Observation and Density Dependence of the Raman Echo in Atomic Thallium Vapor and Noble-Gas Collisional Broadening of the 6P(1/2 - 6P(3/2) Transition of Thallium Measured by Raman Echo.

Working on the 6P(,1/2) - 6P(,3/2) two-photon transition of atomic thallium vapor, we have observed Raman echo generated by excitation and probe fields detuned from the 7S(,1/2) intermediate state by up to about 50 cm(' -1). This constitutes the first time that echoes, generated by pure two-photon excitation, have been observed in an atomic system. The Raman echo intensity is found to increase roughly as the square of the thallium density at low densities, but at higher densities it peaks and subsequently declines. We suggest that two-photon absorption and loss of phase-matching resulting from excitation-pulse-induced population transfer are important in the high density degradation of the echo signal. Then we implement the Raman echo for the first time in collision studies to measure the noble-gas-induced broadening of the 6P(,1/2) - 6P(,3/2) transition of atomic thallium. Except in the case of He, the measured broadening cross sections; He 59(2)(ANGSTROM)('2), Ne 67(2)(ANGSTROM)2, Ar 124(4)(ANGSTROM)('2), Kr 167(5)(ANGSTROM)('2), and Xe 204(6)(ANGSTROM)('2) (T = 840 K), agree with those calculated assuming a pure Van der Waals collisional interaction. Our measurements, although sensitive to collisionally-induced changes in axial velocity of >(, )300 cm/sec, reveal that collisional velocity changes of that magnitude are not important in T1-noble-gas collisions.