selection rule

In EELS, when interband transitions due to Coulomb interactions are considered, if only small-angle scattering is taken into account (small-angle scattering approximation), interband transitions are limited only to the dipole transition (dipole approximation). That is, only the transitions with the change of an angular orbital momentum being ⊿l = ±1 is allowed. The rule which clarifies the allowed and forbidden transitions is called "selection rule." Thus, the transitions from the 1s shell to the unoccupied p states (2p, 3p, etc.) occur. In ELNES, partial density of unoccupied states, instead of total density of states, is obtained. In the low energy-loss region corresponding to valence-electron excitations, scattering at large angles is allowed and this can give rise to transitions in which the selection rule does not hold.

Related Term(s)