The χ (2) (second-order nonlinear optical susceptibility) spectrum of the water surface has been a matter of debate for a few decades. Here, we report that we experimentally measured the isotopic dilution dependence of the χ (2) spectrum and theoretically reproduced it by employing the quantum/classical mixed approach with a new idea to subtract an artifact. The present theoretical framework allows for clarifying the effects of the intramolecular, intermolecular, and Fermi resonance couplings on the OH-stretch vibrational spectra of water at the surface as well as in the bulk.

We present an experimental and theoretical study of Raman spectroscopy of isotopically pure and diluted amorphous as well as crystalline ices at 30 K. Our experiments comprise polarized (VV) and depolarized (VH) spectra in the OHstretch region, whereas our calculations involve molecular dynamics (MD) simulation of the high-and low-density amorphous ices through vitrification and subsequent formulation of vibrational Hamiltonian of coupled OH oscillators in the hydrogen-bond network of the ices. Our theoretical Raman VV and VH spectra at each isotopic dilution are in good agreement with the experimental data. This allows us not only to carry out detailed spectral assignments for the amorphous and crystalline ices from the viewpoint of vibrational phase, delocalization, and density of states but also to confirm the reliability of the MD simulation in which deeply supercooled liquid water spontaneously separates into high-and low-density fluid phases below a liquid-liquid critical point.

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We computed the IR, Raman, and sum frequency generation spectra of water in the OH-stretch region by employing the quantum/classical mixed approach that consists of a vibrational spectroscopic map and molecular dynamics (MD) simulation. We carried out the MD simulation with the TIP4P, TIP4P/2005, and TIP4P/Ice models and applied the map designed for TIP4P by Skinner et al. to each MD trajectory. Although the map is not tuned for TIP4P-like models, TIP4P/2005 and TIP4P/Ice provide the best reproduction of the experimental vibrational spectra of liquid water and crystalline ice, respectively. This result demonstrates the transferability of the map from TIP4P to TIP4P/2005 and TIP4P/Ice, meaning that one can choose an appropriate TIP4P-like model to calculate the vibrational spectra of an aqueous system without rebuilding the map.

We present an experimental and theoretical Raman spectroscopic study of isotopically pure and diluted ice VI at 1 GPa at room temperature. In the experiment, we measured the OHstretch Raman spectra of polycrystalline ice VI pressurized in a diamond anvil cell. We also obtained the theoretical Raman spectra of ice VI as well as ice I h on the basis of molecular dynamics (MD) simulations and a spectroscopic map. We find that the theoretical Raman spectra are in good agreement with the experimental data, which proves the applicability and accuracy of the spectroscopic map and the MD simulations not only in the ambient condition but also at the high pressure. In particular, the theoretical calculation successfully reproduces the experimental observation of the broader bandwidth of ice VI as opposed to ice I h . Through the analysis of the MD trajectories, we attribute this bandwidth difference to the site dependence of the OH-stretch frequency that takes place specifically in ice VI. The present study demonstrates how powerful the combination of experiment and theory in vibrational spectroscopy can be for gaining a microscopic understanding of complex molecular systems.