Terahertz vibration-rotation-tunneling spectroscopy of the water tetramer-d8: combined analysis of vibrational bands at 4.1 and 2.0 THz.

We report the measurement of terahertz vibration-rotation-tunneling spectra of (D(2)O)(4) in the spectral region near 4.13 THz. The characterization of this perpendicular band extends a previously reported study [J. Chem. Phys. 111, 7801 (1999)]. We observed 239 new transitions, each being split into a doublet of constant (approximately 192 MHz) spacing. These are included in a combined fit with the 113 previously measured transitions of the 2.03 THz parallel band using an effective Hamiltonian similar to that used in the global fit of the water trimer. The detailed understanding of the water tetramer evolving from this work underlies our efforts to quantify the contribution of many-body forces to the hydrogen bonding interactions in condensed phase water.