Oscillator strength

In spectroscopy, oscillator strength is a dimensionless quantity that expresses the probability of absorption or emission of electromagnetic radiation in transitions between energy levels of an atom or molecule. The oscillator strength can be thought of as the ratio between the quantum mechanical transition rate and the classical absorption/emission rate of a single electron oscillator with the same frequency as the transition.

An atom or a molecule can absorb light and undergo a transition from one quantum state to another.

The oscillator strength ${\displaystyle f_{12}}$ of a transition from a lower state ${\displaystyle |1\rangle }$ to an upper state ${\displaystyle |2\rangle }$ may be defined by

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