Fourier Transform Infrared Absorption Spectrometry

IRAS detects stretching and bending vibrations of interatomic bonds that involve changing the dipole moments of the molecules, which gives access to hetero-nuclear $(OH, CO, CH, NO)$ and poly-atomic molecular species $(CH_3, CH_4, SiH_4, CO_2, NH_4)$, and larger molecules alcohols, aldehydes, and aromatic compounds. FTIR spectrometer measures the absorption spectrum of a gas, plasma, liquid, or solid sample over a broad range spectral range with high spectral resolution in one sweep. A beam passing through a sample is optically guided through an interferometer and an interferogram is recorded on a cooled detector. The spectrometer software then uses the Fourier Transform to convert the interferogram into a spectrum. The spectra of these and other molecules are well documented in the HITRAN database . The advantage of the FTIR diagnostics for plasma sources is that it allows detection of multiple gas species simultaneously, including multi-atomic species, which are not detectable with optical emission spectroscopy and laser-induced fluorescence. FTIRAS is a fully optical, non-intrusive technique, as opposed to residual gas analyzer or mass spectrometry. The disadvantage of the FTIRAS approach is its relative lack of sensitivity and the fact that it lacks spatial resolution, where the absorption is integrated along the line-of-sight.