Evanescent Waves - Evanescent Waves

A special type of electromagnetic wave that is generated when light is incident from a high refractive index phase to a low refractive index phase at an angle of incidence greater than the critical angle and total (internal) reflection occurs. Its intensity decreases exponentially in the direction from the interface on the low refractive index phase side to the inside of the phase. In the low refractive index phase, the distance from the interface to where the intensity decreases to 1/ e is called the penetration depth. The penetration depth is a function of the optical constants of the two phases and the angle of incidence, and is usually a fraction to several times the wavelength. Spectroscopy using evanescent waves is called attenuated total reflection (ATR) and is used to measure the spectra of chemical species present at interfaces and surfaces in the infrared, visible, and ultraviolet regions.

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

光が高屈折率相から低屈折率相へ臨界角以上の入射角で入射し，全(内部)反射が起こるときに生じる特殊な電磁波のこと．その強度は低屈折率相側の界面から相内部への方向に指数関数的に減少する．低屈折率相において，強度が界面から1/eに減少するまでの距離は侵入の深さとよばれる．侵入の深さは二つの相の光学定数と入射角の関数であり，通常，波長の数分の1から数倍である．エバネッセント波を用いる分光法は減衰全反射法(ATR法)とよばれ，赤外線や可視・紫外線領域で，界面や表面に存在する化学種のスペクトルを測定するために利用される．

出典　森北出版「化学辞典（第2版）」化学辞典 第2版について　情報