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Journal of Physics B Atomic and Molecular Physics

Collisional disalignment of caesium Rydberg states

Leslie Pendrill, George William Series

Published Dec 14, 1978• 6 citations
Publisher

Abstract

Cross sections for the disalignment of excited caesium atoms, by collision with atoms in ground states, have been determined by studying the damping of quantum beats in fluorescence. Atoms in a sealed cell were excited selectively to 122D3/2 and 132S3/2 by a pulsed tunable laser and discharge lamp. Alignment was induced by linear polarisation of the exciting light and monitored correspondingly. The quantum beats originate in the coherent excitation of hyperfine states whose structures have been determined in previous work. Collisional damping is investigated studying the quantum-beat signal over a range of atomic densities. The fluorescence contains a component representing the decay of population as well as of alignment, and extraction of the alignment cross sections relies on previous measurements of the depopulation cross sections. The results for disalignment cross sections are: sigma (2)(122D3/2)=4.1+or-0.8*10-16 m2, sigma (2)(132D3/2)=5.3+or-0.9*10-16 m2, about three times larger than the depopulation cross sections. Radiative lifetimes obtained from the analysis agree with previous determinations.

DOI
10.1088/0022-3700/11/23/015

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Cross sections for the disalignment of excited caesium atoms, by collision with atoms in ground states, have been determined by studying the damping of quantum beats in fluorescence. Atoms in a sealed cell were excited selectively to 122D3/2 and 132S3/2 by a pulsed tunable laser and discharge lamp. Alignment was induced by linear polarisation of the exciting light and monitored correspondingly. The quantum beats originate in the coherent excitation of hyperfine states whose structures have been determined in previous work. Collisional damping is investigated studying the quantum-beat signal over a range of atomic densities. The fluorescence contains a component representing the decay of population as well as of alignment, and extraction of the alignment cross sections relies on previous measurements of the depopulation cross sections. The results for disalignment cross sections are: sigma (2)(122D3/2)=4.1+or-0.810-16 m2, sigma (2)(132D3/2)=5.3+or-0.910-16 m2, about three times larger than the depopulation cross sections. Radiative lifetimes obtained from the analysis agree with previous determinations.
Measurements have been made of time-resolved fluorescence from n 2 S 1/2 and n 2 D 3/2 states of 133 Cs (n=8 to 14) over a range of vapour densities covering the onset of collisional depopulation. Lifetimes for the free atom and depopulation cross sections are deduced. Lifetimes are proportional to (n*) x with x=2.52+or-0.06 (S states) and 2.62+or-0.06 (D 3/2 states). Cross sections are proportional to (n*) 4 . Hyperfine-structure interaction constants for the 2 D 3/2 states determined by the method of quantum beats agree with the recent results of Belin et al. (1976).
The total cross sections for disorientation (σ1) and disalignment (σ2) of 42P3/2 potassium atoms, induced in collisions with the ground-state atoms, have been determined using a modified Zeeman scanning method. Potassium vapor at densities of the order of 1011 cm−3, contained in a fluorescence cell located in a kilogauss variable magnetic field, was irradiated with circularly polarized 7665 Å resonance radiation emitted from a discharge lamp located in a constant field of 5.4 kG. Scans of the variable field permitted selective excitation of single Zeeman components in the absorbing vapor. The σ+ and σ− components of the resulting resonance fluorescence emitted parallel to the scanning field were monitored in relation to the vapor density, as were the π and σ components emitted in the perpendicular direction. As the vapor density increased so did the frequency of the collisions which caused transfers among the Zeeman states in the vapor and thus disorientation and disalignment. The observed dependence of circular and linear depolarization of the fluorescence on the potassium density yielded the cross sections σ1 = 17 × 10−12 cm2 and σ2 = 21 × 10−2 cm2, corrected for imprisonment of radiation.
Applying the cw laser fluorescence method, the cross sections for the fine structure mixing and quenching of the Cs 6P state, induced by collisions with ground-state caesium atoms and molecules, have been measured. Caesium atoms were optically excited to the 5D J states via quadrupole-allowed 6S 1/2 5D J transitions, while the resonance states were populated by the radiative and collisional 5D J 6P J transitions. The relative populations of the Cs 6P J sublevels, as well as ratio of the 5D 3/2 to 6P 3/2 populations, were measured as a function of the caesium ground-state number density. From these measurements we obtained the cross section of (14±5) × 10 -16 cm 2 at T = 585 K for the process Cs(6P 1/2 ) Cs(6P 3/2 ) induced by collisions with ground-state caesium atoms. The applied experimental approach enabled the determination of the effective spontaneous rates for the 6P J states which are in agreement with the predictions of Holstein's theory. The cross sections for quenching of 6P J by caesium atoms and molecules were measured at T = 635 K and the obtained values are (1.6±1.4) × 10 -16 cm 2 and (1210±260) × 10 -16 cm 2 , respectively. Using recently calculated Cs*+Cs potentials we performed an analysis which shows a good agreement between the measured values and the theoretical estimates.
We measure the total collisional cross-sections of caesium Rydberg nD ( n =37–40) states by measuring the corresponding effective decay rates versus Rydberg atoms density in a conventional magneto-optical trap using the state-selective pulse field ionization technique. The results indicate that the large collisional cross-sections mainly come from the contribution of Rydberg atoms interactions. We also obtain the spontaneous radiative lifetimes of different Rydberg states by fitting experimental data, which are in agreement with the existing theoretical values within experimental error.
Collisional transfer of excitation energy from cadmium to caesium was investigated using methods of sensitized fluorescence. A mixture of cadmium and caesium vapors at low pressure was irradiated with Cd 3263.0 Å resonance radiation producing a population of Cd (53P1) atoms whose inelastic collisions with ground-state Cs atoms resulted in a transfer of excitation energy to a close-lying S, P, D and F states in caesium. The subsequent decay of these states manifested itself in the emission of sensitized fluorescence spectrum. Intensity measurements on the components of the spectrum by means of energy calibrated detector yielded some 50 cross-section values whose magnitudes range from 0.2 Å2 to 30.0 Å2 and which exhibit a pronounced resonances with ΔE the energy defect between Cd (53P1) and the appropriate level in caesium.
Measurements of the time-resolved fluorescence from some highly excited states of 133Cs atoms (n2D3/2, n2D5/2 and n2S1/2(n-4)2F; n=9 to 15) have shown multi-exponential behaviour which depends on vapour pressure. This behaviour is the result of mixing of population caused by collisions between the excited and ground-state atoms. The variation of the exponential rate constants with vapour density yields the free-atom lifetimes and collisional transfer cross sections of these excited states. An analogy is drawn between this variation and that of the energy eigenvalues of an atom under external perturbation.