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Physical Review A

Energy-pooling collisions in barium

Jonathan Andrew Neuman, Alan C. Gallagher, J. Cooper

Published Aug 1, 1994• 20 citations
Publisher

Abstract

Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p${(}^{3}$${\mathit{P}}_{1}$) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s${(}^{1}$D) and 5d6s${(}^{3}$D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.

DOI
10.1103/physreva.50.1292

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Physical Review A•Dec 1, 1994•26 citations
Rate coefficients for energy-pooling collisions between low-lying excited states of Ba have been measured. The 6s6p{(}^{3}$${\mathit{P}}_{1}) level in Ba is pumped by a cw diode laser, and radiative decay and collisions with buffer-gas atoms also populate the 5d6s(1{(}^{1}(1D) and 5d6s(3{(}^{3}(3D) metastable levels. The densities of these low-lying excited states are measured by the absorption of lines from a Ba hollow-cathode lamp, and the states populated by energy-pooling collisions are studied by comparing their fluorescence intensity to that of the laser-excited level. The rate coefficients are on the order of gas-kinetic rates, and they are not strongly dependent on energy defect, spin changes, or angular-momentum changes.
Energy-pooling collisions between low-lying excited states of Ba have been studied. We have used a CW dye laser to excite the level in Ba. Radiative decay and collisions with Ba or buffer-gas atoms (Ar, He) populate also the , and metastable levels. The spatial density distributions of these excited levels were measured by the absorption of lines from a Ba hollow-cathode lamp as a function of laser frequency. When the laser was on the exact atomic resonance we observed fluorescence from the , , , , , , and states populated by energy-pooling collisions of Ba( , ). By changing the buffer gas pressure the fluorescence intensity ratio between - and - transitions changes dramatically. This is found to be due to redistribution of population among metastable levels, both in magnitude and space. In addition, we have found that the and metastable levels are also populated when the laser is tuned to the blue from the 553.5 nm line under which conditions a pronounced red-shifted light cone (cone emission) is observed.
We report experimental rate coefficients for the energy-pooling collisions Cs(6P1/2{\mathit{P}}_{1/2}P1/2​)+Cs(6P1/2{\mathit{P}}_{1/2}P1/2​)\ensuremath{\rightarrow}Cs(6S1/2{\mathit{S}}_{1/2}S1/2​)+Cs(${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$) and Cs(6P3/2{\mathit{P}}_{3/2}P3/2​)+Cs(6P3/2{\mathit{P}}_{3/2}P3/2​)\ensuremath{\rightarrow}Cs(6S1/2{\mathit{S}}_{1/2}S1/2​)+Cs(${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$) where ${\mathit{nl}}_{\mathit{J}\ensuremath{'}}$=7P1/2{\mathit{P}}_{1/2}P1/2​, 7P3/2{\mathit{P}}_{3/2}P3/2​, 6D3/2{\mathit{D}}_{3/2}D3/2​, 6D5/2{\mathit{D}}_{5/2}D5/2​, 8S1/2{\mathit{S}}_{1/2}S1/2​, 4F5/2{\mathit{F}}_{5/2}F5/2​, or 4F7/2{\mathit{F}}_{7/2}F7/2​. Atoms were excited to either the 6P1/2{\mathit{P}}_{1/2}P1/2​ or 6P3/2{\mathit{P}}_{3/2}P3/2​ state using a single-mode Ti:sapphire laser. The excited-atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single-mode ring dye laser beam, tuned to either the 6P1/2{\mathit{P}}_{1/2}P1/2​\ensuremath{\rightarrow}8S1/2{\mathit{S}}_{1/2}S1/2​ or 6P3/2{\mathit{P}}_{3/2}P3/2​\ensuremath{\rightarrow}7D3/2,5/2{\mathit{D}}_{3/2,5/2}D3/2,5/2​ transitions, which could be translated parallel to the pump beam. Transmission factors, which describe the average probability that photons emitted within the fluorescence detection region can pass through the optically thick vapor without being absorbed, were calculated for all relevant transitions. Effective lifetimes of levels populated by energy-pooling collisions are modified by radiation trapping, and these factors were calculated using the Molisch theory. These calculated quantities have been combined with the measured excited-atom densities and fluorescence ratios to yield absolute energy-pooling rate coefficients. It was found that the rate for production, in all cases, is greatest for 6D, but that 1/2-1/2 collisions are significantly more efficient than 3/2-3/2 collisions for populating 7P. It was also found that 7P1/2{\mathit{P}}_{1/2}P1/2​ is populated two to three times more efficiently than 7P3/2{\mathit{P}}_{3/2}P3/2​ in 1/2-1/2 collisions, but that the 7P fine-structure levels are approximately equally populated in 3/2-3/2 collisions. \textcopyright{} 1996 The American Physical Society.
