Terahertz laser spectroscopy of the water dimer intermolecular vibrations.II. (H2O)(2)

Terahertz VRT laser spectra of four (H2O)(2) intermolecular vibrations cons isting of 362 transitions have been measured between 87 and 108 cm(-1) with ca. 2 MHz precision. The results differ both qualitatively and quantitativ ely from the predictions of dimer potentials tested. The spectra also revea l an ordering of the intermolecular vibrations which differs dramatically f rom that predicted by normal mode analysis. Strong coupling is indicated be tween the low barrier tunneling motions and the intermolecular vibrations a s well as among different vibrations. In particular the 102.1 cm(-1) (H2O)( 2) vibration assigned as the acceptor wag (nu(8)) exhibits two types of per turbations. In one of these a component of K-a = 1 coupling with a tunnelin g component of K-a = 0 in the 108 cm(-1) acceptor twist (nu(11)) vibration. There is also an indication that the 103.1 cm(-1) (H2O)(2) band assigned a s the donor in-plane bend (nu(6)) is coupled to the acceptor wag resulting in a lower of the in-plane bend frequency and a higher acceptor wag frequen cy. Detailed analysis of the VRT levels confirms the extreme nonrigidity of this complex, indicating that the use of approximate models with reduced d imensionality to calculate its properties are likely to fail. (C) 2000 Amer ican Institute of Physics. [S0021-9606(00)00821-7].