The enhancement of the detectability for trace absorptions by placing samples inside the laser cavity was found to be a factor of 100 for a flash-lamp-pumped dye laser and one thousand for a cw dye laser. High-resolution spectra showed that the holes in the laser output were as narrow as the absorptions that caused them. An approximately linear relationship (rather than the step-function behavior often associated with threshold phenomena) exists between pressures necessary to produce visually identical absorption spectra from samples placed inside and outside of the laser cavity. If such a relationship is of general occurrence, it will greatly facilitate the use of intracavity absorption for quantitative analysis, determination of relative absorption cross sections, and for the study of the kinetics of appearance and disappearance of transient species.
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