Octupole radiation

The intensity of the absorption process is then proportional to the square of the transition matrix element connecting the initial ((pi) and final (delta function which ensures that it satisfies the conservation of energy theorem. The elimination of the spatial dependence of the electric field corresponds to a series expansion of its /l7rz/x dependence up to the first term (linear dependence in r equation (1)) this yields the dipole approximation of the interaction energy between the atom electronic cloud and the X-ray radiation field. Better approximations will include quadrupole, octupole and so forth terms. However, except in few cases, some of them here detailed, the dipole approximation gives a quantitative analysis of the XANES shape and EXAFS oscillations. 

They quanta may carry away different angular momenta (L). IfL= 1,2,3,... the radiations are called dipole, quadrupole, octupole, etc., respectively. Gamma radiation with L = 0 does not exist because the electromagnetic waves have transversal nature (the photons have spin 1). Each multipolarity 2 is characterized by a specific angular distribution. The radiation may be electric or magnetic, depending on the term of the electromagnetic interaction that is responsible for the particular transition. 

The electric and magnetic dipole, quadrupole, octupole, etc., transitions are denoted by El, E2, E3,. .. and Ml, M2, M3,. .., respectively. The selection rules deduced from angular momentum and parity conservation laws for electric and magnetic multipole radiations are summarized in Table 2.6. 

In case of radiation transitions of higher multipolarity (quadrupole with L = 2, octupole with L = 3, and so on) this expression requires a standard substitution... 

---