Review of structures in low-energy bremsstrahlung: Classical and quantum descriptions

A brief review of recent advances in studying structures in energy dependence of the bremsstrahlung cross sections for low incident electron energies is presented. Examples of structures are given in both classical and quantum formalisms. It is shown that the origin of the structures can be formulated as a lack of contribution to the radiation from electrons with certain angular momenta at certain energies. In quantum mechanics the lack of contribution to the total cross section from certain electron angular momenta is due to zeroes in corresponding dipole matrix elements. In classical mechanics summation over angular momentum is replaced by integration and structures are due to suppressed or enhanced contribution from certain intervals of angular momentum. A survey of the known properties of the matrix elements' zeroes is given