Modulation-enhanced spectroscopy

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

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. 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

Specular reflection spectroscopy has been actively used in in situ studies of the formation and optical behaviour of monolayer films on surfaces, and for detecting intermediates and products of heterogeneous chemical and electrochemical reactions. The vibrational spectra of the adsorbed species at electrode surfaces are obtained by surface-enhanced Raman scattering and infrared reflectance spectroscopies. Since the mid-1960s, modulated reflection spectroscopy techniques have been employed in elucidating the optical properties and band structure of solids. In the semiconductor electroreflectance, the reflectance change at the semiconductor surface caused by the perturbation of the dielectric properties of... 

New absorption methods, like intracavity spectroscopy, cavity-ring-down and cavity-enhanced spectroscopy, have demonstrated very high sensitivities in laboratory measurements with DLs. An ultrasensitive technique that combines external cavity enhancement and FM spectroscopy has been developed recently. This method, which has been called NICE-OHMS. or noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, is based on frequency modulation of the laser at the cavity free-spectral-range frequency or its multiple. The MDA of 5x 10 1 X 10 cm ) in the detection of narrow... 

Beden, B., Hahn, E, Leger, J.M., Lanty, C., Perdriel, C.L., De, T.N.R., Lezna, R.O. Arvia, A.J. Electro-modulated infrared spectroscopy of methanol electrooxidation on electrodispersed platinum electrodes. Enhancement of reactive intermediates. J. Electroanal. Chem. Interfacial Electrochem. 301 (1991), pp. 129-138. 

In recent years,3 4 however, there has been renewed interest in the study of the electrode/solution interface due in part to the development of new spectroscopic techniques such as surface-enhanced Raman spectroscopy,5-7 electrochemically modulated infrared reflectance spectroscopy and related techniques,8,9 second-harmonic generation,10-12 and others which give information about the identity and orientation of molecular species in the interfacial... 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

The applicability of the ESE envelope modulation technique has been extended by two recent publications115,1161. Merks and de Beer1151 introduced a two-dimensional Fourier transform technique which is able to circumvent blind spots in the one-dimensional Fourier transformed display of ESE envelope modulation spectra, whereas van Ormondt and Nederveen1161 could enhance the resolution of ESE spectroscopy by applying the maximum entropy method for the spectral analysis of the time domain data. 

Sensitivity and complexity represent challenges for ATR spectroscopy of catalytic solid liquid interfaces. The spectra of the solid liquid interface recorded by ATR can comprise signals from dissolved species, adsorbed species, reactants, reaction intermediates, products, and spectators. It is difficult to discriminate between the various species, and it is therefore often necessary to apply additional specialized techniques. If the system under investigation responds reversibly to a periodic stimulation such as a concentration modulation, then a PSD can be applied, which markedly enhances sensitivity. Furthermore, the method discriminates between species that are affected by the stimulation and those that are not, and it therefore introduces some selectivity. This capability is useful for discrimination between spectator species and those relevant to the catalysis. As with any vibrational spectroscopy, the task of identification of a species on the basis of its vibrational spectrum can be difficult, possibly requiring an assist from quantum chemical calculations. 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

The inhomogeneity of the micellar aggregate also affords assisted spin trapping and the exploitation of magnetic field effects on the charge separated ion pairs [48]. Optical modulation spectroscopy can be used, for example, to follow the decay of radicals formed in homogeneous solution and in SDS micelles. Enhancements of a factor of about 50 in the lifetimes and the steady state concentrations of the radical were observed in the micelle, and a kinetic analysis led to a value of 2 x 103 s 1 for the exit rate constant from the micelle [49]. 

Surface-Enhanced Raman Spectroscopy (SERS) SERS is a technique for increasing the sensitivity of a Raman instrument. This module explores its application to the detection of chemicals indicative of brain injury. 

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