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The theory underlying the use of the phase fluctuation optical heterodyne (PFLOH) technique in studies of molecular relaxation, thermal conduction, and extremely weak absorptions in the gas phase is presented. Several new solutions to the heat conduction equation, with and without mass diffusion, which are appropriate to pulsed, cw, and modulated-cw laser modulation of weakly absorbing gases, are presented. Experiments employing pulsed and modulated-cw excitation have borne out the essential predictions of the theory. PFLOH systems for use in a trace detection application have been built with a demonstrated sensitivity, in terms of measurable absorption by the trace material, below 10(-10) cm(-1).

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