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Physical Review A

Ionization of laser-excited strontium vapor: The dynamics of superelastic collisions with metastable populations

Catherine Bréchignac, Ph. Cahuzac, Anne Débarre

Published May 1, 1985• 11 citations
Publisher

Abstract

We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p ${}^{1}$${P}_{1}$ resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp${;}^{1}$${S}_{0}$ Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.

DOI
10.1103/physreva.31.2950

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Kinetic investigation of energy pooling from Sr [5s5p(3P J )] studied by time-resolved atomic emission following pulsed dye-laser excitation

David Husain, Jie Hong Lei

A kinetic investigation is presented of energy pooling arising from self-annihilation of the optically metastable state, Sr[5s5p(3PJ)], generated by pulsed dye-laser excitation of strontium vapour at λ= 689.3 nm {Sr[5s5p(3P1)]â†� Sr[5s2(1S0)]} at elevated temperature in the presence of excess helium buffer gas. Time-resolved emission was observed for a number of energy-pooled states, namely, Sr[5s5p(1P1)], Sr[5s6s(3S1)], Sr[4d5p(3F2, 3, 4)], Sr[4d5p(1D2)], Sr[5s6p(3P1, 2)], Sr[5s5d(3D2)] and Sr[5p2(3P2)]. This includes characterisation of the time profiles from specific spin–orbit states and significantly extends the range of atomic pooled states reported hitherto derived from the 5s5p(3PJ) energy store. Following rapid energy pooling, the profiles of the ‘pool’ and ‘store’ states are shown to be exponential in character and the first-order decay coefficients are found to be in the ratio of 2 ∶ 1 within experimental error. Further, using the appropriate sensitivity calibration of the optical system, the variation of the ratio of the integrated atomic emission intensities of the energy pooled states to that of the energy store itself were found to be linear and in accord with a mechanism for energy pooling in each case arising from 3PJ+3PJ self-annihilation. The normalised, integrated emission intensities of the energy-pooled states which involved population via endothermic processes from Sr(3PJ)+ Sr(3PJ) were determined and used to calculate the relative distributions of the energy-pooled states on collision. These were plotted semi-logarithmically as a function of the electronic state energy of the appropriate state and yielded a Boltzmann temperature higher than the ambient temperature employed, reflecting the role of atomic collisions with high kinetic energies on energy pooling.

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

Carolina Redondo, Marı́a N. Sánchez Rayo, José Andrés Fernández +2 more

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.

Physical Chemistry Chemical Physics•Jan 1, 2004•1 citation

Collisional energy transfer between excited Sr atoms

H. G. C. Werij, Martin A. Harris, J. Cooper +2 more

The following energy-transfer reactions have been observed in a Sr vapor following pulsed excitation of the 5 $^{1}$${\mathit{P}}_{1}$ state: ${\mathrm{Sr}}^{\mathrm{*}}$(5 $^{1}$${\mathit{P}}_{1}$)+${\mathrm{Sr}}^{\mathrm{*}}$(5 $^{1}$${\mathit{P}}_{1}$)\ensuremath{\rightarrow}Sr(5 $^{1}$${\mathit{S}}_{0}$)+${\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}$(${\mathit{n}}^{2\mathit{S}+1}$L)-\ensuremath{\Delta}E, ${\mathrm{Sr}}^{\mathrm{*}}$(4 $^{1}$${\mathit{D}}_{2}$) +${\mathrm{Sr}}^{\mathrm{*}}$(4 $^{1}$${\mathit{D}}_{2}$)\ensuremath{\rightarrow}Sr(5 $^{1}$${\mathit{S}}_{0}$)+${\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}$(${\mathit{n}}^{2\mathit{S}+1}$L)-\ensuremath{\Delta}E. The 4 $^{1}$${\mathit{D}}_{2}$ state was populated by a nonlinear stimulated process. By measuring the rate coefficients for these reactions for over 40 final states ${\mathit{n}}^{2\mathit{S}+1}$L, we look for insight into how these reactions depend on energy defect, spin, and angular momentum. It is found that there is not a very strong dependence on the energy defect, and that singlet and triplet final states are produced in comparable amounts. The final states tend to be populated more effectively as their angular momentum increases. Total rate coefficients are of the order of gas-kinetic rates.

Physical Review A•Mar 1, 1991•21 citations

Collision energy pooling for Sr(5 3 PJ )+Sr(5 J’3 ) → Sr(6 3,1 S)+Sr(5 1 S)

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

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.

