Fermi resonance interaction spectroscopy

Another potential source of peaks in the NIR is called Fermi resonance. This is where an overtone or combination band interacts strongly with a fundamental band. The math is covered in any good theoretical spectroscopy text, but, in short, the two different-sized, closely located peaks tend to normalize in size and move away from one another. This leads to difficulties in first principle identification of peaks within complex spectra. 

In addition to measurements of lifetime of these vibrational excited states, time-resolved nonlinear IR could also give precise information on the mechanisms of deexcitation of these states. It could thus be shown that relaxation of the first excited state of modes of water molecules in liquid water was mainly due to resonance interactions of these modes with excited bending modes (65). As a result of the analysis of ID IR spectra shown above, Fermi resonance with bending modes allows the energy of the first excited state of to be transferred to the overtone of the bending band. It offers a fast relaxation path toward vibrational levels of a lower energy. Time-resolved nonlinear IR spectroscopy shows that this process is the main relaxation mechanism of and is at the origin of an unexpected increase of the relaxation time when temperature increases (66, 67). 

Figure 5.4 Fermi resonances energies E of the first vibrational levels of a mode q and of a mode in a C-O-H group (upper diagrams) and corresponding IR spectra (lower diagrams). The values E/hc (c, velocity of light in cm sec ) of these two modes are represented when no interaction is present (left upper diagram) and when an anharmonic interaction of the form fqqg is present (right upper drawing / is a constant). Arrows indicate transitions that appear in vibrational spectroscopy (IR or Raman).

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

For atomic H adsorption on surfaces the electronic structure as obtained by UPS studies and DFT calculations on Ni, Pd, and Pt shows a similar picture. There is a strong bonding H-induced feature around 7-9 eV below the Fermi level observed both in UPS and band structure calculations [43]. This has been related to that the H Is level interacts with both the metal -and 7-bands. Since the H Is level is much lower in energy in comparison with the previously discussed adsorbates, for which the outer level was of p character, it is anticipated that the metal s-electrons will be more strongly mixed into the adsorbate bonding resonance. Since no X-ray spectroscopy measurements can be conducted on H it is difficult to derive how much H Is character there is in the 7-band region, respectively, above the Fermi... 

In a carbon-supported metal electrocatalyst, the electronic interaction between metal and carbon support has a significant effect on its electrochemical performance [4], For carbon-supported Pt electrocatalyst, carbon could accelerate the electron transfer at the electrode-electrolyte interface, leading to an accelerated electrode process. Typically, the electrons are transferred from platinum clusters to the oxygen species on the surfece of a carbon support material and the chemical bond formation or the charge transfer process occurs at the contacting phase, which is considered to be beneficial to the enhancement of the catalytic properties in terms of activity and stability of the electrocatalysts. Experimentally, the investigation into the electron interaction between metal catalyst and support materials could be realized by various physical, spectroscopic, and electrochemical approaches. The electron donation behavior of Pt to carbon support materials has been demonstrated by the electron spin resonance (ESR) X-ray photoelectron spectroscopy (XPS) studies, with the conclusion that the electron interaction between Pt and carbon support depends on their Fermi level of electrons. It is considered that the electronic structure change of Pt on carbon support induced by the electron interaction has positive effect toward the enhancement of the catalytic properties and the improvement of the stability of the electrocatalyst system. However, the exact quantitative relationship between electronic interaction of carbon-supported catalyst and its electrocatalytic performance is still not yet fully established [4]. 

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