Recollisions, bremsstrahlung, and attosecond pulses from intense laser fields.

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 ~HHG!; in this process the strongly driven atomic system reradiates energy at odd multiples ~owing to the symmetry of the process! of the driving frequency. Using HHG it has been possible to generate very-short-wavelength light @1# with excellent coherence properties ~which is a consequence of the generation process!. 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 @2#; 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. @3# and by Corkum @4# predicts the cutoff energy to be