15min:

F. N. KEUTSCH, R. S. FELLERS, P. B. PETERSEN, M. R. VIANT, M. G. BROWN AND R. J. SAYKALLY, Department of Chemistry, University of California Berkeley, Berkeley, CA 94720.

We report the observation of a new vibration-rotation-tunneling (VRT) band of (D₂O)₃ at 142.8 cm^-1 and a set of four bands of (H₂O)₃ around 520 cm^-1. These new bands represent the first observation of a translational and librational vibration for a water cluster.

The observed VRT spectrum of (D₂O)₃ at 142.8 cm^-1, in the translational band of the liquid, is assigned to a combination band or mixed level of the asymmetric hydrogen bond stretch and a torsional vibration. The predicted frequencies of the hydrogen bond stretching modes are too high, presumably because calculations fail to include the necessary coupling between stretching and torsional motions .

The bands of (H₂O)₃ around 520 cm^-1 lie in the librational band region of liquid water and are tentatively assigned to the out of plane librational vibration. The observation of at least four bands within 8 cm^-1 is explained by a dramatically increased splitting of the excited state rovibrational levels by bifurcation-tunneling. The experimental results presented should therefore allow for the first exact determination of the height of the bifurcation tunneling barrier. The tunneling time scale is estimated at 2-4ps, similar to those of several important dynamical processes in bulk water .