Role of Vibrational Excitations in High-Order Harmonic Generation

We carry out theoretical calculations to study the role of vibrational excitations in high-order harmonic generation of molecules by intense laser fields. The validity of the Franck-Condon Principle has been investigated in strong field ionization of molecules, as well as in the recombination step. We compare HHG yields of H$_{2}$ and D$_{2}$ using IR lasers with different wavelengths and peak intensities to uncover the dynamics of vibrational states, including their coherence, in the harmonic generation process.