Exact Theory of Nonrelativistic Quadrupole Bremsstrahlung

The quantum mechanical treatment of quadrupole radiation due to bremsstrahlung exact to all orders in the Coulomb interaction of two nonrelativistically colliding charged spin-0 and unpolarized spin-1/2 particles is presented. We calculate the elements of the quadrupole tensor, and present analytical solutions for the double-differential photon emission cross section of identical and nonidentical particles. Contact is made to Born-level and quasi-classical results in the respective kinematic limits and effective energy loss rates are obtained. A generally valid formula for soft-photon emission is established and an approximate formula across the entire kinematic regime is constructed. The results apply to bremsstrahlung emission in the scattering of pairs of electrons and are hence relevant in the study of astrophysical phenomena. A definition of a Gaunt factor is proposed which should facilitate broad applicability of the results.