Multiphoton free-free transitions in a spatially inhomogeneous laser field

Summary The results of an investigation are presented in which it is shown how the spatial inhomogeneity of a laser field modifies the multiphoton free-free transition cross-sections compared to the case of a homogeneous field. This kind of investigation is required to make more close contact with experiments in intense fields, as in these cases the inhomogeneity is produced by the focusing of the laser beam. Furthermore, taking into account the intensity spatial distribution allows us to achieve in an effective way the asymptotic decoupling of the scattered particles from the field, which is very important for theoretical models using asymptotic initial and final states embedded in the field. Differential and total cross-sections are calculated over a wide range of parameters as functions of the scattering angle, of the incoming-particle energy and of the laser intensity and frequency. The laser spatial inhomogeneity is found to wash out most of the oscillating behaviour of multiphoton differential cross-sections, derived within the model of a homogeneous laser. Little modifications are, instead, found in the total cross-sections which are simply scaled to slightly lower values. Crosssections with zero photon exchange are increased, while those with photon exchanges are lowered. This yields the result that the sum of all the ≪inhomogeneous≫ cross-sections is equal to the sum of all the ≪homogeneous≫ ones (sum rule). The multiphoton free-free transition differential cross-sections are found to be very sensitive quantities which may be used to get information on the laser properties and on their nonlinear behaviour, when these are not precisely known. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.