Electron correlation effects in strong laser fields are investigated by using a simplified two electron model to calculate the double ionization rate in helium. In our model we make a correction to the single active electron approximation by including the effect of the outer electron on the inner one through a time-dependent potential. Using this approach we are able to investigate the nonsequential double ionization observed in recent experiments. [S0031-9007(97)

The double-ionization yield of He is calculated with a one-dimensional fully correlated two-electron model for the low laser frequency of recent experiments. Results for a higher laser frequency also indicate a comparable very high double-ionization yield for sufficiently short pulses. It is shown that the Hartree-Fock approximation fails dramatically in describing the two-electron dynamics. Also, in a density functional theory approach, we demonstrate the need for an improved exchange correlation potential and for more accurate density functionals for the ionization probabilities.

We present numerical ab initio calculations for a two-electron quantum system that is a one-dimensional analogue of the He atom in a strong laser field. Electron correlation occurring within this model can be treated exactly. We compare our calculations with a single-electron model that employs a proper effective potential for the `outer' electron. Evidence of fast correlated double ionization is presented and discussed.

We investigate quantum-mechanical effects in the recollision picture of high-harmonic generation, relate these to its classical counterpart, and discuss the generation of attosecond pulses from recollision bremsstrahlung. ͓S1050-2947͑96͒50405-1͔ PACS number͑s͒: 32.80 Rm, 42.65 Ky Over the last two decades the interaction of atoms with intense laser light has been a rich area of research with many interesting and important discoveries. One of these major findings has been high-harmonic generation ͑HHG͒; in this process the strongly driven atomic system reradiates energy at odd multiples ͑owing to the symmetry of the process͒ of the driving frequency. Using HHG it has been possible to generate very-short-wavelength light ͓1͔ with excellent coherence properties ͑which is a consequence of the generation process͒. This has attracted a great deal of interest because of the many possible applications of such light; for example, in atom lithography or imaging of biological samples ͓2͔; one current goal is to provide a sufficiently intense source of these photons.All HHG spectra show the same generic behavior: there is a sharp fall from the driving frequency to a plateau, followed by a cutoff. The position of the cutoff has received much attention as it essentially determines the maximum frequency that can be emitted. A quasiclassical approach involving tunnel ionization, ponderomotive acceleration, and recollision of the electronic wave packet with the parent ionic core, proposed by Kulander et al. ͓3͔ and by Corkum ͓4͔ predicts the cutoff energy to bewhere I p is the ionization potential of the atom and U p the ponderomotive energy of the electron in the incident field. It was specifically found that this empirical law was independent of the form of the potential and so was only really dependent on the form of the field ͓recently it has been found that by adding a second commensurate frequency it is possible to extend the cutoff energy to beyond that defined in Eq. ͑1͒ ͓5͔͔. This classical recollision picture has since been extended to a fully quantum-mechanical treatment of the electron motion in the laser field; according to this treatment the harmonics are emitted at every recollision ͓6͔. However, since the time scale for a ''single'' recollision is only a fraction of the fundamental laser period, problems arise with the onset of coherence in the emission of radiation.In this Rapid Communication we formulate a mathematical model in which the radiation that is emitted during each recollision ͑on an attosecond time scale͒ is simply bremsstrahlung with an appropriate cutoff. The harmonics emerge from the broad bremsstrahlung spectrum through the interference in time of all the single encounters within the incident pulse duration. We investigate the role of quantummechanical effects in the recollision picture of HHG. We first construct a simple model, and compare its predictions with those from both classical and quantum-mechanical wave-packet treatments. In our approach, the ultrashort time scale of the radiation e...