Interferences between En and Mn (n=1,2…) optical transition moments

The operator for interaction between a monochromatic electromagnetic field (light) and molecules is usually presented as two multipolar series expansions V ^E1 +V ^E2 +…(electro-dipole, electro-quadrupole,…) and V ^M1 +V ^M2 +… (magneto-dipole, magneto-quadrupole, …). The optical transition probability is proportional to the complex square of the sum of these series, which contains interference terms like V ^En V ^{En ′} ,V ^Mn V ^Mn′ , and V ^Ep V ^Mk (n≠n ^′ ). In the present study it is shown that all of the terms V ^Ep V ^Mk may be observed experimentally if the molecules are oriented, electrically and magnetically, and the light with appropriate phase shift between electric and magnetic fields is used. Also, a simple operator for the light-molecule interactions is proposed as an alternative to the Taylor series mentioned above.