Molecular High Order Harmonic Generation

Abstract

We present, in this chapter, numerical results of high order harmonic generation spectra (HOHG), in the Born-Oppenheimer approximation, of the diatomic molecular ion H+ 2 and the linear triatomic molecule H2+ 3 under strong laser fields. We revisit, first, the standard electron classical re-collision model. The atomic limit of 3.17 Up for re-collision energy can be considerably exceeded in molecules. HOHG spectra results of H+ 2 at R = 2a.u., under 10 cycles intense (I > 1014W/cm2) 800nm laser field, have shown to be strongly influenced by the orientation of the molecular axis with respect to the laser polarization axis. The interferences between contributions of each nucleus exhibit maximum at certain harmonic orders as a function of molecular orientation. A comparison between the acceleration and dipole formulations of the harmonic emission is presented. Finally, we present numerical results of harmonic spectra of the one electron linear H+ − −H+ 2 molecular system at large internuclear distances R = α and πα/2, where α is the ponderomotive radius, using ultra-short 2 cycles intense (I = 4 × 1014W/cm2) 800nm laser pulse linearly polarized along the internuclear axis. The extended cut-off is shown to be related to the nature of electron transfer, whose direction is shown to depend critically on the absolute carrier envelope phase (CEP) of the ultrashort pulse. Constructive and destructive interferences in the HHG spectrum of coherent superpositions of electronic states in the H+ −H+ 2 system are interpreted in terms of multiple electron trajectories from a Gabor time analysis.