Inverse bremsstrahlung in strong radiation fields - The Born approximation re-examined

The calculation of the energy absorption in a strong radiation field where the electron quiver energy is much larger than the thermal or photon energies is performed using the Born approximation for the collision. The multiphoton sums are evaluated with the aid of asymptotic forms to yield analytic expressions. Comparison with earlier results from the impact (or classical) model shows disagreements. The origin to this difference is ascribed to the breakdown of perturbation theory in this limit, and it is concluded that the impact approximation is more reliable at very high fields.