Probing Orbital Structure of Polyatomic Molecules by High-order Harmonic Generation

Laser driven high-order harmonic generation (HHG) from molecules depends on the particular symmetry of the highest occupied molecular orbital (HOMO) and its orientation with respect to the laser field. High-order harmonic emission is observed in acetylene and allene molecules with 14 fs laser pulses. The molecules are aligned non-adiabatically producing a time dependent modulation of the harmonic signal as the ensemble undergoes the subsequent alignment revivals. At the points of maximum alignment the harmonic signal is measured as a function of the alignment angle showing a behavior that can be related to the structure of the highest occupied molecular orbitals.