Alternative representation of time-dependent Hamiltonians with application to laser-driven systems

A representation of time-dependent Hamiltonians that describe laser-driven systems is presented. Unlike the well-known time-independent dressed potentials that are functions of the characteristic parameter {alpha}{sub 0}={radical}(I)/{omega}{sup 2}, where {omega} and I are the laser frequency and intensity, this approach provides a time-averaged potential that depends explicitly on the field parameters; e.g., I, {omega}, and shape of the laser pulse. The modified dressed potential is ({Dirac_h}/2{pi}) independent and adds a classical time-independent potential barrier to the Kramers-Henneberger dressed potential. We show that this dynamical potential barrier is identical to the Kapitza effective classical potential energy obtained for the motion of a particle in a rapidly oscillating field. As an illustrative numerical example, a simple one-electron effective model Hamiltonian of xenon atom in strong laser field is studied. We show that the zero-order quasienergies obtained by our representation are reasonably accurate and the second order high-frequency perturbation calculations provide quite accurately the lifetime of the photoionized electron for a broad range of laser frequencies.