Modulation IR spectroscopy

The introduction of in-situ infrared spectroscopy to electrochemistry has revolutionised the study of metal/electrolyte interfaces. Modnlation or sampling techniques are applied in order to enhance sensitivity and to separate snrface species from volume species. Methods such as EMIRS (electrochemicaUy modulated IR spectroscopy) and SNIFTIRS (subtractively normalised interfacial Fonrier Transform infrared spectroscopy) have been employed to study electrocatalytic electrodes, for example. There have been surprisingly few studies of the semiconductor/electrolyte interface by infrared spectroscopy. This because up to now little emphasis has been placed on the molecnlar electrochemistry of electrode reactions at semiconductors because the description of charge transfer at semiconductor/electrolyte interfaces is derived from solid-state physics. However, the evident need to identify the chemical identity of snrface species should lead to an increase in the application of in-situ FTIR. 

Figure 3.47. Typical Fourier transform electrochemically modulated IR spectroscopy (FTEMIRS) ATR spectrum of n-Si in acetonitrile-0.1 M tetrabutylammonium perchlorate. Baseline can be analyzed as sum of free-electron and surface state contribution. These two contributions appear in different proportions when potential modulation range is changed, giving direct information on splitting of modulated charge between space charge and surface states. Adapted, by permission, from A. Venkateswara Rao, J.-N. Chazalviel, and F. Ozanam, J. Appl. Phys. 60, 686 (1986), p. 699, Fig. 3. Copyright 1986 American Physical Society.

EMIRS electrochemically modulated IR spectroscopy HCL hollow cathode lamp... 

Blaudez D, Turlet J-M, Dufourcq D, Bard D, Buffeteau T and Desbat B (1996) Investigation at the air-water interface using polarization modulation IR spectroscopy. Journal of the Chemical Society, Faraday Transactions 92 525-530. 

There have been a few other experimental set-ups developed for the IR characterization of surfaces. Photoacoustic (PAS), or, more generally, photothemial IR spectroscopy relies on temperature fluctuations caused by irradiating the sample with a modulated monocliromatic beam the acoustic pressure wave created in the gas layer adjacent to the solid by the adsorption of light is measured as a fiinction of photon wavelength... 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

A long disputed issue of the nature of strongly bound species in this reaction has been recently revived with the vibrational spectroscopy studies of Bewick et al. (30) using EMIRS technique and of Kunimatsu and Kita (31) using polarization modulation IR-reflection-absorption technique. These data indicated the only CO is a strongly bound intermediate. Heitbaum et al. (32) on the other hand advocate COH, and most recently HCO (33), as the poisoning species on the basis of differential electrochemical mass spectroscopy (DEMS). 

In situ studies of catalytic reactions have also been a prime focus of our group. The high-pressure spectroscopic technique used in our research is polarization modulation IR reflection absorption spectroscopy (PM-IRAS). Like SFG, PM-IRAS is a highly surface-sensitive technique that yields vibrational information about adsorbed surface species. Unlike SFG, however, PM-IRAS... 

Infrared (IR) spectroscopy with modulation of the electrode potential was used by Bewick and Kunimatsu to study the change of water structure at the Pt/H2S04 and Au/NaF interfaces. They observed several sharp bands within the OH region, superimposed on a broad absorbance background. The absorption increased at a higher field strength. The observed bands were similar to the bands recorded for small clusters of... 

The aim of this review is to provide an assessment of the state of this field. After a summary of some basic theoretical results, the focus is on experimental aspects, ranging from cell design to specialized techniques such as modulation excitation spectroscopy (MES). We emphasize the opportunities and limitations of ATR-IR spectroscopy in catalysis research. 

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

Photoacoustic Spectroscopy.7 When modulated IR radiation is absorbed by a sample, the substance heats and cools in response to modulated IR energy impinging on it. This thermal hysteresis is converted into pressure waves that can be communicated to surrounding gases and detected by acoustic detectors (essentially a sensitive microphone in the enclosed sample chamber). In such measurements, the acoustic detector replaces the IR detector of the spectrometer. 

---