VIBRATIONAL AND K(A)' DEPENDENCIES OF THE MULTIDIMENSIONAL TUNNELING DYNAMICS IN THE 82.6 CM(-1) INTERMOLECULAR VIBRATION OF THE WATER DIMER-D(4)

Using tunable far infrared laser absorption spectroscopy, 12 vibration -rotation-tunneling (VRT) subbands, consisting of approximately 230 tr ansitions have been measured and analyzed for an 82.6 cm-1 intermolecu lar vibration of the water dimer-d4. Each of the VRT subbands originat e from K(a)'' = 0 and terminate in either K(a)' = 0 or 1. These data p rovide a complete characterization of the tunneling dynamics in the vi brationally excited state as well as definitive symmetry labels for al l VRT energy levels. Furthermore, an accurate value for the A' rotatio nal constant of 122.9 GHz is found to agree well with its correspondin g ground state value. All other excited state rotational constants are fitted, and discussed in terms of the corresponding ground state cons tants. In this vibration, the quantum tunneling motions are determined to exhibit large dependencies with both the K(a)' quantum number and the vibrational coordinate, as is evidenced by the measured tunneling splittings. The generalized internal-axis-method (IAM) treatment, whic h has been developed to model the ground state tunneling dynamics, is considered for the qualitative description of each tunneling pathway, however, the variation of tunneling splittings with vibrational excita tion indicate that the high barrier approximation does not appear to b e applicable in the excited state. The data are consistent with a moti on possessing a' symmetry, and the vibration is assigned as the nu8 ac ceptor bending coordinate. This assignment is in agreement with the vi brational symmetry determined from the data, the results of high level ab initio calculations, and preliminary data assigned to the analogou s vibration in D2O-DOH.