Electric Field Effect on Condensed-Phase Molecular Systems. VII. Vibrational Stark Sensitivity of Spatially Oriented Water Molecules in an Argon Matrix

The susceptibility of a water molecule to electric fields provides fundamental and essential information for understanding the vibrational spectra of water clusters and condensed-phase water. In this study, the Stark sensitivities for the ν ₂ bending and ν ₁ symmetric stretching vibrations of water molecule were experimentally determined. The water molecules isolated in the solid Ar matrix were spatially oriented in the direction of the externally applied field (∼10 ⁸ V m ^-1 ) in the laboratory frame by using the ice-film nanocapacitor method. The signature of the field-induced reorientation of water molecules was observed with reflection-absorption infrared spectroscopy. The Stark sensitivities of the D ₂ O vibrations were determined from the field-induced change of vibrational frequencies of the spatially oriented D ₂ O molecules. The Stark sensitivity of the D ₂ O bending vibration was much larger than that of the symmetric stretching vibration, and the two normal modes showed the opposite signs. Isotope dependence of the Stark sensitivity of the bending vibration was also observed by investigating HDO and H ₂ O molecules. An ab initio calculation was conducted to elaborate on the observed characteristics of the Stark sensitivity of water vibrations.