Spectra of the D2O dimer in the O–D fundamental stretch region: Vibrational dependence of tunneling splittings and lifetimes.

The fundamental O–D stretch region (2600–2800 cm−1) of the fully deuterated water dimer (D2O)2 is studied using a pulsed supersonic slit jet source and a tunable optical parametric oscillator source. Relatively high spectral resolution (0.002 cm−1) enables all six dimer tunneling components to be observed, in most cases, for the acceptor asymmetric O–D stretch, the donor free O–D stretch, and the donor bound O–D stretch vibrations. The dominant acceptor switching tunneling splittings are observed to decrease moderately in the excited O–D stretch states, to roughly 75% of their ground state values, whereas the smaller donor-acceptor interchange splittings show more dramatic and irregular decreases. Excited state predissociation lifetimes, as determined from the observed line broadening, show large variations (0.2 ≤ τ ≤ 5 ns) depending on the vibrational state, K-value, and tunneling symmetry. Another very weak band is tentatively assigned to a combination mode involving an intramolecular O–D stretch plus an intermolecular twist overtone. Asymmetric O–D stretch bands of the mixed isotopologue dimers D2O–DOH and D2O–HOD are also observed and analyzed. [ABSTRACT FROM AUTHOR]