1P069
Towards Heterodyne-Detected Sum-Frequency Generation from
Buried Glass-Water Interface
Molecular Spectroscopy Laboratory, RIKEN, Japan MYALITSIN, Anton;
NIHONYANAGI, Satoshi; YAMAGUCHI, Shoichi; TAHARA, Tahei

Solid-liquid interfaces are ubiquitous in material science and are utilized for awide variety of applications, such as wetting, catalysis and electrochemistry. To
understand and control processes at the surface of the material, it is necessary toto observe the interfacial molecules in situ. However, due to their buried nature,solid-liquid interfaces are surrounded by bulk material and thus make it difficult
to characterize with conventional spectroscopic methods. In contrast,
Sum-Frequency Generation (SFG) can selectively probe the surface of a material
without contribution from the bulk and is therefore suitable for this purpose.
Here we report for the first time the measurement of the complex second ordernon-linear susceptibility χ(2) from the buried glass/water interface, using therecently developed heterodyne-detected vibrational SFG (HD-VSFG) spectroscopy(Figure 1a). This is of particular interest, since the imaginary part of the χ(2)spectrum is directly comparable to the linear absorption spectrum of the watermolecules in the bulk [1]. Additionally, the sign of the imaginary χ(2) provides us
with information about the absolute orientation of the interfacial molecules [2,3].
However, the measurement of the complex χ(2) spectrum requires a carefulconsideration of the phase. Typically, for air/liquid and air/solid surfaces, this isachieved by using the non-resonant signal from a quartz crystal as a reference.
The phase shift Ψ between air/sample and air/quartz is straightforward, if bothsamples are kept at the same height. For the silica/water interface the phase shift,introduced by the glass itself, has to be considered as well. Therefore, weevaporated a 150 nm gold film at one side of the fused silica substrate and used
the sum-frequency signal from the glass/gold interface to normalize theglass/water spectrum.

Figure 1: Schematic of the VSFG setup for buried interface.

χ(2) spectra from the buried silica surface were recorded in a home-built fluid

cell (Figure 1, inset). The gold film at the bottom of the glass was used as areference. Water with different pH could be added and exchanged with a pipette,
without touching the sample. This configuration ensured high stability of thesetup.
A gold film in air shows only an electronically resonant, very broad and mostly
imaginary, χ(2). Under the assumption that the silica/gold χ(2) follows the sameshape, it was 2used to normalize the silica/water spectra. The amplitude of the SFG(2)spectra �𝜒eff � , measured under homodyne and heterodyne conditions, gave thesame results, even though some ambiguity about the phase correction for theburied interface remains. This point is currently under investigation.
(2)

Figure 2 a) and b) show the real and imaginary 𝜒eff of the glass/waterinterfacefor different pH, using glass/gold as reference. The magnitude square2(2)�𝜒eff � is plotted in Figure 2c). By changing the pH of the solution from basic to(2)spectrum of theacidic, we observed an inversion of the sign of the imaginary 𝜒-OH stretch region, which indicates re-orientation of interfacial water at the glass
interface. This flip-flop of the water molecules is explained by the de-protonationof the terminal Si-OH hydroxyl groups to Si-O- of the fused silica at high pH.
Thereby the silica surface becomes negatively charged. This in turn leads to an
inversion of the orientation of the water molecules which re-align with the local
electric field.

(𝟐)

Figure 2: Complex 𝝌𝐞𝐟𝐟 for H2O at different pH and D2O: a) real, b) imaginary, c) magnitude square.
(𝟐)
Flip-flop of interfacial water is observed in the imaginary 𝝌𝐞𝐟𝐟 .
[1] Nihonyanagi, S.; Mondal, J.; Yamaguchi, S.; Tahara, T., Ann. Rev. Phys. Chem., 2013, 64, 579.
[2] Nihonyanagi, S.; Yamaguchi, S.; Tahara, T., J. Chem. Phys., 2009, 130, 204704.
[3] Yamaguchi, S.; Tahara, T., J. Chem. Phys. 2008, 129, 101102.