Response to “Comment on ‘Phase reference in phase-sensitive sum-frequency vibrational spectroscopy”’ [J. Chem. Phys. 145, 167101 (2016)]

Ab initio Modeling of the Vibrational Sum-Frequency Generation Spectrum of Interfacial Water

“…A current hypothesis derived from SFG experiments suggests that the weak low-frequency signal at 3000 cm –1 should come from a nonresonant background (with respect to different references using for the phase determination), and it is unrelated to specific molecular vibrations of interfacial water in this frequency region. − However, according to our findings, probing deeper water layers will enhance the positive 3000 cm –1 signal. It is likely that current SFG experiments only explore a relatively shallow region of the interface, less than ∼3 Å deep.…”

“…It is generally assumed that the dipole orientation of the water molecules with dangling OH bonds preferentially points toward the air, while that of the H-bonded water molecules points toward the bulk phase, due to the fact that the signal of the sharp peak (∼3700 cm –1 ) is positive and that of the broad band (3100–3600 cm –1 ) is negative. In particular, the existence of the positive peak around 3000 cm –1 , which was observed in Im­[χ (2) ] spectra from the Shen group, , lacks an intuitive interpretation, and the origin of this positive signal has been under intense debate. − Now, an accord seems to have been reached that this weak low-frequency signal, even if it exists, is a nonresonant background unrelated to specific molecular vibrations in this frequency region. − …”

Both Inter- and Intramolecular Coupling of O–H Groups Determine the Vibrational Response of the Water/Air Interface

“…For comparison, experimentally determined Im­(χ (2) ) spectra of neat H 2 O and that of HDO are presented in Figure B. The HOD response was obtained from the response of a 3:1 D 2 O/H 2 O mixture, correcting for the statistically present H 2 O molecules in such a mixture. , These Im­(χ (2) ) spectra spectra are slightly different from literature spectra, , which is probably caused by uncertainties in correcting for the phase in the heterodyne experiments. , Qualitatively, the experimental spectra agree with the simulated ones, also showing a decrease in intensity on the low-frequency side of the absorption band upon isotopic dilution associated with the coupling mechanisms discussed above.…”

Proton Order toward the Surface of Ice Ih Revealed by Heterodyne-Detected Sum Frequency Generation Spectroscopy

“…At this sacrifice of the temperature accuracy, the use of the open Dewar vessel enabled the same measurement of the height for the ice surfaces as routinely done for liquid surfaces with the accuracy of 1 μm, ,, resulting in the same reliability of the complex phase of χ (2) . A series of discussion on the “low-frequency band” of the water surface ,,− demonstrates the reliability of the complex phase of χ (2) given by our HD-SFG spectrometer. The experimental data in Figures and were obtained by averaging the results of more than 22 measurements.…”