s4p(3P1)] --> Ca[4s2(1S0) + hv]} was monitored, following rapid Boltzmann equilibration within the Ca(4(3)P(J)) spin-orbit manifold, using Boxcar Integration. The electronically excited atom exhibited exponential decay profiles which were characterised. Time-resolved chemiluminescence was also monitored under identical conditions for both the CaBr(A2-PI-1/2,3/2 --> X2-SIGMA+) and CaBr(B2-SIGMA+ --> X2-SIGMA+) systems, principally via the (0,0) transitions, on account of Franck-Condon limitations. These molecular emissions exhibit decay profiles which are principally single-exponential in form after a short delay and equal in magnitude to those from Ca(4(3)P(J)). In the case of the A - X chemiluminescence, it was possible under some conditions to resolve the spin-orbit components of the (0,0) transitions for A2-PI-1/2 and A2-PI-3/2 --> X2-SIGMA+ whose exponential decay coefficients were also found to be equal in magnitude, and thus indicating that these also are in equilibrium during the kinetic decays. The exponential form and equality in the decay coefficients for the atomic and both A - X and B - X chemiluminescence profiles are in accord with a mechanism involving the production of these electronically excited states by direct reaction: Ca(4(3)P(J)) + CH3Br --> CaBr(A2-PI-1/2,3/2, upsilon' less-than-or-equal-to 2) + CH3 (k2) (2) Ca(4(3)P(J)) + CH3Br --> CaBr(B2-SIGMA+, upsilon' = 0) + CH3 (k3). (3) Generation of the A2-PI and B2-SIGMA+ states between Ca(4(3)P(J)) and CaBr(X2-SIGMA+), which is detected by laser-induced fluorescence, could only involve higher vibrational levels of the ground state, namely, upsilon greater-than-or-equal-to 3 and upsilon greater-than-or-equal-to 4, respectively. It is concluded that these (E, V - E) processes, involving the steady concentrations of CaBr(X2-SIGMA+) from the reaction of Ca(4(1)S0) + CH3Br in the flow, would only make a small contribution to the generation of the A and B states, characterised by the same time-dependences as observed in chemiluminescence, but whose contribution cannot be totally ruled out; a similar mechanism from CaBr(X2-SIGMA+), generated in the time-domain from the reaction between Ca(4(3)P(J)) + CH3Br, would yield double-exponential molecular chemiluminescence decay profiles which are not observed, but some minor distortion of the molecular chemiluminescence profiles at short decay could result from a contribution arising from this mode. The results for reactive channels into specific electronic states involving bromine atom abstraction are compared with analogous processes for Ca(4(3)P(J)) with O-atom, H-atom and Cl-atom abstraction in the time-domain, and halogen atom abstraction studied in molecular beams.