Consistency in the Sum Frequency Generation Intensity and Phase Vibrational Spectra of the Air/Neat Water Interface

Abstract

Substantial progress has been made in the quantitative understanding and interpretation of the hydrogen bonding and ordering structure of the air/water interface since the first sum-frequency generation vibrational spectroscopy (SFG-VS) measurement by Q. Du et al. in 1993 (Phys. Rev. Lett. 1993, 70, 2312-2316). However, there are still disagreements and controversies on the consistency between the different experimental measurements, as well as in the theoretical and computational results. One critical problem lies in the lack of consistency between the SFG-VS intensity measurements and the recently developed SFG-VS phase spectra measurements of the neat air/water interface, which has inspired various theoretical efforts. In this report, the reliability of the SFG-VS intensity spectra of the air/neat water interface is to be quantitatively examined, and possible sources of inaccuracies in the SFG-VS phase spectral measurement are to be discussed based on the nonresonant SHG phase measurements. Solid evidence is shown indicating that the SFG-VS intensity spectra from different laboratories are now quantitatively converging and in agreement with each other. However, the possible inaccuracies and inconsistencies in the SFG-VS phase spectra measurements need to be carefully examined against a properly corrected phase standard to take full advantage of this powerful experimental tool.

Solvation of Calcium–Phosphate Headgroup Complexes at the DPPC/Aqueous Interface

“…It is generally accepted that the lower frequency part of the broad band represents water molecules that form stronger hydrogen bonds than those from the higher frequency part . Other assignments to this broad region have also been proposed . In comparison to the bare air/water interface, the VSFG spectrum of the DPPC monolayer on water shows a significant uneven enhancement in intensity over the entire OH stretching region from 3000 to 3600 cm −1 , particularly at 3200 cm −1 , in accordance with results previously published .…”

Surface Electric Fields of Aqueous Solutions of NH4NO3, Mg(NO3)2, NaNO3, and LiNO3: Implications for Atmospheric Aerosol Chemistry

“…Note that a strict comparison would require taking into account normalization and Fresnel factors correction procedures. 67,68 However, even without these corrections the general trend between salt solutions observed in the VSFG spectra is essentially consistent with the reconstructed power spectra. Only every second and fourth data points are plotted in the conventional VSFG and HD-VSFG spectra, respectively, to avoid spectral clutter.…”

Molecular Structure and Dynamics of Water at the Water–Air Interface Studied with Surface‐Specific Vibrational Spectroscopy

“…This value is obtained from molecular dynamics (MD) simulations showing that there is no preferential water orientation deeper than 5 and that the calculated SFG spectrum does not change anymore by including water below 5from the air-water interface. Intriguingly,a nS FG vibrational spectrum of the water-air interface,f irst reported by Shen and co-workers [29] and subsequently by many others showing av ery similar spectrum, [50] appears to have all three of these modes:the peak at approximately 3700 (2750) cm À1 is generally assigned to water molecules with one O À Hg roup pointing into the gas phase and termed the "free OÀH" group,w hile the peaks at approximately 3200 (2350) and approximately 3400 (2500) cm À1 appear in the frequency region for hydrogenbonded O À Hg roups.T hese latter two peaks originate from the majority of water molecules at the interface,which donate two hydrogen bonds.I na nalogy to the IR absorption spectrum of ice and liquid water, these two peaks in the SFG spectrum have,over the last fifteen years,been taken to be the result of "ice-like" and "liquid-like" interfacial water. Intriguingly,a nS FG vibrational spectrum of the water-air interface,f irst reported by Shen and co-workers [29] and subsequently by many others showing av ery similar spectrum, [50] appears to have all three of these modes:the peak at approximately 3700 (2750) cm À1 is generally assigned to water molecules with one O À Hg roup pointing into the gas phase and termed the "free OÀH" group,w hile the peaks at approximately 3200 (2350) and approximately 3400 (2500) cm À1 appear in the frequency region for hydrogenbonded O À Hg roups.T hese latter two peaks originate from the majority of water molecules at the interface,which donate two hydrogen bonds.I na nalogy to the IR absorption spectrum of ice and liquid water, these two peaks in the SFG spectrum have,over the last fifteen years,been taken to be the result of "ice-like" and "liquid-like" interfacial water.…”