TIME-INDEPENDENT SCATTERING-THEORY FOR TIME-PERIODIC HAMILTONIANS - FORMULATION AND COMPLEX-SCALIGN CALCULATIONS OF ABOVE-THRESHOLD-IONIZATION SPECTRA

A time-independent scattering theory is introduced for time-periodic H amiltonians. The theory is applicable in cases where at infinite time (t = +/- infinity) the relative motion between the colliders is free, and therefore the asymptotic kinetic energy can be defined experimenta lly. This asymptotic condition and the time periodicity of the Hamilto nian lead to time-independent expressions for the distribution of the kinetic energy in the relative motion of the fragments for a half-coll ision and for a full-collision experiment. These expressions are combi ned with the complex-coordinate method to give an efficient numerical algorithm for the calculation of multiphoton ionization-dissociation p robabilities within the framework of the finite-range-potential approx imations. The theory is applied to a model Hamiltonian of an atom in a strong ac field. The obtained above-threshold-ionization spectra are in excellent agreement with theoretical results previously obtained fr om time-dependent calculations.