J. B. Watson

Multielectron response to intense laser fields

In this paper we will describe recent theoretical approaches to calculating the response of multielectronic atoms in intense laser fields. This work is based on the soultion of the time–dependent Schrödinger equation for a single atom driven by a linearly polarized laser field. Results obtained in recent experimental measurements of multiple ionization rates suggest that the interaction between the electrons in helium enhances the double ionization rate. Solving the full two–electron Schrödinger equation is a very computer–intensive problem, and therefore we have introduced a new model that can reproduce the experimental results. This model is described in detail in this paper, and used to investigate the origin of the double ionization.

Generation of a harmonic quasi-continuum from beating laser fields

We present simulations of the two-colour harmonic response of a one-dimensional model atom exposed to a superposition of two laser fields of varying relative intensities and of non-commensurate frequencies. We show that this interaction can be used to efficiently generate a quasi-continuum of very closely spaced harmonics and explain how such a scheme can be implemented experimentally. We show how the generation process can be described in terms of the beat frequencies between the two fields. Finally, we discuss the limitations of the - interaction (which has been studied extensively elsewhere).

Double ionization of helium in an elliptically polarized laser field

In this letter we discuss the double ionization of helium in intense, elliptically polarized laser fields. We extend a model previously used for linear polarizations to the case of a two-dimensional model atom subject to a laser field of arbitrary ellipticity. Our results show a decrease in the efficiency of the non-sequential double ionization as the ellipticity of the driving field is increased, consistent with the recollision picture of double ionization. Our results are in qualitative agreement with previous experimental measurements.

CAN HARMONIC GENERATION CAUSE NON-SEQUENTIAL IONIZATION?

We use a simple two-electron model introduced previously in the literature (Watson et al 1997 Phys. Rev. Lett. 78 1884) to study the process of non-sequential double ionization of atoms. In particular, we discuss the phenomena of non-sequential double ionization of helium and we show that there is a definite relationship between the dipole radiation of the outermost (strongly ionized) electron and the ionization of the inner electron. We also investigate in detail the effect of quantum interference of the outer-electron wavefunction on the non-sequential ionization.

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.

Quantum Signatures in Strong Field Physics

The interaction of atoms with super intense electromagnetic fields has shown to give rise to the emergence of high harmonics.1 Many aspect of this interaction have been explained satisfactorily with purely classical models while others have required a. quantum mechanical treatment of the atom. We investigate the dynamics of these strong field processes more quantitatively by comparing the time dependent Wigner functions with classical Monte Carlo distributions. In particular negativities in the Wigner function will give us a. clear indication for the existence of quantum mechanical behaviour. In an earlier attempt Bestle, Akulin and Schleicli2 have employed a phase space description of the stabilization process in which they concentrated on two dynamic regimes of the system: For a sudden turn-on of the field they predict classical stabilization while for ari adiabatic turn-on the origin of the process is of a purely quantum nature. We here investigate a regime which shows an interesting interplay of both classical and quantum behaviour.

High Harmonic Generation as a Recollision Process

High harmonics are generated when wavepackets created by tunneling recollide with the nucleus after ponderomotive acceleration. We distinguish quantum and classical contributions and identify novel relativistic features.