Creating Giant Oscillating Molecular Dipoles with Strong Electric Fields and Ultrashort Laser Pulses

The dynamics of molecular properties is always associated to the quantum superpositions of Hamiltonian eigenstates. While the nuclear wave function encodes the molecular geometry, the electronic distribution is responsible for the chemical properties. The correlation between electronic and nuclear motions generally manifest as smooth changes of the electronic distribution with respect to changes in the nuclear coordinates, which is the result of the different time scales of electronic and nuclear motions, i.e., of the widespread validity of the Born-Oppenheimer approximation. To date, most of the studied dynamical processes in molecules involve superposition of vibrational states belonging to the same electronic state. We use ultrashort laser pulses to prepare wave packets containing a quantum superposition of both electronic and nuclear degrees of freedom, such that the motion of both electrons and nuclei is highly correlated and occurs in the time-scale of the nuclear motion.

In this work we solve the time-dependent Schrödinger equation on a two-dimensional grid, including both nuclear and electronic degrees of freedom, forcing a 1D motion of the electron in the internuclear axis described by soft-core Coulomb potentials[1]. This model allows the calculation of ionization, dissociation and strong field effects, without invoking the Born-Oppenheimer approximation. The molecular Hydrogen ion is used as a benchmark for the calculations. The strong field is used to orient the H2+ molecules and break the molecular symmetry, therefore favoring one direction for the electronic motion[2]. We exploit the strong transient dipole to allow vibrational trapping in the excited dissociative state up to very large bond distances[3]. We show that we can create large oscillating dipole moments, typically of the order of 10-50 debyes (much larger than any permanent dipole of a diatomic molecule), whose amplitude and period (or the order of 25-300 fs) can be controlled by means of a strong static electric field, while at the same time minimizing the yield of ionization.

The control mechanism proposed in this work has potential applications in controlling the reactivity of a molecule, which is greatly influenced by the electronic density, and in generating electromagnetic radiation of specific frequencies.

[1] K. Kulander, F. Mies, K. Schafer, Phys. Rev. A 53, 2562 (1996).
[2] B. Y. Chang, S. Shin, A. Palacios, F. Martín, I. R. Sola, ChemPhysChem 14, 1405 (2013).
[3] G. Yudin, S. Chelkowski J. Itatani, A. Bandrauk, P. Corkum, Phys. Rev. A 72, 2 (2005).