Abstract-High-precision gigahertz-to-terahertz spectroscopy of aqueous salt solutions as a probe of the femtosecond-to-picosecond dynamics of liquid water

N. Q. Vinh^1,2,3,a), Mark S. Sherwin^1,2, S. James Allen^1,2, D. K. George³, A. J. Rahmani³ and Kevin W. Plaxco^1,4
 HIDE AFFILIATIONS
1 Institute for Terahertz Science and Technology, University of California, Santa Barbara, Santa Barbara, California 93106, USA
2 Department of Physics, University of California, Santa Barbara, Santa Barbara, California 93106, USA
3 Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA
4 Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, USA
a) Author to whom correspondence should be addressed. Electronic mail: vinh@vt.edu
J. Chem. Phys. 142, 164502 (2015); http://dx.doi.org/10.1063/1.4918708

Because it is sensitive to fluctuations occurring over femtoseconds to picoseconds, gigahertz-to-terahertz dielectric relaxation spectroscopy can provide a valuable window into water’s most rapid intermolecular motions. In response, we have built a vector network analyzer dielectricspectrometer capable of measuring absorbance and index of refraction in this frequency regime with unprecedented precision. Using this to determine the complex dielectric responseof water and aqueous salt solutions from 5.9 GHz to 1.12 THz (which we provide in the supplementary material), we have obtained strong new constraints on theories of water’s collective dynamics. For example, while the salt-dependencies we observe for water’s two slower relaxations (8 and 1 ps) are easily reconciled with suggestions that they arise due to rotations of fully and partially hydrogen bonded molecules, respectively, the salt-dependence of the fastest relaxation (180 fs) appears difficult to reconcile with its prior assignment to liberations of single hydrogen bonds