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Journal of Physics B Atomic and Molecular Physics

Collision cross sections for the noble-gas broadening of the Cs 6S-7P doublet using photon echoes

Alan V. Durrant, Joy Manners

Published Nov 14, 1984• 7 citations
Publisher

Abstract

Collisional cross sections and broadening constants are presented for each of the fine-structure components of the second resonance transition in caesium (6S 1 2/-7P 3 2,1/2/, 4555 AA, 4593 AA) perturbed by the five noble gases He-Xe. The results were obtained at perturber pressures of less than 0.5 Torr using the photon echo technique. The cross sections correspond closely with those obtained from spectral line profile measurements, but there are some significant discrepancies.

DOI
10.1088/0022-3700/17/21/003

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Annual Meeting Optical Society of America•Jan 1, 1985•1 citation
Collisional cross sections and broadening constants are presented for each of the fine-structure components of the second resonance transition in caesium (6S 1 2/-7P 3 2,1/2/, 4555 AA, 4593 AA) perturbed by the five noble gases He-Xe. The results were obtained at perturber pressures of less than 0.5 Torr using the photon echo technique. The cross sections correspond closely with those obtained from spectral line profile measurements, but there are some significant discrepancies.
The work described in this thesis is an experimental study of collisional relaxation of two-excitation-pulse optical echoes on the caesium 6S-7P transitions perturbed by noble gases using the echo polarisation rotation discrimination technique. Theoretical preliminaries include the development of a Schrddinger interaction representation theory of the formation of echoes resulting from two resonant excitation pulses applied to a two-state system. Rotation of the echo polarisation enables a polariser to be used to discriminate against the excitation pulses in favour of the echo. To illustrate the mechanisms behind this rotation, a simplified theory based on a nearly degenerate two level system is developed. The theory of collisional relaxation of echoes by phase interrupting and quantum diffractive velocity changing collisions is introduced. In traditional line profile spectroscopy the former type of collision cause line broadening whilst the effects of the latter are unobservable. The experimental work of this thesis involves the measurement of collisional relaxation of two-excitation-pulse echoes on the caesium 6Si/2-7Pi/2 (459nm) and 6S1/2-7P3/2 (455nm) transitions perturbed by low pressure (< 1 torr) of noble gas (He, A t or Xe). The experiments were carried out using a range of interpulse times from about 30 to 200ns. The broadening constants, determined to about 3% from the relaxation measurements, show significant discrepancies with traditional line profile measurements. For the longer interpulse times there is evidence for the quantum diffractive velocity changing aspects of the collisions. The associated cross-sections and average magnitudes of the mean velocity changes are estimated from the data.
Measurements are presented of the collisional relaxation of two-pulse echoes on the 455 nm and 459 nm D-lines of atomic caesium perturbed by low pressures of He, Ar and Xe using the echo-polarisation rotation technique, with interpulse times up to approximately 200 ns. The line broadening constants are measured to about 3% and show significant discrepancies with line profile measurements. For τ ⩾ 100ns there is evidence for the diffractive velocity-changing aspects of the collisions. The mean collisional velocity changes and the cross-sections for diffractive velocity-changing collisions are estimated from the data. The results are found to be insensitive to variations of axial magnetic fields and optical densities.
Cross sections for the transfer of excitation between fine-structure sublevels of the 9D state of caesium due to collisions with noble-gas atoms are measured. The experimental method employs stepwise excitation with a pulsed dye laser in the second step (6 2 P 3/2 to 9 2 D 5/2 ) and time-resolved detection of sensitised fluorescence 9 2 D 3/2 to 6 2 P 1/2 . The cross sections for the process Cs(9 2 D 3/2 )+A to Cs(9 2 D 5/2 )+A are (in units of 10 -14 cm 2 ) 7.6(1.5), 2.5(5), 2.7(5), 3.0(6) and 3.2(6) for A=He, Ne, Ar, Kr and Xe, respectively. For all the noble gases except helium the experimental cross sections are a few times smaller than the ones obtained in a recent semiclassical calculation.
A variant of the trilevel-echo effect is observed and utilized to measure the effective cross section for broadening of the $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition of sodium by the noble gases. The cross section measured here should be the same as the broadening cross section obtained from a direct measurement of the collisionally broadened $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition linewidth (if such a measurement were possible). The new echo, the "inverted-difference-frequency" (IDF) trilevel echo, is well suited to the study of transitions between excited states of the same parity. At 400 K the measured broadening cross sections are He 115(12) A˚2{\mathrm{\AA{}}}^{2}A˚2, Ne 120(12) A˚2{\mathrm{\AA{}}}^{2}A˚2, Ar 234(23) A˚2{\mathrm{\AA{}}}^{2}A˚2, Kr 266(27) A˚2{\mathrm{\AA{}}}^{2}A˚2, and Xe 311(31) A˚2{\mathrm{\AA{}}}^{2}A˚2. With He as the perturber, the cross section for broadening of the $3{P}_{\frac{1}{2}}\ensuremath{-}3{P}_{\frac{3}{2}}$ transition can be calculated from measured depolarization and fine-structure-changing collision cross sections. With the other perturbers, however, collisional phase changes appear to be important. An intuitive diagrammatic technique for the analysis of echoes is applied to the IDF trilevel echo.
A pressure-broadening study of three-photon and one-photon resonances was performed using counterpropagating, linearly polarized laser beams. Self-broadening, as well as the collision-induced effects due to the presence of a second noble gas, was investigated for the 6s, 6s′, 5d and 7s of xenon and the 5s′ of krypton. A multiphoton ionization scheme in a calibrated proportional counter is the basis of a new technique by which collisional processes can be investigated for high pressure regimes which have previously been inaccessible to investigation, particularly for self-broadening where the mean free path of the photons is of the order of 10−3 cm for pressures as low as 1 Torr. Using this technique, we have recorded the collision-induced widths and shifts of the resonance for pressures up to 1000 Torr. In the hundreds of torr range, asymmetric, self-broadening line shapes were observed with full widths and shifts which were greater than five times the laser bandwidth (0.002-0.005 nm).