electric dipole

Dipole Moment

The electric dipole moment for a pair of opposite charges of magnitude q is defined as the magnitude of the charge times the distance between them and the defined direction is toward the positive charge. It is a useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances between the charges are too small to be easily measurable. It is also a useful concept in dielectrics and other applications in solid and liquid materials.

Applications involve the electric field of a dipole and the energy of a dipole when placed in an electric field. 

Electric dipole transition is the dominant effect of an interaction of an electron in an atom with the electromagnetic field.
Following , consider an electron in an atom with quantum Hamiltonian H₀, interacting with a plane electromagnetic wave

Write the Hamiltonian of the electron in this electromagnetic field as

Treating this system by means of time-dependent perturbation theory, one finds that the most likely transitions of the electron from one state to the other occur due to the summand of W(t) written as

Electric dipole transitions are the transitions between energy level in the system with the Hamiltonian H₀ + W_DE(t).
Between certain electron states the electric dipole transition rate may be zero due to selection rule, and then the transitions between such levels are approximated by higher-order transitions.
The next order summand in W(t) is written as

and describes magnetic dipole transitions.
Even smaller contributions to transition rates are given by higher electric and magnetic multipole transitions