Resolving the Controversy over Dipole versus Quadrupole Mechanism of Bend Vibration of Water in Vibrational Sum Frequency Generation Spectra

Abstract

Recently, there has been controversy over whether the HOH bend signal of water in the vibrational sum frequency generation (VSFG) spectrum arises from the conventional dipole mechanism or the quadrupole mechanism. Here, we show that the Im χ (2) (the imaginary part of the second-order nonlinear susceptibility) spectra of the HOH bend mode of water at oppositely charged monolayer/water interfaces all exhibit positive bands, irrespective of the difference in the sign of the charge at the interface. Furthermore, it is found that the peak frequency of the HOH bend band substantially changes depending on the chemical structure of the charged headgroup located at the interface. These results demonstrate that the VSFG signal of the HOH bend vibration is generated from interfacial water with the interfacial quadrupole mechanism that is associated with the large field gradient of incident lights localized in a very thin region at the interface.

Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces

“…Here, the H–O–H bending mode appears as a 1650 cm –1 peak feature. 26 , 28 Upon increasing the salt concentration from 0.1 mM to 1 M, the 1650 cm –1 peak varies substantially.…”

“…Here, we assumed negligible NQEs on the spectral shape, in analogy with previous work. 28 We will discuss the validity of this assumption in the following.…”

“…The H–O–H bending mode of specifically interfacial water molecules has been probed with sum-frequency generation (SFG) spectroscopy. 3 , 22 − 28 Although SFG spectroscopy is surface-specific, the precise origin of the SFG signal has been highly debated. So far, three distinct contributions have been proposed, from interfacial dipoles, bulk quadrupoles, and interfacial quadrupoles.…”

“…Because the frequency shift cannot be accounted for via the bulk quadrupole mechanism, Tahara and co-workers proposed that the bending mode SFG signal is generated by the higher-order term arising from the interface (interfacial quadrupole mechanism). 28 …”

“… 37 This interfacial field and the magnitude of the χ bend (3) contribution has been examined by varying the bulk electrolyte concentration. 38 − 40 However, the χ bend (3) contribution is controversial: Reference ( 26 ) indicated a substantial χ bend (3) contribution, leading to the flipping of the sign of the Im(χ bend (2) ) peak due to the change of the negatively and positively charged interfaces (dipole mechanism), while ref ( 28 ) showed that the χ bend (3) contribution is negligible, leading to the positive Im(χ bend (2) ) peak irrespective of negatively or positively charged interfaces (interfacial quadrupole mechanism).…”