Electric Field Effect on Condensed-Phase Molecular Systems. VIII. Vibrational Stark Effect and Dipolar Inversion in a Carbon Monoxide Crystal

We applied a strong (≤2.6 × 108 V·m–1) external electric field across a carbon monoxide crystal film at 10 K and studied its effect on the sample with reflection–absorption infrared spectroscopy (RAIRS). The vibrational Stark effect (VSE) on the intramolecular CO stretching vibrations of the minor isotopologues (13C16O and 12C18O) reveal the spectral signature of isolated CO vibrations, decoupled from crystal phonons in the solid, as a function of the external electric field magnitude. These so-called molecular CO bands display a VSE with a sensitivity factor of 0.69 ± 0.05 cm–1/(108 V·m–1) in crystalline CO. The VSE on the coupled CO stretching vibrations of the major isotopologue (12C16O) was measured for crystalline and amorphous solid CO films, and the results were analyzed with the help of a classical optics model of RAIRS for thin solid films. In addition to these spectral changes due to VSE, the external electric field facilitates the head-to-tail inversion of CO dipoles in the crystal lattice as a result of electrostatic interactions. This result is the first experimental demonstration of dipole inversion in a molecular crystal induced by a DC electric field. The dipole inversion occurs slowly and irreversibly in crystalline CO, reaching a yield of up to about 20% dipole inversion at an external field strength of 2.6 × 108 V·m–1 at 10 K. The observed yield of dipole inversion is interpreted in terms of a thermodynamic model that accounts for the electrostatic stabilization energy of dipoles and the configurational entropy of the CO crystal. The present study demonstrates that a polarized CO crystal with reduced residual entropy can be formed by applying a strong electric field at low temperature.