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Physical Review A

Laser-induced optical pumping measurements of cross sections for fine- and hyperfine-structure transitions in sodium induced by collisions with helium and argon atoms

Chris C. Dobson, Chi C. Sung

Published May 1, 1999• 3 citations
Publisher

Abstract

Optical pumping of the ground states of sodium can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating measurements of temperature, pressure, etc., which are based on these spectra. Modeling of the fluorescence using rate equations for the eight hyperfine states of the sodium D manifolds can be used to quantify the contribution to the ground state pumping of transitions among the hyperfine excited states induced by collisions with buffer gas atoms. This model is used here to determine, from the shape of experimental spectra, cross sections for $\ensuremath{\Delta}F$ transitions of the ${P}_{3/2}$ state induced by collisions with helium and argon atoms, for a range of values assumed for the ${P}_{1/2}, \ensuremath{\Delta}F$ cross sections. The hyperfine cross sections measured using this method, which to our knowledge is novel, are compared with cross sections for transitions involving polarized magnetic substates ${m}_{F}$ measured previously using polarization sensitive absorption. Also, fine-structure transition $(\ensuremath{\Delta}J)$ cross sections were measured in the pumped vapor, giving agreement with previous measurements made in the absence of pumping.

DOI
10.1103/physreva.59.3402

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Mixture and Quenching of the Rb (5P_J) States Induced by Collisions with Helium Atoms

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Applying the cw laser absorption and fluorescence method,the cross sections for the fine structure mixing and quenching of the Rb(5PJ) state,induced by collisions with He atoms have been measured,Rubidium atoms were optically excited to 5 P3/2states.The excited atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single mode laser beam,tuned to the 5PJ→ 7S1/2 transitions.Transmission factors were calculated for the 5PJ→ 5S transitions.The fluorescence intensity I795 of the sensitized 5P1/2→ 5S1/2emission was measured as a function of He density in the range 0.5×1017 N 4× 1017cm-3at constant temperature T = 340 K.It was found that the quantity N/ I795 exhibited a parabolic dependence on N,confirming that the quenching of the 5PJ states is due to collision with He atoms,not due to Rb ground state atoms.The coefficients of the second-order polynomial fitted through the measured date yielded the cross sectionsσ3/2→1/2=(1.84± 0.61)× 10-17cm2 andσD=(1.07± 0.30)× 10-17cm2 for the 5PJfine-structure mixing and quenching,respectively,due to collision with He atoms.

