More specifically, it is a spectroscopic technique used to identify and study inorganic and organic compounds based on their light absorption in the IR spectrum. Depending on which type of spectrometer you use, the technique detects photons that are transmitted, absorbed, or reflected when a sample is excited using IR light. The chemicals studied can be in solid, liquid, or gas forms.

The infrared region of the electromagnetic spectrum has longer wavelengths and a lower frequency than visible light, making it more suitable for molecular vibration studies. This spectrum is usually divided into three regions: the near-infrared, mid-infrared, and far-infrared, and are named for their relation to the visible spectrum.

While spectroscopy, as a whole, is the study of interactions between matter and radiated energy, spectrometry is the method used to acquire a quantitative measurement. In other words, actually collecting data is made possible with a spectrometer. In the case of IR spectroscopy, an IR spectrometer is used.

Today, one of the most common types of IR spectrometers today is the Fourier-transform infrared spectrometer (FTIR), which employs an interferometer rather than a dispersive monochromator. An FTIR spectrometer produces an infrared spectrum using a broadband IR light source that illuminates the sample. It is eventually visualized as a graph by measuring infrared light absorption and the frequency or wavelengths on their respective axes.

IR spectroscopy relies on the fact that molecules absorb frequencies in a way that is characteristic of their unique structures. This includes the resonant frequencies of each chemical. The absorbed radiation matches the vibrational frequency, which occurs because the energies are affected by the masses of the atoms, the unique shapes of the molecular potential energy surfaces, and the vibronic coupling.

For a sample to be considered IR active, it needs to experience changes in the dipole moment. This refers to vibrational modes, which are a measure of vibrational freedom that varies with the number of atoms and whether the structure is linear or nonlinear.
Other applications of IR spectroscopy, other than organic and inorganic R&D, include chemical analysis, environmental testing, and forensics, which we will cover briefly below.