Absorption coefficient (${\alpha }_{\nu }$)

The absorption coefficient is the fractional reduction of beam intensity per unit path length: $d{I}_{\nu }/ds=-{\alpha }_{\nu }{I}_{\nu }$ when emission and scattering are absent.

A collision or transient molecular pair can possess a dipole even when the isolated molecules do not. The resulting opacity can be important in a dense molecular hydrogen atmosphere. Pair absorption per unit length scales approximately as the product of the two partner number densities, so its mass absorption coefficient is generally density-dependent. It supplies continuum absorption between ordinary spectral lines.