# Infrared Spectroscopy as Molecular Probe of the Macroscopic Metal-Liquid Interface
## 1 Introduction  
Interfaces between metals and fluids are omnipresent. Electrochemistry is the most obvious area, when an electrolyte is in contact with an electrode. This is not only the case in electrocatalysis [1; 2] and energy storage devices like in a battery [3; 4], but also when metals are undergoing corrosion [5; 6]. Despite the importance of these systems, the analysis of the underlying physical and chemical phenomena is still a challenge. This is particularly true for those processes that are happening directly at the interface. In a reacting system, e.g., when an electrochemical cell is under operation, an integral piece of information can be obtained by monitoring the electrical current and voltage, and/or by analyzing the bulk fluid at some distance to the surface. This allows for deducing information about the overall chemical oxidation and reduction reactions [7]. However, details about the molecular interactions at the surface are more difficult to investigate. Moreover, when the fluid (or some of its constituents) and the metal are not reacting, no information can be provided by measurements of current and bulk diagnostics. Non-reacting metal-fluid interfaces call for surface sensitive methods. Unfortunately, the number of possible analytical techniques is rather limited. X-ray and ultraviolet radiation based methods,such as photoelectron spectroscopy, usually require vacuum conditions and hence their application to samples containing volatile liquids is difficult [8]. Scattering and diffraction methods are usually limited to crystalline materials, and often even to atomically flat single crystal surfaces. Suitable approaches, on the other hand, can be found in the vibrational spectroscopy toolbox. Reflection-absorption infrared spectroscopy can be used to study heterogeneous catalytic processes, e.g., to identify intermediate species at the gas-solid interface [9; 10]. However, its application to aqueous systems, e.g., a salt solution on a metal surface, is difficult due to the strong absorption of water in the mid-infrared. Second-order (or even-order, in general) nonlinear effects are advantageous for studying surfaces and interfaces [11]. The even-order susceptibility is zero in the bulk of a fluid, and hence only the molecules in the optically anisotropic interfacial layer contribute to the signal, in particular when the molecules at the interface are oriented in a certain manner. In other words, the signal of even-order methods is highly surface-specific, while methods utilizing odd-order effects like infrared absorption and Raman scattering may be biased by signals from the bulk. Second-order vibrational spectroscopy can, for instance, be performed in terms of sum-frequency generation (SFG) [11; 12]. However, when the IR radiation must travel through a highly absorbing medium like water, the resulting signal levels may be low. The strong absorption of water can be avoided in Raman spectroscopy because the excitation wavelength may basically be chosen arbitrarily [13], and thus it can be spectrally separated from any absorption bands. Consequently, surface-enhanced Raman scattering (SERS) can be a solution if the metal is capable of coupling with the electric field of an incident laser beam to result in the plasmonic enhancement of the inherently weak Raman signal [14]. However, the application of SERS and the data evaluation is not straightforward due to a limited reproducibility. As a possible approach that can overcome all the above mentioned difficulties, we propose the use of attenuated total reflection infrared (ATR-IR) spectroscopy. ATR techniques are generally advantageous for studying highly absorbing media, as the radiation interacts with the sample only in an evanescent field, and hence the attenuation of the intensity is moderate [15; 16]. ATR-IR has proven its potential for studying molecular phenomena at liquid-liquid and liquid-solid interfaces, including aqueous systems in the past [17; 18; 19].