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Canadian Journal of Physics

Polarization transfer and relaxation induced in collisions of excited alkali atoms

E. I. Dashevskaya, E. E. Nikitin

Published Mar 15, 1976• 10 citations
Publisher

Abstract

We present an approximate theory of intramultiplet mixing, relaxation, and polarization transfer in excited alkali atoms, valid for all magnitudes of the fine-structure splitting. The theory is developed as the generalization of the previously postulated theory of fine-structure mixing induced in adiabatic and quasiresonant collisions. In the adiabatic region, the theory predicts a change in the sign of the coefficient for polarization transfer between the components j = 1/2 and,j = 3/2 of the 2P resonance doublet, as the Massey parameter for this transition changes.

DOI
10.1139/p76-079

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Sensitized Fluorescence in Vapors of Alkali Atoms. XIII. 62P1/2−62P3/2 Excitation Transfer in Rubidium, Induced in Collisions with Noble Gas Atoms

I. N. Siara, E. S. Hrycyshyn, L. Krause

The cross sections for excitation transfer between the 62P fine-structure substates in rubidium, induced in collisions with noble gas atoms, have been determined in a series of sensitized fluorescence experiments. Mixtures of rubidium vapor and noble gases at pressures varying in the range 0–5 Torr were irradiated with each component of the second 2P rubidium doublet in turn and the following cross sections for 2P mixing were obtained from measurements of sensitised-to-resonance fluorescent intensity ratios. Rb–He: Q12(2P1/2 → 2P3/2) = 29.3 Å2; Q21(2P1/2 ← 2P3/2) = 19.0 Å2. Rb–Ne: Q12 = 10.3 Å2; Q21 = 6.4 Å2. Rb–Ar: Q12 = 24.0 Å2; Q21 = 14.9 Å2. Rb–Kr: Q12 = 23.2 Å2; Q21 = 14.6 Å2. Rb–Xe: Q12 = 43.9 Å2; Q21 = 27.7 Å2 In their dependence on the magnitude of the fine-structure splitting, the values are consistent with previously determined cross sections for mixing in the first and third 2P doublets of alkali atoms.

