  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 observed line broadening, show large variations (0.2 to 5 nanoseconds)
depending on 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.
