Hyperfine splitting resolution

A monochromatic beam of X-rays with about 1 eV bandwidth is produced by the standard beamline equipment, the undulator and the high-heat-load premonochromator being the most important parts among them. Further monochromatiza-tion down to approximately the millielectronvolt bandwidth is achieved with the high-resolution monochromator. The width of a band of a millielectronvolt, however, is much more than the inherent linewidth of the Fe y-radiation, F 10 eV, or the full range of hyperfine-split Mossbauer lines, A m 10 eV. Yet, NFS is detectable because the coherent excitation of the nuclei is caused in the... 

Such a short spin-equilibrium relaxation time raises the question of whether discrete spin state isomers exist. Their existence is affirmed by two observations. One is the persistence of electronic spectral bands typical of the low-spin 2E state over a wide temperature range in solid samples (98). The other is the observation of EPR signals characteristic of the 2E state in both solids and solutions between 4 and 293 K (98,139). At very low temperatures EPR signals of both spin states can be observed simultaneously (98). At low temperatures hyperfine splitting into eight lines is observed from coupling with the 1 = 7/2 Co nucleus. As the temperature is raised the spectral features broaden and the hyperfine resolution is lost. This implies a relaxation process on the EPR time scale of 1010 sec-1, or a relaxation time of the order 0.1 nsec, consistent with the upper limit set by the ultrasonic experiments. 

The resolution of the molecular beam experiments is high enough to observe even rather small nuclear hyperfine interactions such as the spin-spin and spin-rotation interactions as well as the larger quadrupole coupling interactions. The largest terms in the Hamiltonian for the hyperfine splittings are given below 66) ... 

Electron-Nuclear Double Resonance (ENDOR) Spectroscopy. This observes a spin resonance transition after a nuclear resonance transition has been saturated by a radio-frequency pulse (Fig. 11.65) so as to invert the relative populations of the y.ey. > and y,./) .y> spin states this forces the populations of the aeaN> and fSey > states to be different and thus offers the opportunity to measure hyperfine splittings much more carefully, with better resolution than in standard EPR. 

Figure 11.23. Energy level diagram and observed transitions for SrF in its X2 + state, showing the 19F hyperfine splitting [43], observed because of the higher resolution obtained with the microwave/optical polarisation method. This diagram may be compared with figure 11.21, appropriate for the earlier lower resolution studies employing conventional fluorescence detection.

Several techniques allow further elucidation of the information contained in an EPR spectrum. Through single-crystal EPR it is possible to determine the orientations of the g and A tensors relative both to each other and to the internal coordinates of a structurally defined active site. The use of several microwave frequencies can be particularly informative. While the spectrum shown in Fig. 2 (middle) was taken with an X-band spectrometer (u = 9 GHz), Q-band (v 35 GHz) and S-band (u = 3 GHz) should also be employed. The high microwave frequency leads to increased resolution by spreading the g values over a wider magnetic field range with little effect on hyperfine splitting. Low... 

The results demonstrate the value of being able to select the matrix independently of the radical precursor. The e.s.r. spectra of alkyl radicals trapped in their parent hydrocarbon or halohydrocarbon are often poorly resolved, and only the major hyperfine splitting constants can be determined. By using a bulky hydrocarbon such as camphane or adamantane as a matrix a marked increase in resolution has been observed for the spectra of many alkyl radicals up to Ce and all of the cyclo-alkyl radicals up to Cy. As an example, the spectra of the cyclohexyl radical trapped in matrices of cyclohexane and camphane are shown in Fig. 7. In cyclohexane the a and major )3-proton hyperfine splittings are not well resolved, and the small )3 splitting is not observed at all, whereas in camphane all three hyperfine splittings are readily observed. 

The hyperfine splittings in the e.s.r. spectra of radicals of the allylic type are considerably less than those of alkyl radicals, and for radicals trapped in their parent compounds the resolution is insufficient to determine all the hyperfine coupling constants. However, by use of the rotating cryostat, the unsubstituted radical and three methyl-substituted allyl radicals have been prepared in a matrix of adamantane and it has been possible to resolve all the hyperfine couplings. 

Figure 3 Energy-level diagram and corresponding EPR signals at 9.6 GHz (X-band) with maximized resolution for an unpaired electron in an imposed magnetic field that interacts with a) a proton (nuclear spin / = 1/2), b) a nucleus (/= ), and c) a nucleus in which the energy levels are additionally split by a nearby proton. The spectra were produced by the EPR simulation program SimFonia for illustration of the hyperfine splitting purpose.

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