Transition rule

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The transition rules (or selection rules) describe possible state transitions of a quantum mechanical system, expressed by changes of the quantum numbers. Transition rules exist for various types of quantum numbers:

• Transitions of an electron orbiting an atom.

For the difference between the initial state described by the quantum numbers <math>(n,l,m)</math> and the final state <math>(n',l',m')</math>, the following conditions have to hold:

<math>\Delta l = (l - l') = \pm 1</math> and <math>\Delta m = (m - m') = 0 , \pm 1</math>

• Vibrational transitions of diatomic molecules. Change of vibrational quantum number <math>v</math>:

<math>\Delta v = \pm 1</math>

• Rotational transitions of molecules. Change of the rotational quantum number <math>J</math>:

<math>\Delta J = \pm 1</math>

The transition rules determine the structure of atomic spectra.