Rotationally-resolved spectroscopy of the donor bending mode of (D ₂O)₂

High-resolution spectra of the intramolecular bending modes of deuterated water dimer, (D₂O)₂, have been measured using a quantum cascade laser based cavity ringdown spectrometer. Two perpendicular bands have been observed and are assigned as the K_a = 1 ← 0 and K _a = 2 ← 1 bands of the bending mode of the hydrogen bond donor. The tunneling splittings in the complex are well-resolved, and it is found that excitation of the donor bend has little effect on tunneling of the hydrogen bond acceptor, but causes significant perturbations on the tunneling motion which exchanges the roles of hydrogen bond donor and acceptor. The presence of this perturbation has prevented a detailed assignment of the tunneling levels in the excited state at this time. An accurate value for the band center of the donor bend is calculated to be 1182.2 cm^-1, which is in good agreement with previous theoretical calculations performed on an ab initio potential energy surface.