The term dark state
refers to state of an atom or molecule which can no longer absorb (or emit) photons and therefore appears dark. All atoms and molecules are described by quantum state
s -- different states can have different energies and a system can make a transition from one energy level
A quantum mechanical system or particle that is bound -- that is, confined spatially—can only take on certain discrete values of energy. This contrasts with classical particles, which can have any energy. These discrete values are called energy levels...
to another by, for example, emitting or absorbing a photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
. However, not all transitions between arbitrary states are allowed. A state that cannot be accessed by absorbing a photon is called a dark state. This can occur in experiments using laser light to induce transitions between energy levels, when atoms can spontaneously decay into a state that is not coupled to any other level by the laser light, preventing the atom from absorbing or emitting light from that state.
Experiments in atomic physics are often done with a laser of a specific frequency
(meaning the photons have a specific energy), so they only couple one set of states with a particular energy
to another set of states with an energy
. The atom can however still decay spontaneously into a third state by emitting a photon of a different frequency. The new state with energy
of the atom no longer interacts with the laser simply because no photons of the right energy are present to induce a transition to a different level. In practice, the term dark state is often used for a state that is not accessible by the specific laser in use even though transitions from this state are in principle allowed.
Whether or not we say a transition between a state
and a state
is allowed often depends on how detailed the model is that we use for the atom-light interaction. From a particular model follow a set of selection rules that determine which transitions are allowed and which are not. Often these selection rules can be boiled down to conservation of angular momentum (the photon has angular momentum). In most cases we only consider an atom interacting with the electric dipole field of the photon. Then some transitions are not allowed at all, others are only allowed for photons of a certain polarization.
Let's consider for example the hydrogen atom. The transition from the state
to the state
is only allowed for light with polarization along the z axis (quantization axis) of the atom. The state
therefore appears dark for light of other polarizations.
Transitions from the 2S
level to the 1S
level are not allowed at all. The 2S
state can not decay to the ground state by emitting a single photon. It can only decay by collisions with other atoms or by emitting multiple photons. Since these events are rare, the atom can remain in this excited state for a very long time, such an excited state is called a metastable state.