Shinjang dashösi ilmiy jurnili•Jan 1, 2007•0 citations
Optical pumping of the ground states of sodium can radically alter the shape of the laser-induced fluorescence excitation spectrum, complicating measurements of temperature, pressure, etc., which are based on these spectra. Modeling of the fluorescence using rate equations for the eight hyperfine states of the sodium D manifolds can be used to quantify the contribution to the ground state pumping of transitions among the hyperfine excited states induced by collisions with buffer gas atoms. This model is used here to determine, from the shape of experimental spectra, cross sections for \ensuremathΔF\ensuremath{\Delta}F\ensuremathΔF transitions of the P3/2{P}_{3/2}P3/2​ state induced by collisions with helium and argon atoms, for a range of values assumed for the P1/2,\ensuremathΔF{P}_{1/2}, \ensuremath{\Delta}FP1/2​,\ensuremathΔF cross sections. The hyperfine cross sections measured using this method, which to our knowledge is novel, are compared with cross sections for transitions involving polarized magnetic substates mF{m}_{F}mF​ measured previously using polarization sensitive absorption. Also, fine-structure transition (\ensuremathΔJ)(\ensuremath{\Delta}J)(\ensuremathΔJ) cross sections were measured in the pumped vapor, giving agreement with previous measurements made in the absence of pumping.
The cross sections for the excitation energy transfer in the collisional processes Na*(3P 1 2/)+M(nS 1 2/) to or from Na*(3p 3 2/)+m(Ns 3 2/), m being ground-state Rb (n=5) or Cs (n=6) atom, have been measured by laser fluorescence method. With either of the two sodium 3P fine-structure states optically excited, the intensity ratio of the sensitized to direct fluorescence was measured. The polarization effects due to partially polarized and anisotropic direct fluorescence were investigated. The measured cross sections (T=530 K) for the sodium 3P fine-structure excitation transfer induced by collisions with Rb are: sigma ( 1 / 2 to 3 / 2 )=170 AA 2 , with an overall accuracy of 20%. The corresponding values in the Cs case are: sigma ( 1 / 2 to 3 / 2 )=140 AA 2 , sigma ( 3 / 2 to 1 / 2 )=83 AA 2 . The theoretical calculations reveal the decisive role of the dipole-quadrupole interaction. The obtained values are in satisfactory agreement with the experiment only if both the dipole-dipole and dipole-quadrupole interactions are taken into account, while the calculations involving the dipole-dipole term alone yield the cross section values which are as much as one order of magnitude lower than the experimentally observed ones.
Cross sections for 6^D, 7^D and 8^0 fine-structure mixing in Rb, induced in collisions with various ground-state atoms and N2 molecules, have been determined using methods of atomic fluorescence.Rb vapour, pure or mixed with a buffer gas, was irradiated in a glass fluorescence cell with pulses of radiation from a N2 laser-pumped dye laser, populating a fine-structure state by two-photon absorption.The resulting fluorescence included a direct component emitted in the decay of .the optically excited state and a sensitized component due to the col1isional1y populated fine-structure state.Measure ments of relative intensities of the two components in relation to buffer gas (or Rb vapour) pressure yielded the following cross sections.-14 2 Inelastic Cross sections (10 cm ) process Rb He Ne Ar Kr Xe I would like to express sincere thanks to Professor L.
The cross sections for excitation transfer between the 62P fine structure levels of rubidium, induced in collisions with ground state rubidium atoms, have been measured using a nitrogen laser pumped dye laser as the optical excitation source in a fluorescence experiment. Rubidium vapor was irradiated with each component of the 2P rubidium doublet in turn, and measurements of the relative intensities of fluorescence yielded the following cross sections: [Formula: see text] These results are consistent with the empirical relationship between the magnitude of the cross sections and the fine structure splitting that has previously been established for the alkalis.
We report the results of an experimental study of the effects of velocity-changing collisions on two-photon and stepwise-absorption line shapes. Excitation spectra for the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}\ensuremath{\rightarrow}4{D}_{\frac{1}{2}}$ transitions of sodium atoms undergoing collisions with foreign gas perturbers are obtained. These spectra are obtained with two cw dye lasers. One laser, the pump laser, is tuned 1.6 GHz below the $3{S}_{\frac{1}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ transition frequency and excites a nonthermal longitudinal velocity distribution of excited 3P123{P}_{\frac{1}{2}}3P21​​ atoms in the vapor. Absorption of the second (probe) laser is used to monitor the steady-state excited-state distribution which is a result of collisions with rare gas atoms. The spectra are obtained for various pressures of He, Ne, and Kr gases and are fit to a theoretical model which utilizes either the phenomenological Keilson-St\"orer or the classical hardsphere collision kernel. The theoretical model includes the effects of collisionally aided excitation of the 3P123{P}_{\frac{1}{2}}3P21​​ state as well as effects due to fine-structure state-changing collisions. Although both kernels are found to predict line shapes which are in reasonable agreement with the experimental results, the hard-sphere kernel is found superior as it gives a better description of the effects of large-angle scattering for heavy perturbers. Neither kernel provides a fully adequate description over the entire line profile. The experimental data is used to extract effective hard-sphere collision cross sections for collisions between sodium 3P123{P}_{\frac{1}{2}}3P21​​ atoms and helium, neon, and krypton perturbers.
We report on the measurement of the linear polarization degree of the 3s2S1/23s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}3s2S1/2​ $\ensuremath{\rightarrow}$ 3p2P3/23p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{3/2}3p2P3/2​ $\ensuremath{\rightarrow}$ 5s2S1/25s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}5s2S1/2​ transition in atomic sodium. A linearly polarized pump laser creates an alignment in the excited 23{}^{23}23Na 3p2P3/23p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{3/2}3p2P3/2​ level. From the spectra, we extracted the inelastic collision cross section for the disalignment of the excited atoms by collisions with ground-level argon or xenon atoms. Measurements were done by monitoring the intensity of the cascade fluorescence from the 4p2P1/24p{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{P}_{1/2}4p2P1/2​ $\ensuremath{\rightarrow}$ 3s2S1/23s{\phantom{\rule{0.16em}{0ex}}}^{2}\phantom{\rule{-0.16em}{0ex}}{S}_{1/2}3s2S1/2​ transition at 330.29 nm, in the presence of Ar and Xe for pressures ranging from 5.3 to 146.6 mbar. We demonstrated that the polarization measurement can provide important information about the alignment-dependent inelastic process in alkali-metal--rare-gas collisions. A depolarization cross-section value of 196 (22) \AA{}2{}^{2}2 for Na-Ar and 357 (21) \AA{}2{}^{2}2 for Na-Xe are obtained.
Applying the cw laser absorption and fluorescence method,the cross sections for the fine structure mixing and quenching of the Rb(5PJ) state,induced by collisions with He atoms have been measured,Rubidium atoms were optically excited to 5 P3/2states.The excited atom density and spatial distribution were mapped by monitoring the absorption of a counterpropagating single mode laser beam,tuned to the 5PJ→ 7S1/2 transitions.Transmission factors were calculated for the 5PJ→ 5S transitions.The fluorescence intensity I795 of the sensitized 5P1/2→ 5S1/2emission was measured as a function of He density in the range 0.5×1017 N 4× 1017cm-3at constant temperature T = 340 K.It was found that the quantity N/ I795 exhibited a parabolic dependence on N,confirming that the quenching of the 5PJ states is due to collision with He atoms,not due to Rb ground state atoms.The coefficients of the second-order polynomial fitted through the measured date yielded the cross sectionsσ3/2→1/2=(1.84± 0.61)× 10-17cm2 andσD=(1.07± 0.30)× 10-17cm2 for the 5PJfine-structure mixing and quenching,respectively,due to collision with He atoms.