Relaxation time measurements in frequency and time-domain photoacoustic spectroscopy of condensed phases

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Abstract
The use of the photoacoustic spectroscopy (PAS) technique to measure nonradiative lifetimes in either the frequency or time-domain mode is discussed. Expressions are presented for the dependence of the cell pressure upon the lifetime, and the modifications introduced by the microphone transfer function are discussed. It is shown that in both measurement modes thermally and optically thick specimens are preferred. Other sample types have contributions to the measured phase or time delay arising from the optical absorption depth and sample cell parameters. The model presented suggests that the PAS technique is suitable for the measurement of lifetimes in the range between 10−5 and 10−1 s, with the limits arising from the microphone transfer function, the finite sound velocity, and thermal transport velocity in the transducer gas.