Figure 1

(a) VSFG spectra of the heavy water-air interface for ssp polarization. Fits to the data are shown by the solid lines. Simulated data for the “icelike” and “waterlike” hypothesis are shown by the dotted line. Peak positions are indicated in the graph. For pure ${\mathrm{D}}_2\mathrm{O}$ [ratio $\mathrm{D}/(\mathrm{D}+\mathrm{H})=1$, closed circles], two resonances are clearly identified in the O-D stretch region. For a sample with $\mathrm{D}/(\mathrm{D}+\mathrm{H})=0.33$ (open circles), the contribution from ${\mathrm{D}}_2\mathrm{O}$ modes is less than 10%, and HDO modes dominate the spectrum. (b) Energy level diagram for ${\mathrm{D}}_2\mathrm{O}$ and HDO water molecules. The fundamental O-D stretch vibration is coupled intramolecularly, giving rise to symmetric (ss) and asymmetric (as) modes. Because of Fermi resonance with the overtone of the bending mode (${\delta }_2$), the ss mode is further split into a low and a high frequency band (labeled here ${\nu }_1$ and ${\nu }_2$). For an HDO water molecule, the intramolecular modes are decoupled, allowing the determination of the fundamental O-D stretch vibration. The corresponding frequency of each energy level is specified, except for the as mode, in ${\mathrm{cm}}^{-1}$.Reuse & Permissions

Figure 2

(a) VSFG spectra of the lipid-water interface for ssp and pss polarization combination. The ssp spectra have been offset for clarity. Fits to the data are shown by the thin solid lines. The positions of the peaks are indicated, in ${\mathrm{cm}}^{-1}$. (b) Variation of the amplitudes of the nonlinear susceptibility components of the three modes as a function of the mole fraction of deuterium in the water subphase. Upper panel: the amplitudes of 2370 and $2520{\mathrm{cm}}^{-1}$ peaks and the concentration of ${\mathrm{D}}_2\mathrm{O}$ (dashed line). Lower panel: the amplitude of $2470{\mathrm{cm}}^{-1}$ peak and the concentration of HDO (dashed lines). The isolated O-D stretch peaks around $\mathrm{D}/(\mathrm{D}+\mathrm{H})=0.5$, since the HDO concentration peaks at this mole fraction.Reuse & Permissions