13.17 : 衰减全反射（ATR）红外光谱法：概述

Internal-reflection spectroscopy, or attenuated total reflectance infrared spectroscopy, is an analytical technique to obtain infrared spectra of challenging samples such as solids with limited solubility, films, threads, pastes, and adhesives.

ATR is based on total internal reflection, which occurs when a beam of light passes from a crystal, representing a highly dense medium, to a less dense sample medium at an angle greater than the critical angle.

During this process, the beam penetrates a short distance beyond the crystal into the sample, generating an evanescent wave. The sample absorbs this short-lived radiation at specific wavelengths, resulting in beam attenuation, known as attenuated total reflectance.

The penetration depth depends on factors such as beam wavelength, refractive indices of the crystal and sample, and the angle of the beam relative to the interface.

The ATR spectrum comprises the same bands as a conventional absorption spectrum but differs in relative intensities.