Infrared Spectroscopy

Abstract

Coblentz determined the relation between infrared spectra and molecular identity. Infrared spectroscopy became a tool for chemical analysis widely used in analytical chemistry laboratories. Prism or grating “dispersive” spectrometers fulfill this function well. Molecular spectroscopy was first practiced in the visible and UV range using large high-resolution spectrographs to record the fine spectral lines of vapor phase molecules. Extension to the infrared region was impeded by the lack of sensitivity of single-element infrared detectors and the challenge of making ever larger spectrometers to achieve high resolution. Dispersive spectrometers were superseded by “interferometric” spectrometers that possessed a much larger optical throughput and, when combined with the Fourier technique, permitted multiplexing the entire spectrum efficiently on a single detector enabling extending high-resolution molecular spectroscopy to the infrared region. As Fourier transform spectroscopy technology matured, measurement accuracy and reproducibility were also enabled. Quantitative infrared spectroscopy has opened a wide range of new applications in industry, environmental monitoring, and monitoring of atmospheric variations in relation to weather forecasting. Space probes use Fourier transform spectrometers to determine the chemical nature of planetary atmospheres and surfaces and even the measurement of the cosmic background radiation remnant. Low-cost Fourier transform spectrometers have also replaced dispersive spectrometers in analytical chemistry laboratories. Given its importance in a wide range of applications, a detailed description of the technique is provided.