Abstract

In the Floquet method, the dynamics of an atom in a monochromatic laser field is described within a time-independent framework. The atomic state vector, which satisfies the time-dependent Schrodinger equation, is replaced by exp (–iEt/ħ)|F(t)), where E is the quasienergy of the atom and where |F(t)〉 is periodic in t with period equal to the cycle time of the laser field. If |F(t)〉 is expanded in a Fourier series with terms exp (–inωt)t)|Fn〉, where ω is the laser frequency, the harmonic components |Fn〉 satisfy a set of time-independent coupled equations. The nth harmonic component represents the (real or virtual) absorption of n photons by the atom. In an ionization process (the active electron is initially bound) the quasienergy is complex; it has a real part which includes the AC Stark shift, and an imaginary part which is proportional to the total ionization rate. Large scale calculations of total ionization rates for hydrogen and helium have been performed. Departures from perturbation theory can be substantial. A knowledge of the harmonic components gives the time-dependent atomic dipole moment, which has been used to calculate harmonic generation rates in hydrogen. Further, absorption cross sections for electrons scattering from protons have been calculated.

© 1989 Optical Society of America