Peak fine structure

The spectrum of Fig. 5.11 also reveals within each diagonal and crosspeak fine structure that is equivalent to the structure seen within the one-dimensional multiplets. Whether such fine structure is always resolved depends on the experimental settings used in the acquisition of the data. Whilst in the illustrative 

Recall also that following the second pulse, some magnetisation remains associated with the original spin . Thinking back to the discussions of polarisation transfer in the INEPT experiment, it was shown that the basic requirement for the transfer of polarisation was an anti-phase disposition of the doublet vectors of the source spin, which for INEPT was generated by a spin-echo sequence. Magnetisation components that were in-phase just before the second 90° pulse would not contribute to the transfer, hence the A period was optimised to maximise the anti-phase component. The same condition applies for 

F ure 5.15. A schematic energy level diagram for the coupled two-spin AX system. 

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Higher-resolution display than basic COSY. Additional information on magnitudes of coupling constants may be extracted from 2D peak fine-structure. Singlets suppressed. 

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

The spectrum of Figure lb is a fingerprint of the presence of a CO molecule, since it is different in detail from that of any other molecule. UPS can therefore be used to identify molecules, either in the gas phase or present at surfaces, provided a data bank of molecular spectra is available, and provided that the spectral features are sufficiently well resolved to distinguish between molecules. By now the gas phase spectra of most molecules have been recorded and can be found in the literature. Since one is using a pattern of peaks spread over only a few eV for identification purposes, mixtures of molecules present will produce overlapping patterns. How well mixtures can be analyzed depends, obviously, on how well overlapping peaks can be resolved. For molecules with well-resolved fine structure (vibrational) in the spectra (see Figure lb), this can be done much more successfiilly than for the broad. 

Fine structure extending several hundred eV in kinetic energy below a CEELS peak, analogous to EXAFS, have been observed in REELS. Bond lengths of adsorbed species can be determined from Surface Electron Energy-Loss Fine Structure (SEELFS) using a modified EXAFS formalism. 

The photoelectron spectrum of nitrogen (N2) has several peaks, a pattern indicating that electrons can be found in several energy levels in the molecule. Each main group of lines corresponds to the energy of a molecular orbital. The additional "fine structure" on some of the groups of lines is due to the excitation of molecular vibration when an electron is expelled. 

The fine structure of the spectrum is the splitting of the resonance into sharp peaks. Note that the methyl resonance in ethanol at 8 = 1 consists of three peaks with intensities in the ratio 1 2 1. The fine structure arises from the presence of other magnetic nuclei close to the protons undergoing resonance. The fine structure of the methyl group in ethanol, for instance, arises from the presence of the protons in the neighboring methylene group. 

Geometrical cis-trans isomers — The UV-Vis spectra of most cis isomers are similar to those of the corresponding dll-trans isomer. However, a few consistent differences can be found in the spectra of cis isomers as compared to those from the corresponding a -trans compound a hypsochromic shift (2 to 6 nm for mono-dY through 34 nm for tetra-c ), a decrease in absorbance, a reduction of the spectral fine structure, and tlie appearance of a new absorption band known as a cis peak. For example, in a study in which the structures were confirmed by NMR, the isomers... 

FIGURE 6.2.3 Calculations of spectral fine structure (% in/II) and intensity of cis peak (%... 

The presence of o-qulnone surface waves seems, at the present time, to be coincidental to activation particularly In the case of ascorbic acid oxidation. On the other hand. Its presence may serve as a criterion of cleanliness and activation. Thus, the surface waves at 0.250 and 0.190 are Indicators or signatures for active GCE electrodes and should be used as diagnostic for a clean GCE surface as Is the hydrogen fine structure for platinum (31). It Is unfortunate that the o-qulnone peaks do not appear to be proportional to the surface area as Is the platinum fine structure. 

Linear absorption measurements can therefore give the first indication of possible alloy formation. Nevertheless, in systems containing transition metals (Pd-Ag, Co-Ni,. ..) such a simple technique is no longer effective as interband transitions completely mask the SPR peak, resulting in a structurless absorption, which hinders any unambiguous identification of the alloy. In such cases, one has to rely on structural techniques like TEM (selected-area electron diffraction, SAED and energy-dispersive X-ray spectroscopy, EDS) or EXAFS (extended X-ray absorption fine structure) to establish alloy formation. 

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