Question

what is the physical interpretation of a selection rule?

Answer 1

The transitions among quantum levels due to absorption or emission of electromagnetic radiation is specified by the selection rule. An oscillating electric field E₀cos(t) that interacts with a transition dipole is as a result of the incident electromagnetic radiation.

A dipolar polarization is created by an interaction of electromagnetic radiation with a molecule. It is called transition dipole moment, <sub>z

  z = 1^* z 2</sub> d

In an experiment we present an electric field along the z axis (in the laboratory frame) and we may consider specifically the interaction between the transition dipole along the x, y, or z axis of the molecule with this radiation. If m_z is zero then a transition is forbidden. The selection rule is a statement of when m_z is non-zero. in the section on electronic transitions, these selection rules also apply to the orbital angular momentum
( l=±1l=±1 , m=0m=0 ).