Effect of Electric Field on Condensed-Phase Molecular Systems. II. Stark Effect on the Hydroxyl Stretch Vibration of Ice

We studied the Stark effect on the hydroxyl stretching vibration of water molecules in ice under the influence of an external electric field. Electric fields with strengths in the range from 6.4 × 10⁷ to 2.3 × 10⁸ V·m^-1 were applied to an ice sample using the ice film capacitor method. Reflection absorption infrared spectroscopy was used to monitor the field-induced spectral changes of vibrationally decoupled O-H and O-D bands of dilute HOD in D₂O and H₂O-ice, respectively. The spectral changes of the hydroxyl bands under applied field were analyzed using a model that simulates the absorption of a collection of Stark-shifted oscillators. The analysis shows that the Stark tuning rate of ν(O-D) is 6.4-12 cm^-1/(MV·cm^-1) at a field strength from 1.8 × 10⁸ to 6.4 × 10⁷ V·m^-1, and the Stark tuning rate of ν(O-H) is 10-16 cm^-1/(MV·cm^-1) at a field strength from 2.3 × 10⁸ to 9.2 × 10⁷ V·m^-1. These values are uniquely large compared to the Stark tuning rates of carbonyl or nitrile vibrations in other frozen molecular solids. Quantum mechanical calculations for the vibrations of isolated water and water clusters show that the vibrational Stark effect increases with the formation of intermolecular hydrogen bonds. This suggests that that the large Stark tuning rate of ice is due to its hydrogen-bonding network, which increases anharmonicity of the potential curve along the O-H bond and the ability to shift the electron density under applied electric field.