D. G. Lappas

Electron correlation effects in the double ionization of He

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.

Orientation dependence of high-order harmonic generation in hydrogen molecular ions

We show that the direction of laser polarization with respect to the molecular axis of a linear molecule can have a significant effect on high-harmonic generation. Numerical integration of the two-dimensional Schrodinger equation of the H2+ and the H32+ model molecular ions shows that the efficiency of some high harmonics can be higher by up to three orders of magnitude when the laser is polarized in the perpendicular direction rather than parallel to the molecular axis. We discuss experiments to test this prediction.

Strong field ionization in arbitrary laser polarizations

Summary form only given. Numerical models have been developed in the 1990s; however, most theoretical investigations have dealt only with linearly polarized light. The reason for this is that al least two dimensions are needed for the case of arbitrary polarization, and current three dimensional approaches are unable to deal with the enormous basis set needed to represent (for arbitrary polarization) the evolving wave function. Consequently, little work has been carried out for the general case of atomic response in the plane of an arbitrary laser polarization. We have developed a method for numerically integrating the two-dimensional Schrodinger equation without using a restrictive basis expansion, enabling us to study light of arbitrary polarization and with realistic frequencies, interacting within a two-dimensional representation of a hydrogenic atom.

Recollisions and high-harmonic generation

We describe the Recollision Model of high-harmonic generation, in which electrons escape from atomic Coulomb binding in intense laser fields by tunneling, are ponderomotively accelerated, and recollide on return to the parent ionic core. High harmonics are generated by Bremsstrahlung, modulated by the coherent periodic nature of the tunneling process. Harmonic plateaux involve the time-dependent dipole induced by the returning wave packets. We show how atomic wave packets are generated and manipulated by strong laser driving fields, and link the recollision to nonsequential ionization.