Abstract
For pt.I see ibid., vol.13, p.2817, (1980). The behaviour of a relativistic free electron in an external plane wave field is analysed and a review of the existing solutions of the corresponding Dirac equation is presented. Completeness and orthogonality of the Volkov states are also proved. Based on the exact wavefunction obtained, a relativistic generalisation of the perturbation method proposed in pt.I is elaborated as a means of treating intense field problems in a covariant manner.