Nonlinear rotational spectroscopy reveals many-body interactions in water molecules

TL;DR

This study uses advanced two-dimensional terahertz rotational spectroscopy to detect many-body interactions and metastable complexes in water molecules, revealing new insights into water's molecular dynamics and interactions.

Contribution

The paper introduces the application of 2D terahertz rotational spectroscopy to water, uncovering high-order coherences and metastable complexes not observable with previous methods.

Findings

Detection of high-order rotational coherences in water

Observation of metastable water complexes with >100 ps lifetimes

Identification of multiple preferred bimolecular geometries

Abstract

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.