In condensed matter physics and atomic physics, the recoil temperature is a fundamental lower limit of temperature attainable by some laser cooling schemes. When an atom decays from an excited electronic state at rest to a lower energy electronic state by the spontaneous emission of a photon, due to conservation of momentum, the atom gains momentum equivalent to the momentum of the photon. This kinetic energy gain corresponds to the recoil temperature of the atom. [1] The recoil temperature is
Trecoil=
| \hbar2k2 | |
| mkB |
=
| p2 | |
| mkB |
,
\hbar
p=\hbark
In general, the recoil temperature is below the Doppler cooling limit for atoms and molecules, so sub-Doppler cooling techniques such as Sisyphus cooling[2] are necessary to reach it. For example, the recoil temperature for the D2 lines of alkali atoms is typically on the order of 1 μK, in contrast with a Doppler cooling limit on the order of 100 μK.[3] However, the narrow-linewidth intercombination transitions of alkaline earth atoms such as strontium can have Doppler limits that are below their recoil limits, allowing laser cooling in narrow-line magneto-optical traps to the recoil limit without sub-Doppler cooling.[4]
Cooling beyond the recoil limit is possible using specific schemes such as Raman cooling.[5] Sub-recoil temperatures can also occur in the Lamb Dicke regime, where the recoil energy of a photon is smaller than a motional energy quantum; therefore the atom's state is effectively unchanged by recoil photons. [6]
See also: Doppler cooling, sub-Doppler cooling, Raman cooling and Mössbauer effect.