Floquet calculations of atomic photo-electron spectra in intense laser fields

Abstract

We present the results of Floquet calculations of rates of photo-detachment from a short-range three-dimensional model atom in an intense high-frequency laser field. We find that beyond a certain intensity the atom becomes progressively more stable against ionisation and that at this intensity the ATI spectrum exhibits a plateau of the kind that has recently been reported in the literature [101]. We also discuss the angular distribution of the photo- electrons and examine the special case of a laser-induced degeneracy in the frequency-intensity plane. In the Floquet method one represents the time-dependent Schrodinger equation by an infinite series of coupled time-independent equations, although in practice one must truncate these to a system of finite size. We study the consequences of this truncation by performing a series of calculations for the rate of (resonant) multiphoton detachment from a one-dimensional model atom in a laser field. We find that if the wavefunction is modified to take full account of the truncation then the number of equations which must be retained in order to obtain accurate results is significantly reduced. In performing a Floquet calculation for a real atomic system it is generally assumed that the atom remains at all times in a single diabatic Floquet state, rather than in a superposition of such states. By constructing a suitable two- state model we investigate the validity of this approximation and discuss the usefulness of the Floquet method in modelling an actual experiment. We present results for the multiphoton ionisation of H(1s) by a monochromatic circularly polarised field and by a linearly polarised bichromatic field of commensurable frequencies. In the monochromatic case we draw qualitative comparisons with the predictions of Keldysh theory and use the concept of a propensity rule to explain why the angular distributions remain essentially perturbative at high intensities. In the bichromatic case we study the structure of the ATI spectrum and in particular the role played by a relative phase. The angular distributions are found to be strongly affected by the low- frequency field even when its intensity is too small to cause any appreciable ionisation of the system.