Inverse bremsstrahlung absorption in large radiation fields during binary collisions-the classical theory as an asymptotic limit of the Born approximation

The relationship between the classical and quantum theory of inverse bremsstrahlung absorption was discussed in a semi-quantitative manner in an earlier publication (see J. Phys. A., vol.5, p.1221 (1972)). In this paper the formal derivation of the classical theory as an asymptotic limit using the saddle point approximation is given. Although the derivation is only given for radiation absorption, the application of the method to other collisional phenomena in strong fields is discussed. The use of cut-offs in Coulomb collisions is also investigated.