Evanescent Wave
Definition and meaning of Evanescent Wave in chemistry.
An evanescent wave is a non-propagating electromagnetic wave that forms at the interface between two media when light undergoes total internal reflection, with its intensity decaying exponentially into the medium of lower refractive index.
In more detail
Although total internal reflection sends essentially all light back into the denser medium, Maxwell's equations require a field that extends a short distance across the boundary; this evanescent field carries no net energy away unless something absorbs it. Its penetration depth is typically a fraction of the light's wavelength (on the order of 100-300 nm) and increases as the incidence angle approaches the critical angle. Because it samples only a thin layer near the surface, the evanescent wave is exploited to probe surfaces and interfaces without interference from the bulk material.
Key facts
| Field | Analytical Chemistry |
|---|---|
| Typical penetration depth | ~100-300 nm (fraction of the wavelength) |
| Key techniques | ATR-FTIR spectroscopy, TIRF microscopy |
| Origin | Total internal reflection at a boundary between two media |
In attenuated total reflectance (ATR) infrared spectroscopy, an IR beam totally internally reflects within a high-index crystal (such as diamond or germanium); the evanescent wave extending a few micrometers into a sample pressed against the crystal is selectively absorbed at the sample's characteristic vibrational frequencies, generating its IR spectrum.
Frequently asked questions
Does an evanescent wave transfer energy across the interface?
Not on its own. Under total internal reflection, no net energy propagates away from the boundary, but if an absorbing sample sits within the evanescent wave's penetration depth, it can absorb energy from the field, as occurs in ATR spectroscopy.
What controls the penetration depth of an evanescent wave?
Penetration depth depends on the wavelength of light, the angle of incidence, and the refractive indices of the two media, growing larger as the incidence angle nears the critical angle.