Concentration-modulated absorption spectroscopy. Part 3.—The effect of finite spectral linewidths

Abstract

Finite spectral linewidths of radiation sources in absorption spectroscopy lead to marked deviations from the Beer–Lambert relationship when quantitative studies are carried out on spectrally narrow transitions. Such deviations are even more marked in concentration-modulated absorption spectroscopy, a newly developed quantitative technique which takes advantage of the high power of laser sources to perturb the concentrations of absorbing species in localized regions of space. However, when advantage is taken of the formal relationship which exists between probe laser gain in concentration-modulated absorption spectroscopy and sample absorbance in order to determine species concentration, the results appear to be free of the effects of source linewidth. The reasons for this surprising observation are accounted for quantitatively in this study and it is shown that even for a laser linewidth an order of magnitude broader than the absorption line, little error is introduced in the measured sample concentrations. Since the analysis also demonstrates a method for correcting experimental absorbance data for the influence of finite resolution effects, it is possible to use the measured concentrations with the corrected absorbance data to determine accurate values for absorption cross-sections.