Experimental and Theoretical Heterodyne-Detected Sum Frequency Generation Spectroscopy of Isotopically Pure and Diluted Water Surfaces.

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.