Fourier-Transform Infrared (FTIR) Spectroscopy

In FTIR absorption spectroscopy, two spectrally broad and overlapping beams of light are passed through a sample and onto a detector. A Michelson interferometer is used to vary (in time) the interference pattern created by the two overlapping beams of light. This generates an interferogram in the detector signal which is converted to a measured transmission spectrum (intensity of light vs wavelength) by performing an inverse Fourier transform on the interferogram. This technique enables an extremely large portion of the infrared spectrum to be studied in a single measurement. Using our FTIR we can measure absorption spectra at wavelengths from 1 to 20 μm in a single measurement. This is illustrated in the figure below which shows an example transmission spectrum of ambient air. The spectrum illustrates several absorption bands of H2O and CO2, which are composed of thousands of individual absorption transitions. The inset shows CO2‘s fundamental absorption band for its bending-mode vibration which more clearly illustrates the quantized nature of molecular energy levels and absorption transitions.

Currently we are using this diagnostic to characterize the absorption spectra of solid, liquid, and gaseous samples of fuels, propellants and combustion products. This fundamental data enables us to quantitatively predict the amount of light absorbed by specific molecules and how this varies with thermodynamic conditions. This information is vital to the development of our laser-based diagnostics that we use to characterize thermochemically dynamic environments.