Optical absorption spectroscopy probes water wire and its ordering in a hydrogen-bond network

TL;DR

This study demonstrates that optical absorption spectroscopy, combined with advanced ab initio theory, can effectively detect and characterize water wires and their ordering in hydrogen-bond networks, including in biological and ice phases.

Contribution

It introduces a novel application of optical absorption spectroscopy to directly probe water wire structures and their ordering, supported by ab initio many-body calculations.

Findings

Charge transfer exciton dominates the spectrum in water and ice.

Spectral intensity correlates with water wire ordering length.

Maximum spectral signal observed in ice XI with long-range order.

Abstract

Water wires, quasi-one-dimensional chains composed of hydrogen-bonded (H-bonded) water molecules, play a fundamental role in numerous chemical, physical, and physiological processes. Yet direct experimental detection of water wires has been elusive so far. Based on advanced $ab$ $initio$ many-body theory that includes electron-hole interactions, we report that optical absorption spectroscopy can serve as a sensitive probe of water wires and their ordering. In both liquid and solid water, the main peak of the spectrum is discovered to be a charge transfer exciton. In water, the charge transfer exciton is strongly coupled to the H-bonding environment where the exciton is excited between H-bonded water molecules with a large spectral intensity. In regular ice, the spectral weight of the charge transfer exciton is enhanced by a collective excitation occurring on proton-ordered water wires, whose spectral intensity scales with the ordering length of water wire. The spectral intensity and excitonic interaction strength reaches its maximum in ice XI, where the long-range ordering length yields the most pronounced spectral signal. Our findings suggest that water wires, which widely exist in important physiological and biological systems and other phases of ice, can be directly probed by this approach.