An optical echo study of caesium-noble gas collisions

Godfrey, Keith William (1989). An optical echo study of caesium-noble gas collisions. PhD thesis The Open University.



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.

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