Physical Review A•Aug 1, 1988•36 citations
We report on the observation and absolute measurements of the population dynamics for several Sr i and Sr ii levels when the vapor is illuminated by a short laser pulse. When the laser beam excites the 5s5p {}^{1}$${P}_{1} resonance level, we show that a pool of metastable population is created, allowing an efficient heating of the medium by superelastic collisions with seed electrons, and a strong final ionization is achieved. When the laser source excites the 5s10s 7thinsp{;}^{1}$${S}_{0} Rydberg level, superelastic collisions do not occur and a drastic change in the population behavior is observed. In both cases the dynamics of the energy transfer towards the plasma is detailed.
The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.
A kinetic investigation is presented of energy pooling arising from self-annihilation of the optically metastable state, Sr[5s5p(3PJ)], generated by pulsed dye-laser excitation of strontium vapour at λ= 689.3 nm {Sr[5s5p(3P1)]â†� Sr[5s2(1S0)]} at elevated temperature in the presence of excess helium buffer gas. Time-resolved emission was observed for a number of energy-pooled states, namely, Sr[5s5p(1P1)], Sr[5s6s(3S1)], Sr[4d5p(3F2, 3, 4)], Sr[4d5p(1D2)], Sr[5s6p(3P1, 2)], Sr[5s5d(3D2)] and Sr[5p2(3P2)]. This includes characterisation of the time profiles from specific spin–orbit states and significantly extends the range of atomic pooled states reported hitherto derived from the 5s5p(3PJ) energy store. Following rapid energy pooling, the profiles of the ‘pool’ and ‘store’ states are shown to be exponential in character and the first-order decay coefficients are found to be in the ratio of 2 ∶ 1 within experimental error. Further, using the appropriate sensitivity calibration of the optical system, the variation of the ratio of the integrated atomic emission intensities of the energy pooled states to that of the energy store itself were found to be linear and in accord with a mechanism for energy pooling in each case arising from 3PJ+3PJ self-annihilation. The normalised, integrated emission intensities of the energy-pooled states which involved population via endothermic processes from Sr(3PJ)+ Sr(3PJ) were determined and used to calculate the relative distributions of the energy-pooled states on collision. These were plotted semi-logarithmically as a function of the electronic state energy of the appropriate state and yielded a Boltzmann temperature higher than the ambient temperature employed, reflecting the role of atomic collisions with high kinetic energies on energy pooling.
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.
The following energy-transfer reactions have been observed in a Sr vapor following pulsed excitation of the 5 1^{1}1P1{\mathit{P}}_{1}P1​ state: Sr∗{\mathrm{Sr}}^{\mathrm{*}}Sr∗(5 1^{1}1P1{\mathit{P}}_{1}P1​)+Sr∗{\mathrm{Sr}}^{\mathrm{*}}Sr∗(5 1^{1}1P1{\mathit{P}}_{1}P1​)\ensuremath{\rightarrow}Sr(5 1^{1}1S0{\mathit{S}}_{0}S0​)+Sr∗∗{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}Sr∗∗(n2S+1{\mathit{n}}^{2\mathit{S}+1}n2S+1L)-\ensuremath{\Delta}E, Sr∗{\mathrm{Sr}}^{\mathrm{*}}Sr∗(4 1^{1}1D2{\mathit{D}}_{2}D2​) +Sr∗{\mathrm{Sr}}^{\mathrm{*}}Sr∗(4 1^{1}1D2{\mathit{D}}_{2}D2​)\ensuremath{\rightarrow}Sr(5 1^{1}1S0{\mathit{S}}_{0}S0​)+Sr∗∗{\mathrm{Sr}}^{\mathrm{*}\mathrm{*}}Sr∗∗(n2S+1{\mathit{n}}^{2\mathit{S}+1}n2S+1L)-\ensuremath{\Delta}E. The 4 1^{1}1D2{\mathit{D}}_{2}D2​ state was populated by a nonlinear stimulated process. By measuring the rate coefficients for these reactions for over 40 final states n2S+1{\mathit{n}}^{2\mathit{S}+1}n2S+1L, we look for insight into how these reactions depend on energy defect, spin, and angular momentum. It is found that there is not a very strong dependence on the energy defect, and that singlet and triplet final states are produced in comparable amounts. The final states tend to be populated more effectively as their angular momentum increases. Total rate coefficients are of the order of gas-kinetic rates.
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.