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

Abstract

We applied a strong (<= 2.6 X 10(8) 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 ((CO)-C-13-O-16 and (CO)-C-12-O-18) 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) /(10 8 V.m(-1)) in crystalline CO. The VSE on the coupled CO stretching vibrations of the major isotopologue ((CO)-C-12-O-16) 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 X 10(8) 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.