F. Ceccherini

Dynamical symmetries and harmonic generation

We discuss harmonic generation in the case of laser field-dressed Hamiltonians that are invariant under so-called dynamical symmetry operations. Examples for such systems are molecules which exhibit a discrete rotational symmetry of order N (e.g. benzene with N = 6) interacting with a circularly polarized laser field and single atoms in a bichromatic field, with the two lasers having circular polarizations. Within a general group theory approach we study the harmonics one obtains from the interaction of a laser pulse and a circular molecule. When the system is in a pure field-dressed state the known selection rule kN ± 1, k = 1, 2, 3 ,... results. However, other lines are observed when recombinations with states of a symmetry different from the initial one become important. This is the case for realistic laser pulses (i.e. with a finite duration), in particular when the fundamental laser frequency (or one of its multiples) is resonant with a transition between field-dressed states. Numerical ab initio simulations, confirming our analytical calculations and illustrating the power of the group theory approach, are presented.

PARTIAL PHOTOIONIZATION CROSS SECTION MEASUREMENT IN A RB MAGNETO-OPTICAL TRAP

We report an experiment in which relative measurements of the partial photoionization cross sections are performed on trapped rubidium atoms. This experimental procedure allows us, together with recent new methods to determine the absolute total cross section, to make a complete quantitative study of the photoionization process in an atomic trap.

A numerical ab initio study of harmonic generation from a ring-shaped model molecule in laser fields

When a laser pulse impinges on a molecule which is invariant under certain symmetry operations selection rules for harmonic generation (HG) arise. In other words, symmetry controls which channels are open for the deposition and emission of laser energy-with the possible application of filtering or amplification. We review the derivation of HG selection rules and study numerically the interaction of laser pulses with an effectively one-dimensional ring-shaped model molecule. The harmonic yields obtained from that model and their dependence on laser frequency and intensity are discussed. In a real experiment obvious candidates for such molecules are benzene, other aromatic compounds, or even nanotubes.

Fullerenes and molecules in strong laser pulses: Collective dynamics, field ionization and harmonic generation

Computational studies of many-electron systems in strong laser fields are presented. The ionization and nonlinear dipole response of the ball-shaped C60 fullerene molecule are investigated with a time-dependent density functional approach and a jellium approximation for the ionic background. We find that C60 ionization at 800 nm wavelength occurs multiphoton-like rather than via excitation of a “giant” resonance. Harmonic generation from the interaction of a circularly polarized laser field and a molecule with a discrete rotational symmetry is also studied, and spectra and selection rules are interpreted via a general group theory approach.