Theory of Molecular Quantum Dynamics in High-Harmonic Generation

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Intensive research efforts are currently devoted to high-harmonic generation, i.e. the generation of high-frequency photons by laser irradiation of atoms or molecules. In the widely accepted three-step model, harmonic radiation originates from a sequence of ionization, free acceleration of the electron in the laser field, and recollision with the core. This recollision is of great interest since it allows to probe the state of a molecule with its own electron. Previously, the influence of molecular dynamics on high-harmonic generation was described in an approximation where only the active electron feels the laser force while the dynamics of the core is field free. Here, we develop a theory to overcome this limitation and take the interaction of the molecular core with the laser field into account. This involves the propagation of nuclear wave packets in laser-coupled Born-Oppenheimer surfaces. The approach is applicable to diatomic as well as polyatomic molecules. The signatures in the harmonic spectrum due to nuclear motion contain information on the instantaneous laser-induced distortion of Born-Oppenheimer potentials, thus providing a way to investigate these distortions experimentally. We investigate schemes to control high harmonic generation by exploiting the nuclear motion. Predictions of harmonic spectra not only for the usual Ti:Sapphire laser wavelength but also for longer infrared wavelengths are planned.