Canadian Journal of Physics•Aug 15, 1972•19 citations
We present an approximate theory of intramultiplet mixing, relaxation, and polarization transfer in excited alkali atoms, valid for all magnitudes of the fine-structure splitting. The theory is developed as the generalization of the previously postulated theory of fine-structure mixing induced in adiabatic and quasiresonant collisions. In the adiabatic region, the theory predicts a change in the sign of the coefficient for polarization transfer between the components j = 1/2 and,j = 3/2 of the 2P resonance doublet, as the Massey parameter for this transition changes.
Fine-structure mixing cross-sections for the alkalis in collisions with the rare gases are reviewed. Included in the review are all the rare gases in collisions with all of the first excited state of the alkalis, the second excited state for K, Rb and Cs and the third excited state for Rb and Cs. The cross-sections are converted to probabilities for energy transfer using a quantum-defect calculated cross-section and are then presented as a function of adiabaticity. The data shows a clear decreasing trend with adiabaticity but secondary factors prevent the probabilities from decreasing as quickly as expected. Polarizability is introduced as a proxy for the secondary influences on the data as it increases with both rare gas partner and alkali excited state. The polarizability is shown to cause the probability of fine structure transition to be higher than expected. An empirical model is introduced and fit to the data. Future work will develop a model using time-independent perturbation theory in order to further develop a physical rational for the dependence of fine structure cross sections on adiabaticity and to further understand the secondary influences on the probability for fine structure transition.
The collision cross sections for alkali-metal--rare-gas spin orbit mixing between the ${n}^{2}{P}_{3/2}\ensuremath{\rightarrow}{n}^{2}{P}_{1/2}$ levels trend strongly with the Massey parameter, or adiabaticity of the collisions. The strength of the interaction, as characterized by the C6{C}_{6}C6​ dispersion coefficient, is a secondary influence on the rates. An analytic expression for the probability of energy transfer in alkali-metal--rare-gas collisions is derived using time-dependent perturbation theory. The model agrees well with a broad literature survey of the observed temperature-dependent cross sections. A simple interaction potential successfully organizes the alkali-metal--rare-gas database. The rates become very large for high-lying states, as the collisions are quite sudden and the radius of the valence electron is large. In contrast, the highly adiabatic cesium 62P{6}^{2}P62P mixing rates are six to eight orders of magnitude smaller. The mixing rate for the Rb-He diode pumped alkali laser system varies from $0.20--1.53\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}11}\mathrm{cm}{}^{3}/\mathrm{at}.\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ for T=279−−893KT=279--893\phantom{\rule{0.222222em}{0ex}}\mathrm{K}T=279−−893K.
Calculated potentials for the NaHe molecule are used to obtain collision-induced widths and shifts for the sodium DDD lines together with cross sections for fine-structure transitions and the relaxation of the multipole polarizations of the resonance levels. The atomic collision is treated in an adiabatic approximation in which the colliding atoms form a molecular system which rotates during the collision. We find the resonance lines to be broadened equally and to be approximately 70% wider, and with associated shifts of an order of magnitude smaller, than expected on the basis of van der Waals forces. The dependence of the cross sections for relaxation of the multipole polarizations of the resonance levels on the molecular-coupling conditions and the rotation of the system during collision is discussed and indicates a limitation on the sum rule for these cross sections. Good over-all agreement with the existing experimental data is obtained.
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.
This paper examines two collisional mechanisms by which cold alkali-metal atoms can escape from a neutral-atom trap, due to collison of excited- and ground-state atoms. One is fine-structure-changing collisions, by which the atoms are heated by the amount of the fine-structure splitting. The other produces hot ground-state atoms following emission of a red-shifted photon during the course of a collision. A rate expression is obtained that applies to both normal and ultracold temperatures (1 mK). This expression assumes a canonical distribution of initial states, low-intensity excitation, and a semiclassical treatment of the survival probability relative to excited-state decay during the long time of the ultracold collision, and uses fine-structure-changing probabilities found by quantum scattering calculations. The known properties of the attractive molecular states of the alkaline-metal dimers are used to identify and calculate the probabilities for the specific mechanisms. We conclude that the earlier semiclassical analysis by Dashevskaya [Opt. Spectrosk. 46, 423 (1979)] of the fine-structure-changing mechanisms for the various alkali-metal species is qualitatively correct. We present quantum-mechanical calculations of the rate coefficient for fine-structure-changing collisions between ground- and excited-state Cs atoms from 1000 K to 100 \ensuremath{\mu}K. Collision-rate coefficients for the trap-loss processes are calculated for pairs of Li, Na, K, Rb, and Cs atoms at low temperature. There is a wide variation of predicted loss rate among alkali-metal species from the fastest for K and the slowest for Li. Retardation corrections to molecular lifetimes must be taken into account to predict the correct rate coefficients for Na, K, and Rb. Good agreement is obtained with the observed trap loss rate in a Cs trap.
The cross sections for excitation transfer between the 62P fine-structure substates in rubidium, induced in collisions with noble gas atoms, have been determined in a series of sensitized fluorescence experiments. Mixtures of rubidium vapor and noble gases at pressures varying in the range 0–5 Torr were irradiated with each component of the second 2P rubidium doublet in turn and the following cross sections for 2P mixing were obtained from measurements of sensitised-to-resonance fluorescent intensity ratios. Rb–He: Q12(2P1/2 → 2P3/2) = 29.3 Å2; Q21(2P1/2 ← 2P3/2) = 19.0 Å2. Rb–Ne: Q12 = 10.3 Å2; Q21 = 6.4 Å2. Rb–Ar: Q12 = 24.0 Å2; Q21 = 14.9 Å2. Rb–Kr: Q12 = 23.2 Å2; Q21 = 14.6 Å2. Rb–Xe: Q12 = 43.9 Å2; Q21 = 27.7 Å2 In their dependence on the magnitude of the fine-structure splitting, the values are consistent with previously determined cross sections for mixing in the first and third 2P doublets of alkali atoms.