Terahertz laser vibration-rotation-tunneling spectrum of the water pentamer-d₁₀. Constraints on the bifurcation tunneling dynamics

The vibration-rotation-tunneling (VRT) spectrum of a low-frequency intermolecular vibration of (D₂O)₅ was recorded near 0.9 THz (30.2 cm^-1). From an analysis of the relative intensities in the compact Q-branch region, the ground-state C-rotational constant is estimated to be 975 ± 60 MHz, consistent with ab initio structural predictions. The precisely determined B-rotational constant (B = 1750.96 ± 0.20 MHz) agrees well with previous results. Efforts to resolve possible bifurcation tunneling fine structure, such as that observed in VRT spectra of (D₂O)₃, revealed no such effects. This constrains the splittings to be less than 450 kHz, or roughly 3 times smaller than required by previous results.