The dipole moment of water: Stark effects in D2O and HDO

Abstract

The dipole moment of D₂O has been determined from Stark effect measurements for the 3₁₃–2₂₀ and 4₄₁–5₃₂ microwave transitions as 1·857 ± 0·006 and 1·869 ± 0·005D respectively. A rotational dependence of dipole moment has also been established for HDO through μ_a in the 2₂₀–2₂₁ and 5₃₂–5₃₃ transitions; μ_a was determined as 0·662 ± 0·001 and 0·644 ± 0·001D respectively. The total dipole moment for HDO has been determined from the 3₂₁–4₁₄ transition to be 1·85 ± 0·01D and to lie within 0·1° of the bisector of the HOD angle. High resolution Stark spectroscopy has been performed on the 6₂₄–6₁₅ transition of D₂O with improved precision using the 337 μm emission line of the HCN laser. This experiment has confirmed the dipole results from the microwave work and the frequency of the 6₂₄–6₁₅ transition in D₂O has been determined as 890 395 ± 3 MHz. The slight increase of dipole moment with deuteration is consistent with the dipole moment for H₂O determined from the dielectric constant. This increase is discussed for the vibrational ground state (as for ammonia) in terms of anharmonicity in the bending vibration. The change of μ with rotational transition is interpreted in terms of large changes in molecular geometry for certain rotational states due to centrifugal distortion.