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.
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.
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.
Electronically excited strontium atoms in the low-lying 5s5p(3PJ) state, 1.8 eV above the 5 s2(1S0) ground state, have been studied in the time-resolved mode by recording the forbidden emission at λ= 689.3 nm [Sr(53P1)→ Sr(51S0)+hν]. Sr(53P1) was generated by the repetitive pulsed dye-laser excitation of ground-state strontium atoms, in equilibrium with the solid at elevated temperatures, in a slow-flow system kinetically equivalent to a static system. The resulting resonance fluorescence at λ= 689.3 nm, following rapid Boltzmann equilibration within the 53PJ spin-orbit manifold, was then monitored by means of boxcar integration. Particular consideration was given to the development of a pre-trigger photomultiplier gating system which permitted decay measurements necessarily to be made on time scales considerably shorter than achieved hitherto with this type of investigation. Measurement of the fluorescence decays in the noble gases and across a range of atomic strontium concentration revealed significant effects due to radiation trapping. Extrapolation of the decay data to [Sr(51S0)]= 0 yielded the mean radiative lifetime τe= 20.1 ± 0.4 µs for the λ= 689.3 nm transition. This is compared with the range of values reported previously. The following upper limits are reported for the collisional-quenching constants of Sr(53PJ) by the noble gases (kQ/cm3 atom–1 s–1, T= 950 K): He, <5.5 × 10–15; Ne, <3.5 × 10–15; Ar, <3.9 × 10–15; Kr, <5.5 × 10–15 and Xe, <3.1 × 10–15, which are compared with data derived from the phase-shift technique. A limit is also reported of the rate constant for quenching of Sr(53PJ) by Sr(51S0) of <2 × 10–13 cm3 atom–1 s–1(T= 850–1000 K). The quenching of Sr(53PJ) by H2 and D2 is studied in detail, yielding the following results: kH2= 3.7 ± 0.4 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ), kD2= 3.0 ± 0.2 × 10–12 cm3 molecule–1 s–1(950 K, 1 σ). The results for these two isotopes are compared with analogous data reported from other measurements for Ca(43PJ) and Mg(33PJ).
Inert-gas-induced collisional transfer between the first four excited states of Sr has been measured by determining excited-state densities following pulsed laser excitation of the 5 1P state. We observe 5 1P collisional transfer exclusively to the 4 3D state, with (\ensuremath{\sim}800 K) rate coefficients ranging from 0.24 to 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for the various gases. We also measure 4 1D2 transfer primarily to the 4 3D state, with rate coefficients of 0.65 to 6.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The 4 3D state radiative lifetime is observed to be 2.9\ifmmode\pm\else\textpm\fi{}0.2 \ensuremath{\mu}s, collisional mixing between the 4 3Dj fine-structure levels is essentially gas kinetic (k\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$), and k${10}^{\mathrm{\ensuremath{-}}13}$ cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ for 4 3D\ensuremath{\rightarrow}5 3P collisional transfer.
The kinetics of Ca(4 3PJ), Sr(5 3PJ) and Sr(6 3S1) have been studied by time-resolved emission following pulsed dye-laser excitation. Ground-state calcium atoms, Ca[4s2(1S0)], in the presence of Sr[5s2(1S0)], were optically excited to the first electronically excited state, Ca[4s4p(3P1)], by means of a dye-laser operating at λ= 657.3 nm [Ca(4 3P1)â†� Ca(4 1S0)]. Following rapid Boltzmann equilibration within the Ca(43PJ) manifold, fast, near-resonant energy transfer took place with Sr(5 1S0) to yield Sr(5 3PJ), together with energy pooling to yield Sr(6 3S1). The following transitions were monitored in the time-resolved mode using pre-trigger photomultiplier gating and boxcar integration with repetitive pulsing on a slow-flow system, kinetically equivalent to a static system: Ca(4 3P1)→ Ca(4 1S0)+hν(λ= 657.3 nm), Sr(5 3P1)→ Sr(5 1S0)+hν(λ= 689.3 nm) and Sr(6 3S1)→ Sr(5 3P0)+hν(λ= 679.1 nm). Detailed measurements of the time dependence of the emission profiles at λ= 657.3 and 689.3 nm demonstrated the rapid establishment of the equilibrium Ca(4 3PJ)+ Sr(5 1S0)⇌ Ca(4 1S0)+ Sr(5 3PJ) and its sustenance throughout kinetic decays. A further comparison of the first-order decay coefficients for the decays of Ca(4 3PJ) and Sr(5 3PJ) with those for Sr(6 3S1) demonstrated energy pooling between Ca(4 3PJ)+ Sr(5 3PJ) and Sr(5 3PJ)+ Sr(5 3PJ). Finally, the measured first-order decay coefficients, k′, for Ca(4 3PJ) and Sr(5 3PJ) in chemical equilibrium with one another are seperated quantitatively into their component parts of spontaneous emission, diffusion, the equilibrium constant connecting the 3PJ states and collisional quenching rate constants. Rate constants for quenching of Ca(4 3PJ) and Sr(5 3PJ) by Kr, Xe, H2 and D2 derived from earlier measurements on the individual atoms are found to be quantitatively consistent with the decay profiles under equilibrium conditions.
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 53P${\mathrm{\ifmmode^\circ\else\textdegree\fi{}}}_{2}$, 0\ensuremath{\rightarrow}6 3S1 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.
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 1P1 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;1S0 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.