Transition dipole moment

The Transition dipole moment or Transition moment, usually denoted ${\displaystyle \scriptstyle {\mathbf {d} _{nm}}}$ for a transition between an initial state, ${\displaystyle \scriptstyle {m}}$, and a final state, ${\displaystyle \scriptstyle {n}}$, is the electric dipole moment associated with the transition between the two states. In general the transition dipole moment is a complex vector quantity that includes the phase factors associated with the two states. Its direction gives the polarization of the transition, which determines how the system will interact with an electromagnetic wave of a given polarization, while the square of the magnitude gives the strength of the interaction due to the distribution of charge within the system. The SI unit of the transition dipole moment is the Coulomb-meter (Cm); a more conveniently sized unit is the Debye (D).

...
Wikipedia