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Radiation meeting the face of the crystal at an angle of incidence above the critical angle is totally internally reflected but creates an electric field beyond the surface. This evanescent field can interact with an infrared absorbing material on the surface of the crystal, reducing the reflected energy and so generating an infrared spectrum of the material. This is called an attenuated internal reflection (ATR) spectrum. The evanescent field decays exponentially beyond the surface of the crystal so the spectrum is generated in the region close to the surface of the sample. ATR has become increasingly used for measuring IR spectra of solids and liquids as it is more convenient than transmission measurements for most samples. While an ATR spectrum may contain the same features as one measured in transmission there are significant differences which are important to appreciate when comparing spectra with those obtained by transmission measurement. Originally ATR measurements typically involved a parallel-sided plate with the sample clamped to one or both faces. The IR beam was reflected between the surfaces several times to increase the intensity of absorption bands. The technique was sometimes called Multiple Internal Reflection (MIR) or Frustrated Multiple Internal Reflection (FMIR). The most common high-refractive index material was a thallium bromoiodide glass known as KRS5 with a refractive index of 2.37. This is a relatively soft material which allowed good contact with solids such as many polymers but was easily damaged by hard materials. To allow adequate light throughput the ATR element was typically about 1.5cm high by 5cm. Samples were usually of similar size, the most common applications being to polymer films. The advent of FTIR spectrometers with higher sensitivity led to the use of single reflection devices with small crystals, typically about 2mm square. Hard crystal materials such as diamond and germanium are used and good contact can be achieved using much less force than with a large crystal. This has allowed applications to be extended to a much wider range of materials including solid powders and drops of liquid. The inert nature and hardness of diamond means that ATR is sometimes described as a universal measurement method for solids and liquids.

Factors that affect the appearance of an ATR spectrum are the wavelength, the refractive indices of the crystal and the sample, and the angle of incidence. Band intensities are proportional to the wavelength so that bands at longer wavelength appear stronger relative to those at shorter wavelength than in transmission spectra. With a germanium crystal, refractive index 4.0, absorption bands generally appear symmetrical. Refractive index effects become significant if the refractive index of the sample approaches that of the crystal or the angle of incidence is close to the critical angle. Refractive index changes across an absorption band can result in bands appearing asymmetrical and also lead to bands appearing at slightly longer wavelength than in transmission. However with a diamond crystal these effects are generally insignificant for most organic materials.