Kinetic investigation of the time-resolved atomic fluorescence Sr(5 3P1→ 5 1S0) and molecular chemiluminescence SrCl(A 2Π, B 2Σ+→ X 3Σ+ following the pulsed dye-laser generation of Sr(5 3PJ) in the presence of CH3Cl and CF3Cl

Abstract

A kinetic study of the vibronic energy distribution in SrCl following the collision of the optically metastable, electronically excited strontium atom, Sr[(5s)¹(5p)¹, ³P_J], 1.807 eV above its (5s)², ¹S₀ electronic ground state, in the presence of CH₃Cl and CF₃Cl is presented. This is investigated in the time-resolved mode following the generation of the excited atom by pulsed dye-laser excitation at elevated temperature from ground-state strontium vapour in the presence of reactants and an excess of helium buffer gas in a slow-flow system, kinetically equivalent to a static system. The decay of Sr(5 ³P_J) is monitored by time-resolved atomic fluorescence at the resonance wavelength [λ= 689.3 nm, Sr(5 ³P₁)→ Sr(5 ¹S₀)+hν], following rapid Boltzmann equilibrium within the 5 ³P_J spin–orbit manifold using boxcar integration. Electronically excited SrCl in the A ²Π_{1/2, 3/2} and B ²Σ⁺ states, respectively 1.855 eV (179.0 kJ mol^–1) and 1.948 eV (188.0 kJ mol^–1) above the X ²Σ⁺ ground state, was also monitored by time-resolved molecular chemiluminescence of the A, B–X systems under identical conditions to those employed for characterising the decay profiles of Sr(5 ³P_J). Both atomic and molecular chemiluminescence emissions showed exponential decay profiles, characterised by first-order decay coefficients which were found to be equal under the same experimental conditions within experimental error. SrCl(A ²Π) and SrCl(B ²Σ⁺) are thus shown to arise from direct production on the collision of Sr(5 ³P_J) with CH₃Cl and CF₃Cl where these processes are energetically favourable. These results from measurements in the time-domain are considered with analogous data for Ca(4 ³P_J) collisions involving F, Cl, Br and I atom abstraction reactions and with chemiluminescence from SrCl(A, B–X) systems in the single-collision condition following the reaction of Sr(5 ³P_J) with various Cl-containing molecules.