a Three Pulse Optical Echo Study of Depolarising Collisions in Caesium.

Available from UMI in association with The British Library. The work described in this thesis is an experimental study, using three excitation pulse photon echoes, of collisional relaxation of caesium atoms perturbed by noble gases. The theoretical aspects of this thesis include the development of a density matrix theory to describe the formation of echoes formed from a sequence of three excitation pulses when applied to a multi-level system. The manner in which the resultant echoes store information about optical coherences, Zeeman coherences and state populations is discussed. The theory of collisional relaxation of three excitation pulse echoes by depolarising collisions and velocity changing collisions is introduced. Measurements of collision cross sections for depolarising collisions and velocity changing collisions for atoms in a single state are possible by the three excitation pulse echo techniques. The experimental work of this thesis involves the measurement of collisional relaxation of three pulse echoes formed on the caesium 6S_{1 over2} rightarrow 7P_ {3over2} (455 nm) and 6S _{1over2} rightarrow 7P_{1over2} (459 nm) transitions, perturbed by low pressure (below 1 torr) helium, argon and xenon gas. One series of experiments measured the collision cross section for optical coherence destroying collisions, and the results obtained are in good agreement with previous two pulse echo measurements. The remaining experimental work determined values of collision cross sections due to depolarising collisions for ground state caesium atoms perturbed by noble gas. This marks the first measurement of this parameter for 6S_{1 over2} ground state caesium atoms.