EXPRESS: Photoacoustic Detection of Weak Absorption Bands in Infrared Spectra of Calcite.

Photoacoustic spectroscopic (PAS) detection of infrared absorption often produces spectra with enhanced intensities for weaker peaks, enabling the detection of features due to overtones and combinations, as well as less-abundant isotopic species. To illustrate this phenomenon, we present and discuss PAS infrared spectra of calcite (CaCO3). We use linearization of rapid-scan spectra, as well as comparing step-scan and rapid-scan spectra, to demonstrate that saturation is not the driving force behind these enhanced intensities. Our results point to a new knowledge gap, since a theoretical basis for the enhancement of these weak bands has not yet been developed.