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Physical Review A

Collisional transfer within the Sr(5 °J3 ) multiplet due to nearly adiabatic collisions with noble gases

James Floyd Kelly, Martin A. Harris, Alan C. Gallagher

Published Apr 1, 1988• 53 citations
Publisher

Abstract

A time-resolved study of the Sr(5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}$) intramultiplet collisional mixing, following optical excitation of the $^{3}\mathrm{\ifmmode^\circ\else\textdegree\fi{}}_{1}$ state, is presented. The degeneracy-averaged rate coefficients of Sr(5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}$\ensuremath{\rightarrow}5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{J\mathcal{'}}$) due to collisions with each of the noble gases in a quasiequilibrium cell are derived from least-squares fittings of the triexponential time behavior of 5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}$ fluorescence and ${5}^{3}$P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{2}$, 0\ensuremath{\rightarrow}6 $^{3}\mathrm{S}_{1}$ absorption data. A 5 $^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}$ radiative rate of 22\ifmmode\pm\else\textpm\fi{}0.5 \ensuremath{\mu}s is also obtained. The dependence of the rates upon buffer pressure demonstrated the presence of both linear and quadratic terms, due to two- and three-body reactions, respectively. The behavior of the binary rate coefficients with varying noble-gas species and temperature is qualitatively consistent with St\"uckelberg's model for noncrossing levels.

DOI
10.1103/physreva.37.2354

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Energy pooling (EP) has been observed in Sr vapor following pulsed optical excitation to the 5${}^{3}$${P}_{1}$ state. From the time-dependent radiative decay of 6${}^{3}$${S}_{1}$ and of ${6}^{1}$${\mathit{S}}_{0}$ via the cascade resonance line, the EP rate coefficients have been investigated for the following: Sr(5 $^{3}P_{J}$)+Sr(5${}^{3}$${\mathit{P}}_{\mathit{J}\mathcal{'}}$) ${(5}^{1}$${S}_{0}$) +Sr(6 $^{3}S_{1}$ or ${6}^{1}$${S}_{0}$). The values of ${k}_{11}$ have been measured, as well as ${k}_{\mathrm{JJ}\mathcal{'}}$ averaged over a thermal distribution of 5 $^{3}P_{J}$ populations. The time-dependent Sr(${5}^{3}$${P}_{J}$) densities were measured by absorption of lines from a Sr lamp crossing the excitation region. The rate coefficients are surprisingly large; in spite of the small statistical weights of these product states, they are nearly gas-kinetic. The dependence of the rates upon spin, J, and energy defect \ensuremath{\Delta}E has also been examined.

