Toward Unraveling the Puzzle of Sum Frequency Generation Spectra at Interface of Aqueous Methanol Solution: Effects of Concentration-Dependent Hyperpolarizability

The air/liquid interface of aqueous methanol solutions, one of the earliest benchmark liquid interfaces ever studied by surface-selective sum frequency generation vibrational spectroscopy (SFG-VS), is known to display a turnover of SFG-VS intensities at the intermediate methanol concentrations, the origin of which has been under debate for the past 25 years. In this work, we discover that the hyperpolarizability of methanol, which was previously assumed to be a constant, in fact strongly depends on the bulk mole fractions of the methanol molecules. Furthermore, such concentration-dependence of the methanol hyperpolarizability is shown to play a critical role in the nonmonotonic changes of the methanol SFG-VS intensities as a function of the bulk mole fractions. By comparing the results from the molecular dynamics simulations and the newly interpreted SFG-VS spectra, a consistent surface model is obtained, showing that the average tilt angles of the asymmetrically oriented interfacial methanol molecules that actually contribute to the net SFG-VS signals are independent of the concentrations.