Above-threshold ionization

HHG Angular distributions for above-threshold ionization caused by an infrared field and an attosecond pulse train. For details see C. Figueira de Morisson Faria, P. Salieres, P.Villain and M. Lewenstein, Phys. Rev. A 74, 053416 (2006)
Above-threshold ionization (ATI) is a strong-field phenomenon in which an atom absorbs more photons than the necessary amount for it to ionize. The resulting photoelectron spectra exhibit features which can be described semiclassically, in terms of contributions of direct and rescattered electrons. The former reach the detector without recolliding with its parent ion, whereas the latter are driven back by the field and rescattered by it. The maximal energies of the direct and rescattered electrons are 2Up and 10Up, respectively, where Up is the ponderomotive energy, proportional to the intensity of the driving field. In the photoelectron yield, these energies appear as cutoffs, and, between them, there is a wide plateau.

Our studies on above-threshold ionization are mainly related to the quantum interference between the possible trajectories along which the electron returns to its parent ion. In order to address this issue, we have used a uniform saddle-point approximation for the first time in strong-field laser physics, whose only applicability requirement  is that the saddles occur in pairs (see C. Figueira de Morisson Faria, H. Schomerus and W. Becker, Physical Review A 66, 043413 (2002)).

Current areas of research involve ATI in the following context:

Additional attosecond pulses

The initial conditions of a wavepacket being injected in the continuum, and hence the ATI spectra, can be manipulated by using a time-delayed attosecond pulse train superposed to a strong, infra-red field. We have investigated how the ATI spectra and the angular electron momentum distributions can be influenced by such a pulse within an almost entirely analytic framework. We traced all the features observed to the quantum interference effects.
Our results are in agreement with the outcome of purely numerical methods and experiments.

Interference effects in molecules

In molecules in strong fields, an electron may be freed, or rescatter with spatially separated centers. Quantum mechanically, these processes will interfere. Hence, the structure of the molecule can be traced back to interference patterns in the ATI spectra . Our work is related to the interference of one- and two-center scenarios in ATI of diatomic molecules, and how these scenarios  lead to the spectra.

Excitation in nonsequential double ionization

Further studies involve excitation processes in laser-induced nonsequential double ionization, which can be reduced to rescattered and direct ATI-like processes.