Semiclassical treatment for molecular rotation spectra in high electric fields

TL;DR

This paper explores the behavior of molecular rotation spectra under high electric fields, introducing an approximate method, analyzing static and oscillating fields, and discussing implications for polar materials and high-power laser interactions.

Contribution

It presents a novel approximate method for analyzing molecular rotation spectra in high electric fields and discusses the emergence of new polarization modes called "dipolons."

Findings

Separation of azimuthal and zenithal motions simplifies spectral analysis.

Strong electric fields induce large macroscopic polarization and parametric resonances.

High-frequency oscillating fields effectively behave as static fields with reduced strength.

Abstract

Molecular rotation spectra, generated by the coupling of the molecular electric-dipole moments to an external time-dependent electric field, are discussed in a few particular conditions which can be of some experimental interest. First, the spherical-pendulum molecular model is reviewed, with the aim of introducing an approximate method which consists in the separation of the azimuthal and zenithal motions. Second, rotation spectra are considered in the presence of a static electric field. Two particular cases are analyzed, corresponding to strong and weak fields. In both cases the dipoles perform rotations and vibrations about equilibrium positions, which may exhibit parametric resonances. For strong fields a large macroscopic electric polarization may appear. This situation may be relevant for polar matter (like pyroelectrics, ferroelectrics), or for heavy impurities embedded in a polar solid. The dipolar interaction is analyzed in polar condensed matter, where it is shown that new polarization modes appear for a spontaneous macroscopic electric polarization (these modes are tentatively called "dipolons"); one of the polarization modes is related to parametric resonances. The extension of these considerations to magnetic dipoles is briefly discussed. The treatment is extended to strong electric fields which oscillate with a high frequency, as those provided by high-power lasers. It is shown that the effect of such fields on molecular dynamics is governed by a much weaker, effective, renormalized, static electric field.