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So my physics teacher was recently teaching me about lasers. I remember that she explained lasers with the help of a meta stable state. This essentially means that once an atom is excited the electrons move to a excited state. This state has a very short period. Below this energy state a slightly stable state exists where the electron has a slightly longer time staying there. In lasers the the excited atoms are stimulated with light which then go to this ‘meta stable’ state and then fall down and release 2 photons of light. I have 2 questions:

  1. Does this so called meta stable state actually exist?
  2. Why would the electron want to go to that state when it could directly fall to the ground energy level and release a 2 photons?
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Yes, metastable states exist. One can study their lifetime and other properties. Metastability is due to fast optical dipole transitions to the ground state being forbidden. An example are transitions that require spin flips, for example from a triplet state to a singlet ground state.

In a three-level system, the transition from the high level to the ground state can occur when this state is excited by light from the ground state. But when the probability of a transition to the metastable state is high enough, this metastable state will get populated. For example, the high state can be a triplet state that can be dipole-allowed to some extent because of spin-orbit coupling.

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  • $\begingroup$ Thanks for the reply. But under what circumstances does the probability of transition to the meta stable state actually increase? $\endgroup$ Commented Mar 16, 2018 at 16:21
  • $\begingroup$ In the example that I mentioned, after excitation the transition to the triplet metastable state would be spin-allowed and fast. $\endgroup$
    – user137289
    Commented Mar 16, 2018 at 18:55

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