The collisional quenching of [formula omitted] by [formula omitted] following pulsed dye-laser excitation of atomic calcium

Abstract

The collisional quenching of electronically excited calcium atoms, Ca[4s4p(3PJ), 1.888eV above the 4s2(1S0) ground state by ground state calcium atoms, Ca[4s2(1S0)], has been investigated. Ca[4s4p(3P1)] was generated by the pulsed dye-laser excitation of ground state calcium atoms at λ=657.3nm {Ca[4s4p(3P1)]←Ca[4s2(1S0)]} over a range of elevated temperatures in the presence of excess helium buffer gas. Measurements were then made of the atomic resonance fluorescence from Ca(43P1→41S0) after Boltzmann equilibration within Ca(43P0,1,2). First-order decay coefficients for the atomic emission profiles were quantified both as a function of temperature and hence, atomic density of Ca(41S0) and of helium density itself. Consideration of the terms contributing to the measured decay coefficients, viz. emission, diffusion, and collisional quenching of Ca(43PJ) by Ca(41S0) and He yields the collisional rate data. The absolute second-order rate constant for quenching of Ca(3P) by Ca(1S) is found to be kCa=(2.4±0.3)×10−12cm3 per atom s−1 (810–997K) marginally lower than obtained by phase angle modulation following dye-laser excitation for a similar temperature range. It is significantly larger than that obtained from earlier estimates derived from the departure of a T1.5 dependence of the temperature variation of the diffusion coefficient, D12(Ca3P-He), measured in the time-domain. A related analysis of the first-order decay coefficients for Ca(43PJ) yielded the rate constant for the collisional quenching of Ca(3P) by He, viz. kHe=(2.3±0.3)×10−16cm3 per atom s−1 (T=940K). This is in agreement with the upper limit obtained earlier in the time-domain but significantly lower than that resulting from phase angle modulation measurements across a temperature range in this region.