Dicke effect

In spectroscopy, the Dicke effect, also known as Dicke narrowing or sometimes collisional narrowing, named after Robert H. Dicke, refers to narrowing of the Doppler broadening of a spectral line due to collisions the emitting species experiences with other particles.

Mechanism

When the mean [free path] of an atom is much smaller than the wavelength of the atomic [spectral line|radiative transition], the atom changes velocity and direction many times during the emission or absorption of a photon. This causes an averaging over different Doppler states and results in an atomic linewidth that is narrower than the Doppler width.