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Edited by Andrei Tokmakoff, The University of Chicago, Chicago, IL, and accepted by the Editorial Board November 10, 2013 (received for review September 16, 2013)
Edited by Andrei Tokmakoff, The University of Chicago, Chicago, IL, and accepted by the Editorial Board November 10, 2013 (received for review September 16, 2013)
Significance
The discussion about the structure of water becomes increasingly more controversial, and appears to be stuck because the major techniques used to study it (small-angle X-ray scattering emphasizing the structure, multidimensional infrared spectroscopy emphasizing the dynamics, and molecular dynamics simulations using questionable force fields) do not seem to provide mutually converging results. This calls for a different experimental approach, which we offer here with 2D Raman-terahertz spectroscopy. The work extends multidimensional vibrational spectroscopy into the far-IR regime where thermally excited soft modes are found that are directly responsible for molecular dynamics (in contrast with the high-frequency spectator modes investigated in 2D-IR spectroscopy, which per se are quiet at room temperature and sense the environment only indirectly).
Abstract
Two-dimensional Raman-terahertz (THz) spectroscopy is presented as a multidimensional spectroscopy directly in the far-IR regime. The method is used to explore the dynamics of the collective intermolecular modes of liquid water at ambient temperatures that emerge from the hydrogen-bond networks water forming. Two-dimensional Raman-THz spectroscopy interrogates these modes twice and as such can elucidate couplings and inhomogeneities of the various degrees of freedoms. An echo in the 2D Raman-THz response is indeed identified, indicating that a heterogeneous distribution of hydrogen-bond networks exists, albeit only on a very short 100-fs timescale. This timescale appears to be too short to be compatible with more extended, persistent structures assumed within a two-state model of water.
Footnotes
- 1To whom correspondence should be addressed. E-mail: phamm@pci.uzh.ch.
- Author contributions: J.S. and P.H. designed research; J.S. and S.A. performed research; J.S., S.A., and P.H. analyzed data; and J.S. and P.H. wrote the paper.
- The authors declare no conflict of interest.
- This article is a PNAS Direct Submission. A.T. is a guest editor invited by the Editorial Board.
- This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1317459110/-/DCSupplemental.
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