Constraints on the bifurcation tunneling dynamics

Ž . Ž . The vibration–rotation–tunneling VRT spectrum of a low-frequency intermolecular vibration of D O was recorded 2 5 Ž y1. near 0.9 THz 30.2 cm . 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 Ž . determined B-rotational constant Bs1750.96"0.20 MHz agrees well with previous results. Efforts to resolve possible Ž . bifurcation tunneling fine structure, such as that observed in VRT spectra of D O , revealed no such effects. This 2 3 constrains the splittings to be less than 450 kHz, or roughly 3 times smaller than required by previous results. q 1998 Published by Elsevier Science B.V. All rights reserved.