M. Protopapas

Intense laser - atom dynamics with the two-dimensional Dirac equation

A time-dependent numerical analysis is made of atomic wavefunctions evolving under the influence of laser pulses sufficiently intense that relativistic effects are important. The investigation utilizes the two-dimensional Dirac equation to identify relativistic effects such as the contribution of the magnetic field and spin dynamics.

TIME-DEPENDENT PULSE AND FREQUENCY EFFECTS IN POPULATION TRAPPING VIA THE CONTINUUM

We investigate the effects of smooth laser pulses in both an autoionization system and in the laser-induced continuum structure scheme. We are particularly interested in the phenomenon of population trapping in these systems and discuss the effects of time-dependent pulses on this trapping. We present analytical solutions to both systems which hold for any pulse shape, provided the adiabatic approximation holds. We also pay special attention to the effects of large pulse energies on the trapped population, which we find to be detrimental and propose a possible solution to this problem by using chirped (time-dependent) frequencies.

Relativistic mass shift effects in adiabatic intense laser field stabilization of atoms

We investigate the effect of the relativistic mass shift of the electron on adiabatic (high intensity, high frequency) stabilization by numerically integrating the relativistic Schrodinger equation in one dimension. We find that a relativistic treatment implies better stabilization than would be expected non-relativistically. We propose an explanation which involves consideration of the force acting on the electron within the Kramers - Henneberger frame.

Phase control of a two-channel ionization system

A theoretical analysis of the phase control of a two-channel atomic ionization scheme is given. We report results on phase-selective excitation, both with square and smooth laser pulses. We show that the phenomenon of population trapping, which is known to occur in this system, can be greatly influenced by the phase difference of the lasers involved in the excitation. As a result, the time dependences of the ionization probability, the populations in the bound states and the form of the photoelectron energy spectra can all be controlled by varying the phase difference of the lasers. We also show that for specific atomic and phase parameters population can be entirely trapped within a superposition state, totally immune to decay, with no ionization whatsoever.

Relativistic laser-atom interaction: dynamics and radiation

Summary form only given. The experimental availability of lasers in the intensity regime around and above 10/sup 19/ W/cm/sup 2/ has opened a new era of research for atom-laser interactions. Because the ponderomotive energy of electrons driven by such radiation is no longer negligible as compared to the electron rest mass, conventional nonrelativistic theoretical descriptions lose their validity and it is imperative to invoke relativistic ideas. We focus on the relativistic theory of atoms interacting with strong laser fields in the framework of the numerical investigation of the two-dimensional Dirac equation, three-dimensional classical Monte Carlo simulations, and analytical laser-assisted Mott scattering.