Time-resolved measurements of fluorescence and level populations have been made on barium vapour illuminated by an intense pulse of laser radiation (1 MW cm -2 , 50 ns FWHM) tuned to the resonance transition of Ba I, 6s 2 1 S 0 -6s6p 1 P 1 , at 5535 AA. Barium number densities between 10 13 -10 15 cm -3 were investigated, using a buffer gas of helium at 100 Torr. Fluorescence was measured with two different geometrical arrangements, one allowing partial spatial resolution. In neither case did the observed fluorescence saturate with increasing laser power, although the maximum flux was more than sufficient to burn through the vapour column, as confirmed by transmission measurements. The power loss from the laser beam was found to be compatible with the scattered radiation from the pumped column in the absence of radiation trapping. In contrast to other similar experiments, no significant ionisation was detected (<5%). Time-resolved population measurements by anomalous dispersion on the 6s6p 3 P 2 and 6s5d 1 D 2 metastable levels yielded quenching cross sections for both Ba-Ba and Ba-He collisions, but gave anomalous result for the peak populations. A qualitative explanation of the fluorescence anomaly is proposed on the basis of the AC Stark effect, and it is shown that the same effect can inhibit the onset of stimulated emission to the metastable D levels.
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.
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.
We describe a set of experiments that investigate excitation transfer among the barium 6s5d 3^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ metastable levels due to collisions with argon perturber atoms, and quenching of atoms in these levels by nitrogen perturber molecules. The metastable levels were populated through optical pumping of the 6s2{\mathit{s}}^{2}s2 1^{1}1S0{\mathit{S}}_{0}S0​\ensuremath{\rightarrow}6s6p 3^{3}3P1o{\mathit{P}}_{1}^{\mathit{o}}P1o​ intercombination transition with a pulsed laser, followed by stimulated emission or stimulated Raman scattering into the 6s5d 3^{3}3D1,2{\mathit{D}}_{1,2}D1,2​ levels. Collisional mixing then distributed population throughout the 3^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ levels. Time-dependent absorption coefficients were obtained by measuring the transmission of a weak cw probe laser beam (after the pulsed laser fired) at several perturber number densities. Argon buffer gas was used to investigate fine-structure mixing, and nitrogen was used to study quenching. The time-dependent absorption coefficients were fitted to appropriate fitting functions, obtained by solving the set of coupled rate equations used to model the time evolution of the metastable level populations. The buildup rates of the initially unpopulated 3^{3}3D3{\mathit{D}}_{3}D3​ and underpopulated 3^{3}3D1{\mathit{D}}_{1}D1​ levels were used to determine the collisional mixing rates, while the measured decay rates of the 3^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ level populations yielded the quenching rates. From plots of buildup rates versus argon number density for the 3^{3}3D1,3{\mathit{D}}_{1,3}D1,3​ states, we were able to determine four of the six fine-structure mixing rate coefficients kij{\mathit{k}}_{\mathit{i}\mathit{j}}kij​.The results are k12{\mathit{k}}_{12}k12​=(1.71\ifmmode\pm\else\textpm\fi{}0.18), k21{\mathit{k}}_{21}k21​=(1.39\ifmmode\pm\else\textpm\fi{}0.13), k23{\mathit{k}}_{23}k23​=(1.42\ifmmode\pm\else\textpm\fi{}0.13), and k32{\mathit{k}}_{32}k32​=(1.92\ifmmode\pm\else\textpm\fi{}0.16)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}13}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. The experiment is not sensitive to the values of k13{\mathit{k}}_{13}k13​ and k31{\mathit{k}}_{31}k31​. Estimated rate coefficients for excitation transfer among the 3^{3}3DJ{\mathit{D}}_{\mathit{J}}DJ​ levels due to collisions with barium atoms are k21Ba{\mathit{k}}_{21}^{\mathrm{Ba}}k21Ba​=1.3\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and k23Ba{\mathit{k}}_{23}^{\mathrm{Ba}}k23Ba​=5.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}10}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. A plot of the population decay rates versus N2{\mathrm{N}}_{2}N2​ number density for each of the three metastable levels yielded the average nitrogen quenching rate coefficient kq{\mathit{k}}_{\mathit{q}}kq​=(3.14\ifmmode\pm\else\textpm\fi{}0.19)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}11}$ cm3{\mathrm{cm}}^{3}cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$.