Time-resolved investigation of radiation trapping for the transition [formula omitted] following the generation of Sr[5s5p(3PJ)] by pulsed dye-laser excitation at elevated temperatures

Abstract

We present a kinetic investigation of radiation trapping for the transition Sr[5s5p(3P1)] → Sr[5s2(1S0)] + hν at λ = 689.3 nm following the pulsed dye-laser generation of Sr[5s5p(3PJ)] at elevated temperature following excitation of ground state atomic strontium, Sr[5s2(1S0)], at the resonance wavelength. Sr[5s5p(3PJ)], 1.807 eV above the 1S0 ground state, was produced following laser photolysis to the 3P1 state in a slow flow system, kinetically equivalent to a static system, in the presence of helium buffer gas and at temperatures in the range 670–900 K. Following rapid Boltzmann equilibration within the Sr(5s5p(3P0,1,2)] spin-orbit manifold, time-resolved emission profiles at λ = 689.3 nm were recorded using signal averaging and computerised analysis techniques under a range of conditions of temperature and pressure, emission from the 3P0.2 levels being negligible, these being so-called “reservoir states”. First-order decay coefficients (κ′(3PJ)) were characterized for the emission at the resonance wavelength as well as integrated atomic fluorescence emission intensities using a sensitivity calibration of the total optical detection system. The resulting mean radiative lifetime for the transition (τc = 20.7 μs) was found to be in accord with the results of previous investigations, and the variation of the integrated atomic fluorescence emission intensities with the pressure of helium in accord with considerations of pressure broadening. Furthermore, the absolute second-order rate constant for the quenching of Sr(53PJ) by He (κQ = (2.9 ± 0.2) × 10−15cm3 atoms−1 s−1T = 840 K) was found to be in accord with the previous upper limit reported for this quantity by time-resolved emission investigations and the value reported from phase-shift techniques. The experimental curve of κ′(3PJ) versus temperature and hence Sr(51S0) density was constructed and compared with the results for radiation trapping according to the theory of (a) Milne, employing the diffusion theory of radiation for low “equivalent opacity” and infinite slab geometry of a thickness comparable with that of the laser beam and (b) Holstein, calculated as a general transport problem by solving a Boltzmann-type integro-differential equation in terms of a transmission coefficient. The results are found to be in accord with the calculations using the theory of Milne. This approach extends the previous empirical corrections that have been made for this effect with Sr(53P1). In more general terms it establishes from fundamentals the importance of radiation trapping for this transition at λ = 689.3 nm, which, whilst characterised by a relatively large mean radiative lifetime by comparison with that for a fully allowed electric-dipole atomic emission, is, nevertheless, subject to radiation trapping at the densities typically employed with time-resolved measurements following pulsed dye-laser excitation.