Kinetic investigation of the time-resolved atomic fluorescence Ca(43P1→ 41S0) and molecular chemiluminescence CaCl(A2Π, B2Σ+→ X2Σ+) following the pulsed dye-laser generation of Ca(43PJ) in the presence of CH3Cl

Abstract

We present a kinetic study of the collisional behaviour of the optically metastable, electronically excited Ca[4s4p(3PJ)], 1.888 eV above its 4s2(1S0) electronic ground state, with molecular CH3Cl. Ca[4s4p(3P1)] was generated by pulsed dye-laser excitation of calcium vapour at elevated temperatures (900 K) and in the presence of CH3Cl and an excess of helium buffer gas at λ= 657.3 nm [Ca(4s4p(3P1)]â†� Ca[4s2(1S0)] in a slow-flow system, kinetically equivalent to a static system. Atomic fluorescence at the resonance wavelength following rapid Boltzmann equilibration within the Ca(4 3PJ) spin–orbit manifold was monitored using boxcar integration and exhibited exponential decay profiles which were characterised. Time-resolved chemiluminescence was also monitored under identical conditions for both the CaCl(A 2Π1/2, 3/2→ X 2Σ+) and CaCl(B 2Σ+→ X 2Σ+) systems, principally via the (0,0) transitions, on account of Franck–Condon limitations for which the X 2Σ+, A 2Π1/2, 3/2(2.000 eV) and B 2Σ+(2.089 eV) states are characterised by internuclear separations which are close in magnitude. These emissions also exhibit single-exponential decay profiles which are equal in magnitude to those from Ca(4 3PJ). 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 A 2Π1/2 and A 2Π3/2→ X 2Σ+, 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 first-order 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 3PJ)+ CH3Cl → CaCl(A 2Π1/2, 3/2v′⩽ 9)+ CH3(k2)(2), Ca(4 3PJ)+ CH3Cl → CaCl(B 2Σ+v′⩽ 7)+ CH3(k3)(3) Generation of the A 2Π and B 2Σ+ states from collision between Ca(4 3PJ) and CaCl(X 2Σ+), which is detected by laser-induced fluorescence, could only involve higher vibrational levels of the ground state, namely, v 3 and v 5, respectively. It is concluded that these (E, V–E) processes, involving the steady concentrations of CaCl(X 2Σ+) from the reaction of Ca(4 1S0)+ CH3Cl in the flow, would make only 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 CaCl(X 2Σ+) generated in the time domain from the reaction between Ca(4 3PJ) and CH3Cl would yield double-exponential molecular chemiluminescence decay profiles, which are not observed. The results for reactive channels into specific electronic states involving halogen abstraction are compared with analogous processes for Ca(4 3PJ) with O-atom and H-atom abstraction in the time domain, and halogen atom abstraction studied in molecular beams.