  Because of their central importance in chemistry and biology, water molecules
have been the subject of decades of intense spectroscopic investigations.
Rotational spectroscopy of water vapor has yielded detailed information about
the structure and dynamics of isolated water molecules as well as water dimers
and clusters. Nonlinear rotational spectroscopy in the terahertz regime has
been developed recently to investigate the rotational dynamics of linear and
symmetric-top molecules whose rotational energy levels are regularly spaced,
but it has not previously been applied to water or other lower-symmetry
molecules with irregularly spaced levels. We report the use of recently
developed two-dimensional terahertz rotational spectroscopy to observe
high-order rotational coherences and correlations between rotational
transitions that could not be observed previously. The results include
two-quantum (2Q) peaks at frequencies that are shifted slightly from the sums
of distinct rotational transitions on two molecules, which directly reveal the
presence of previously unseen metastable water complexes with lifetimes of 100
ps or longer. Several such peaks observed at distinct 2Q frequencies indicate
that the complexes have multiple preferred bimolecular geometries. Our results
demonstrate sensitivity of rotational correlations measured in 2D THz
spectroscopy to molecular interactions and complexation in the gas phase.
