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Journal of Modern Optics

Collisional Depolarization in Caesium (6S 1/2 -7P 1/2) by Noble Gases Using Three Pulse Echoes

Alan V. Durrant, Joy Manners, William Thomason Dove

Published Jun 1, 1992• 0 citations
Publisher

Abstract

Three pulse echoes are used to measure the collisional destruction of Zeeman coherences in caesium (6S1/2−7P1/2) perturbed by low pressures of helium, argon and xenon. Anomalously large collision rates associated with the 6S1/2 ground state are obtained. It is suggested that the transfer of Zeeman coherences by spontaneous decay plays a role.

DOI
10.1080/713823536

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Three pulse echoes are used to measure the collisional destruction of Zeeman coherences in caesium (6S1/2−7P1/2) perturbed by low pressures of helium, argon and xenon. Anomalously large collision rates associated with the 6S1/2 ground state are obtained. It is suggested that the transfer of Zeeman coherences by spontaneous decay plays a role.
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.
Photon echoes generated from visible transitions in the noble gases helium, neon, and argon are observed and the collisional relaxation rates are measured. The photon echoes are excited on transitions initiating from metastable states which are populated by electron impact in an rf discharge. The photon-echo intensity is measured as a function of time after the discharge is turned off and as a function of gas pressure. The reduction of the echo intensity in these two experiments is attributed to relaxation of the metastable population in the afterglow and dephasing collisions, respectively. The dependence of the photon-echo damping time on transition wavelength and perturber gas is interpreted by means of a simple van der Waals model for the excited-atom and perturber-gas interaction.
An experimental investigation of the Cs 6s {sup 2}S{sub 1/2}{yields}6p {sup 2}P{sub 3/2}{yields}10s {sup 2}S{sub 1/2} double-resonance two-photon polarization spectrum has been made. In the investigation, a linear polarization degree was measured to extract the alignment depolarization cross section in the cesium 6p {sup 2}P{sub 3/2} level due to collisions between ground-level argon and excited 6p {sup 2}P{sub 3/2} cesium atoms over the Zeeman coherences. The spectra show strong depolarization of the Cs 6p {sup 2}P{sub 3/2} level. The measured alignment depolarization cross section value of 186 (58) A{sup 2} is in good agreement with the experiment and theoretical predictions.