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The low-wavenumber spectra of CCl 4 , Milli-Q water and aqueous solution of a-chymotrypsin protein were measured using four-photon coherent spectroscopy in the subterahertz range. Good coincidence of narrow peak positions in the four-photon spectra of liquid CCl 4 with the rotational frequencies in the gas phase was observed for the first time. A new sharp (width ∼0.6 cm −1 ) resonance at ∼3 cm −1 was observed in the aqueous protein solution, in contrast with water. This difference was interpreted as an exhibition of low-wavenumber vibrations of large fragments of the protein molecule.

The terahertz and subterahertz four-photon spectroscopy of liquids gives the opportunity to study large volume samples in native conditions. We present the experimental measurements by this technique of the low-wavenumber molecular motion in water and some aqueous solution. In fact, the coherent librations of H 2 O molecules in liquid water as well as aqueous solutions of H 2 O 2 and a protein in the microwave range (0.1-100 cm −1 ) are observed. A satisfactory correlation of the observed resonances with the rotational spectra of H 2 O molecule in the gas phase is found. Moreover the lines of ortho-and para-isomers of H 2 O molecule were identified. We have found that the protein molecules in water interact selectively with the spin isomers of H 2 O. There is a strong suppression of the rotational motion of the para-isomers and their respective line intensities, with a corresponding magnification by a factor of 8 by the ortho species.

Four-photon spectroscopy of the Rayleigh wing was applied to study distilled and Milli-Q water, liquid carbon tetrachloride (CCl 4 ), 30% aqueous solution of hydrogen peroxide (H 2 O 2 ) and 0.4 mM aqueous solution of a-chymotrypsin protein, in the spectral range from −10 to 10 cm −1 . The observed spectra allowed the measurement of the fracton dimensionality as d = 2.87 ± 0.01 in Milli-Q water, 2.97 ± 0.02 in the protein solution, 3.00 ± 0.02 in CCl 4 and H 2 O 2 and 3.15 ± 0.05 in distilled water. The variation of fractal dimensionality can be interpreted as indication of the structural differences in the investigated liquids. In the presence of protein and hydrogen peroxide molecules, this difference indicates that the hydrogen-bond network of Milli-Q water is restructured in the solution.

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