HARMONIC-GENERATION IN IONIZING SYSTEMS BY THE TIME-DEPENDENT COMPLEXCOORDINATE FLOQUET METHOD

The time-dependent complex coordinate Floquet method was developed for the study of non-linear phenomena observed for atoms (or small molecu les) in strong laser fields. On the basis of this method and by taking use of the special definition of the inner product for non-Hermitian operators a simple time-independent expression for the probability to obtain high harmonics was derived. This novel formula is used to inves tigate the phenomenon of harmonic generation for a model potential rep resenting a single electron in the presence of an intense laser field. The parameters of the one-dimensional potential are chosen to fit two electronic states of the xenon atom. Numerical results indicate the e xistence of a correlation between harmonic generation and other non-li near effects occurring in this system such as avoided crossings of Flo quet resonance states as the field strength amplitude is varied. The h armonic generation spectra obtained by the model presented here qualit atively agree with experimental results on rare gas atoms placed in st rong laser fields. For strong laser fields (> 10(14) W cm-2), very-hig h-order harmonic generation is observed in numerical studies using bot h complex Floquet analysis and a simulation by a pseudo-spectral grid solution to the time-dependent Schrodinger equation with a non-periodi c Hamiltonian. For extremely strong laser fields, numerical results pr edict that the most intense frequency emitted by an atom interacting w ith a monochromatic laser field might not be the irradiating laser fre quency, but rather a high multiple of it.