infrared Infrared radiation extends from the nominal red edge of the visible spectrum at 700 nanometers (nm) to 1 mm. NameWavelength Gamma rayless than 0.01 nm X-ray0.01 nm – 10 nm Ultraviolet10 nm – 380 nm Visible380 nm–700 nm Infrared700 nm – 1 mm Microwave1 mm – 1 meter Radio1 mm – 100,000 km

ISO scheme DesignationAbbreviationWavelength Near-InfraredNIR0.78–3 µm Mid-InfraredMIR3–50 µm Far-InfraredFIR50–1000 µm

Natural infrared At zenith, sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation.

Infrared spectroscopy Infrared spectroscopy is a simple and reliable technique widely used in both organic and inorganic chemistry, in research and industry.

Infrared spectroscopy A basic IR spectrum is essentially a graph of infrared light absorbance (or transmittance) on the vertical axis vs. frequency or wavelength on the horizontal axis. Typical units of frequency used in IR spectra are reciprocal centimeters (sometimes called wave numbers), with the symbol cm −1. The absorptions are resonant frequencies. The frequency of the absorbed radiation matches the transition energy of the bond or group that vibrates.

Number of vibrational modes A molecule can vibrate in many ways, and each way is called a vibrational mode. For molecules with N number of atoms in them, linear molecules have 3N – 5 degrees of vibrational modes, whereas nonlinear molecules have 3N – 6 degrees of vibrational modes (also called vibrational degrees of freedom). As an example H 2 O, a non-linear molecule, will have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.

The atoms in a CH 2 X 2 group, commonly found in organic compounds and where X can represent any other atom, can vibrate in nine different ways. Six of these involve only the CH 2 portion: symmetric and antisymmetric stretching, scissoring, rocking, wagging and twisting, as shown below. (Note, that because CH 2 is attached to X 2 it has 6 modes, unlike H 2 O, which only has 3 modes. The rocking, wagging, and twisting modes do not exist for H 2 O, since they are rigid body translations and no relative displacements exist.)

Practical IR spectroscopy The infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample. When the frequency of the IR is the same as the vibrational frequency of a bond, absorption occurs. Analysis of the position, shape and intensity of peaks in this spectrum reveals details about the molecular structure of the sample. To take the infrared spectrum of a sample, it is necessary to measure both the sample and a "reference" (or "control").

Absorption bands IR spectroscopy is often used to identify structures because functional groups give rise to characteristic bands both in terms of intensity and position (frequency).

傅立葉轉換紅外線儀 (FTIR) A common laboratory instrument that uses this technique is a Fourier transform infrared (FTIR) spectrometer. A data-processing technique called Fourier transform turns the raw data into the desired result (the sample's spectrum): Light output as a function of infrared wavelength (or equivalently, wavenumber).