Absorption	Occurs when the infrared frequency equals the natural vibrational frequency of bond stretching and bending. Only bonds possessing a dipole moment absorb this energy.
Bending	A type of vibration that gives rise to energy absorptions in specific ranges in the IR spectrum that correspond to lower energy levels. There are four types of bending: in-plane scissoring, in-plane rocking, out-of-plane wagging, and out-of-plane twisting.
Detector	Part of a spectrophotometer that measures the difference in intensities between the sample beam vs. the reference beam to yield just the sample contribution.
Diffraction grating	Individual wavelengths of the combined IR beam that are passed to the detector one at a time (varies the wavelength of radiation that reaches the detector).
Dipole moment	A measure of the separation of charge in a bond or in a molecule.
Double Beam	IR radiation from a source is split into two beams of equal intensity. One of the beams passes through the sample while the other beam passes through a reference.
Electromagnetic radiation	A form of energy that has wave characteristics and that propagates through a vacuum at the speed of light.
Fermi resonance	Coupled vibration caused by the combination of a fundamental vibration with an overtone band.
Fundamental absorptions	Stronger absorptions that arise from excitation of bonds from the ground state to the first excited or lowest energy state.
Fourier Transform (FT)	A computer program that takes the wave-like pattern of the interferogram and separates the individual absorption frequencies into a spectrum.
Infrared radiation	Energy that corresponds to the vibrational region of the electromagnetic spectrum that extends from 4000 cm^-1 to 400 cm^-1. This energy causes “vibrational” motion within covalent bonds that have a dipole moment.
Infrared spectrum	Refers to the infrared absorption pattern that results from the different vibrational motion of the bonds in a molecule.
Interferogram	A plot of intensity vs. time. A complex signal with a wave-like pattern that contains all of the frequencies between 4000 cm^-1 to 400 cm^-1. (The FID for short)
KBr pellet	An IR sample that is prepared for solid compounds. A small amount of solid sample and KBr powder is mixed together and melted under high pressure. A thin, solid matrix is produced and can be placed in the sample holder and a spectrum obtained.
Laser beam splitter	The laser produces a red light of a known frequency at a known moving mirror position. This frequency enters into the FT for the final spectrum.
Overtones	Weak absorptions that are caused from excitation from the ground state to a higher energy state.
Reduced mass	The product of the masses of two nuclei divided by the sum of the masses of the two nuclei.
Salt plates	A pair of polished KBr or NaCl plates. A small amount of the liquid compound is placed between the plates to produce a thin film. A beam is passed through this sample to acquire the IR spectrum.
Selector	Causes both the reference and sample beams to be passed to the diffraction grating as a combined beam.
Single beam	The IR radiation from a source is sent to a stationary mirror and a moving mirror. The moving mirror accomplishes the same as a reference cell. It creates an interface beam to the sample beam that subtracts out the background.
Stretching	A type of vibration that gives rise to energy absorptions in a specific range of the IR spectrum. There are two types of stretching- symmetric and asymmetric. The later corresponds to lower energy and the former corresponds to higher energy.
Wavelength	Wavelength (λ) is inversely proportional to frequency (ν) based upon the relationship that ν = c/ λ, where c = the speed of light.
Wavenumber	The unit used to quantify the energy of the vibrational infrared portion of the electromagnetic spectrum. It is expressed as reciprocal cm (cm^-1). Wavenumbers are directly proportional to energy (Planks constant).