Radiative lifetimes, diffusion and energy pooling of Sr(5s5p( 3P J)) and Sr(5s4d( 1D 2)) studied by time-resolved atomic emission following pulsed dye-laser excitation

Abstract

A study is presented of the kinetic behaviour of the optically metastable states Sr(5s5p( 3P J )) and Sr(5s4d( 1D 2)), each separately generated by pulsed dye-laser excitation at λ = 689.3 nm (Sr(5s5p( 3P 1))←Sr(5s 2( 1S 0))) and λ = 496.1 nm (Sr(5s4d( 1D 2))←Sr(5s 2( 1S 0))) in the presence of helium buffer gas. Radiative and diffusional loss of both metastable states in helium, including the effect of radiation trapping for Sr(5s5p( 3P J )), have been investigated as a function of temperature and pressure by time-resolved emission measurements. We report the following radiative lifetimes: τ e(Sr( 3P 1)) = (19.6 −0.5 +0.6) μs for fluorescence via the spin-forbidden transition to the ground state; and τ(Sr( 1D 2)) = (412 −9 +10) μs for radiative loss to all lower-lying states. Both values are compared with the results of previous experimental work and theoretical calculations as appropriate. When expressed in the form D 12 ∝ T n , the measured temperature dependence of the diffusion coefficients D 12(Sr( 3P J )-He) and D 12(Sr( 1D 2-He) yielded n = 1.59±0.51 and n = 1.83±0.89, respectively. Rate constants for self-quenching of Sr( 3P J ) and Sr( 1D 2) atoms by ground-state Sr( 1S 0) vapour are estimated to be k Sr = (2.9−6.4)×10 −14 cm 3 atom −1 s −1 and k′ Sr = (3.4±0.9)×10 −14 cm 3 atom −1 s −1 respectively. Whilst an upper limit of k He⩽(2.6±0.6)×10 −15 cm 3 atom −1 s −1 is determined for collisional quenching of Sr( 3P J ) atoms by He, no such effect could be observed for Sr( 1D 2) atoms. Energy pooling processes involving Sr(5s5p( 3P J )) atoms to yield the higher-lying 5s5p( 1P 1), 5s4d( 1D 2) and 5s6s( 3S 1) states are also investigated in the time domain, where the mechanisms for pooling formation are established from a quantitative comparison of the time dependence of atomic emission from the energy store and pooled states. For Sr(5s4d( 1D 2)), fluorescence from some 22 energy-pooled atomic states is reported, mostly arising from bimolecular collisions between two Sr( 1D 2) atoms with subsequent emission of radiation. Approximate relative yields into all these pooled states, estimated from extrapolation of their time-dependent decay profiles coupled with appropriate corrections for Einstein coefficients and experimental sensitivity, indicate that energy pooling collisions constitute a small “bleed-off” from the store states which does not affect their time profiles. Finally, quantum yields for emission from Sr(5s5p( 3P J )) and Sr(5s4d( 1D 2)) atoms and branching ratios into the 3P J levels following laser excitation of the 1D 2 state are reported. These are shown to be quantitatively consistent with a mechanism based on removal of both excited states by radiative decay and diffusion, as seen by reference to previous calculated Einstein coefficients for the relevant radiative transitions. Using experimental data reported in an earlier publication, quantum yields for emission from Ca(4s4p( 3P J )) and Ca(4s3d( 1D 2)) atoms and the branching ratio for production of Ca(4s4p( 3P J )) following laser excitation of Ca(4s3d( 1D 2)) are also determined, from which a mechanism analogous to that for atomic strontium is established.