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A time-resolved study of the Sr(5 3\ifmmode∘\else\textdegree\fiJ^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}3\ifmmode∘\else\textdegree\fiJ​) intramultiplet collisional mixing, following optical excitation of the 3\ifmmode∘\else\textdegree\fi1^{3}\mathrm{\ifmmode^\circ\else\textdegree\fi{}}_{1}3\ifmmode∘\else\textdegree\fi1​ state, is presented. The degeneracy-averaged rate coefficients of Sr(5 3\ifmmode∘\else\textdegree\fiJ^{3}\ifmmode^\circ\else\textdegree\fi{}_{J}3\ifmmode∘\else\textdegree\fiJ​\ensuremath{\rightarrow}5 3\ifmmode∘\else\textdegree\fiJ′^{3}\ifmmode^\circ\else\textdegree\fi{}_{J\mathcal{'}}3\ifmmode∘\else\textdegree\fiJ′​) due to collisions with each of the noble gases in a quasiequilibrium cell are derived from least-squares fittings of the triexponential time behavior of 5 3\ifmmode∘\else\textdegree\fi1^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}3\ifmmode∘\else\textdegree\fi1​ fluorescence and 53{5}^{3}53P\ifmmode∘\else\textdegree\fi2{\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{2}\ifmmode∘\else\textdegree\fi2​, 0\ensuremath{\rightarrow}6 3S1^{3}\mathrm{S}_{1}3S1​ absorption data. A 5 3\ifmmode∘\else\textdegree\fi1^{3}\ifmmode^\circ\else\textdegree\fi{}_{1}3\ifmmode∘\else\textdegree\fi1​ radiative rate of 22\ifmmode\pm\else\textpm\fi{}0.5 \ensuremath{\mu}s is also obtained. The dependence of the rates upon buffer pressure demonstrated the presence of both linear and quadratic terms, due to two- and three-body reactions, respectively. The behavior of the binary rate coefficients with varying noble-gas species and temperature is qualitatively consistent with St"uckelberg's model for noncrossing levels.
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Collisional JJJ-mixing rates between the J=0,1,2J=0, 1, 2J=0,1,2 states of the Sr(53PJ)\mathrm{Sr}(5^{3}P_{J})Sr(53PJ​) multiplet have been measured in the presence of rare-gas perturbers. For Kr and Xe pressures above 100 Torr, the rates are dominated by a component which is quadratic in pressure. We believe this very unusual behavior is brought about by the simultaneous interaction of a Sr atom with two perturbers and not by excimer formation.
An experimental study is reported on the dynamics of the set of energy states below the 5s5p(1P1) state of atomic strontium, namely 1D2, 3DJ, 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(3DJ) 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(3P0,1,2)] 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 3DJ) with negligible yields of both Sr(1D2) and Sr(5 3PJ). The model uses relative values of Einstein coefficients from the averaged Sr(5 3DJ) state to any of the three Sr(5 3P0,1,2) 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 3P0,1,2), is shown to fit well the observed population profiles and also shows collisional propensity behaviour.
We present a time resolved study of excitation transfer processes between the low lying excited states of barium (6s6p 3^{3}3PJ{\mathit{P}}_{\mathit{J}}PJ​, 6s5d 1^{1}1D2{\mathit{D}}_{2}D2​, 6s5d 3^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​) induced by collisions with noble gases. The transient excited state populations have been measured by means of fluorescence and absorption spectroscopy. Cross sections for spin-changing collisions from 3^{3}3P1{\mathit{P}}_{1}P1​ to 1^{1}1D2{\mathit{D}}_{2}D2​ and from 1^{1}1D2{\mathit{D}}_{2}D2​ to 3^{3}3D3{\mathit{D}}_{3}D3​ as well as for fine-structure mixing within the 3^{3}3D multiplet were obtained by fitting the numerical solution of a seven-state system of rate equations including diffusion to the experimental data. We compare our results to experimental findings for other alkaline earth metals and report an improved value of the lifetime of the 6s6p 3^{3}3P1{\mathit{P}}_{1}P1​ state of barium.
Measurements of the mixing rates and cross sections for collisional excitation transfer between the 5P 1/2 and 5P 3/2 states of rubidium (Rb) in the presence of 4 He buffer gas are presented. Selected pulses from a high repetition rate, mode-locked femtosecond laser are used to excite either Rb state with the fluorescence due to collisional excitation transfer observed by time-correlated single-photon counting. The time dependence of this fluorescence is fitted to the solution of rate equations which include the mixing rate, atomic lifetimes and any quenching processes. The variation in the mixing rate over a large range of buffer gas densities allows the determination of both the binary collisional transfer cross section and a three-body collisional transfer rate. We do not observe any collisional quenching effects at 4 He pressures up to 6 atm and discuss in detail other systematic effects considered in the experiment.
Energy pooling (EP) has been observed in Sr vapor following pulsed optical excitation to the 53{}^{3}3P1{P}_{1}P1​ state. From the time-dependent radiative decay of 63{}^{3}3S1{S}_{1}S1​ and of 61{6}^{1}61S0{\mathit{S}}_{0}S0​ via the cascade resonance line, the EP rate coefficients have been investigated for the following: Sr(5 3PJ^{3}P_{J}3PJ​)+Sr(53{}^{3}3PJ′{\mathit{P}}_{\mathit{J}\mathcal{'}}PJ′​) (51{(5}^{1}(51S0{S}_{0}S0​) +Sr(6 3S1^{3}S_{1}3S1​ or 61{6}^{1}61S0{S}_{0}S0​). The values of k11{k}_{11}k11​ have been measured, as well as kJJ′{k}_{\mathrm{JJ}\mathcal{'}}kJJ′​ averaged over a thermal distribution of 5 3PJ^{3}P_{J}3PJ​ populations. The time-dependent Sr(53{5}^{3}53PJ{P}_{J}PJ​) densities were measured by absorption of lines from a Sr lamp crossing the excitation region. The rate coefficients are surprisingly large; in spite of the small statistical weights of these product states, they are nearly gas-kinetic. The dependence of the rates upon spin, J, and energy defect \ensuremath{\Delta}E has also been examined.