Aligning and Orienting Molecules Trapped in Octahedral Crystal Fields

Abstract

The effect of external fields on directional states of a linear molecule trapped in a crystal field of octahedral symmetry is studied numerically. Adiabatic field-dressed energy levels are obtained by solving the time-independent Schrödinger equation for the rotational degrees of freedom of the confined molecule. In the absence of external fields, the internal, octahedral crystal field serves to transform free-rotor states to angularly confined librational states of defined parity which arrange in near-degenerate sets of high multiplicity. Interaction of a linearly polarized, nonresonant laser field with the polarizability or of a static electric field with the dipole moment create alignment or orientation of the molecular axis, respectively. In the latter case, the combined effect of internal (octahedral) and external static field is instrumental in creating orientation by coupling different tunneling states. Depending on the polarization direction of the external fields with respect to the symmetry axes provided by the crystal field, cooperative and competitive effects are distinguished. If the direction of the external field coincides with the minima of the crystal field, high degrees of alignment or orientation can be achieved for specific states, even for low field strengths. Otherwise, high efficiency of this mechanism is restricted to high fields and low temperatures. Strategies for an experimental realization are outlined.