Enhanced Stimulated Raman Scattering by a Pressure-Controlled Shock Wave in Liquid Water

Stimulated Raman scattering (SRS) is observed using a Nd:YAG laser in liquid water at both forward and backward directions under different pressures. The spectra at atmospheric pressure and high pressure exhibit different characteristic features. For high pressure, the main SRS peak (about 3400 cm^–1) of liquid water shifts to low frequency. Interestingly, a new peak is observed in both directions. The position of the new peak is lower than that at atmospheric pressure, which belongs to strong hydrogen bonds. Especially, a low peak is obtained at around 3140 cm^–1 in the backward direction at 400 MPa, indicating the formation of an ice-like structure. In addition, the normalized SRS intensity of high pressure is higher than that of atmospheric pressure. These results indicate that high pressure can significantly enhance the SRS of water molecules. The enhancement mechanism is attributed to the long duration and slightly slow velocity of the shock wave induced by high pressure.