Collisional dynamics of low energy states of atomic strontium following the generation of Sr(5s5p 1P1) in the presence of He and Ar

Abstract

An experimental study is reported on the dynamics of the set of energy states below the 5s5p(¹P₁) state of atomic strontium, namely ¹D₂, ³D_J, ³P₂, ³P₁ and ³P₀, in the presence of the rare gases He and Ar and at temperatures from 673 to 760 K. These were generated following the nanosecond pulsed laser excitation of ground state Sr vapour at λ = 460.07 nm {Sr[5s5p(¹P₁)] ← Sr[5s²(¹S₀)]}, intentionally employing radiation trapping to lengthen the effective lifetime of Sr(¹P₁) and so facilitate population of these lower lying states by emission and collision. The populations of the lower states were then interrogated by laser-induced fluorescence (LIF) at selected delays and, where possible, also by spontaneous emission methods. The evolution of the populations with time have been modelled with a set of coupled linear differential equations yielding a consistent set of collisional rate data and emission lifetimes. The individual populations of the closely spaced spin–orbit states within Sr(³D_J) cannot be separated on the time scale of the pulsed lasers and are treated as a single state. By contrast, for most rare gases, intramultiplet mixing within Sr[5s5p(³P_0,1,2)] is slow and the J states are treated separately. The collisions of Sr(5 ¹P₁) with Ar and He have been shown to yield mainly Sr(5 ³D_J) with negligible yields of both Sr(¹D₂) and Sr(5 ³P_J). The model uses relative values of Einstein coefficients from the averaged Sr(5 ³D_J) state to any of the three Sr(5 ³P_0,1,2) states, calculated assuming an LS coupling approximation for Sr and normalised to the overall experimental Einstein coefficient. The six-state kinetic model proposed, constructed incorporating earlier measurements specifically on Sr(5 ³P_0,1,2), is shown to fit well the observed population profiles and also shows collisional propensity behaviour.