Terahertz laser spectroscopy of the water dimer intermolecular vibrations.I. (D2O)(2)

Terahertz laser VRT spectra of the water dimer consisting of 731 transition s measured with an average precision of 2 MHz and involving four (D2O)(2) i ntermolecular vibrations (one previously published) have been measured betw een 65 and 104 cm(-1). The precisely determined energy level patterns diffe r both qualitatively and quantitatively from the predictions of several dim er potentials tested, and reveal an ordering of the intermolecular vibratio ns which differs dramatically from that predicted by standard normal mode a nalysis. Strong coupling is indicated between the low barrier tunneling mot ions and the intermolecular vibrations as well as among different vibration s. Particularly, the 83 cm(-1) (acceptor wag) and 90 cm(-1) (D2O)(2) (accep tor twist) vibrations interact through a Coriolis perturbation. These spect ra provide the basis for our recent determination of the water pair potenti al. The corresponding data set for (H2O)(2) is presented in an accompanying paper. (C) 2000 American Institute of Physics. [S0021-9606(00)00721-2].