Entanglement of Molecular Orientation and Vibronic Degrees of Freedom by Ultrafast Photoexcitation in an Ensemble of Initially Randomly Oriented Molecules

When exciting an ensemble of initially randomly oriented molecules, a linearly polarized few cycle, few femtosecond, or UV or NIR pulse entangles the molecular degrees of freedom with the orientations of the molecule during the fast excitation step. We show, using fully quantum dynamical studies of an ensemble of initially randomly oriented LiH molecules, that the entanglement is not maximal and varies significantly with the pulse parameters. For a few-cycle NIR multiphoton excitation, a dozen dominant orientations suffice to describe the ensemble coherent dynamics, while only a few are needed for a one-photon UV process. Each principal orientation is correlated with a principal molecular vector made of a superposition of Σ or Π electronic states because of the cylindrical symmetry. For each principal molecular vector, the oscillation of specific electronic coherences drives charge migration and forces on the nuclei.