Splitting hyperfine structure

With this procedure, as with the double-resonance methods in atomic physics, Zeeman and Stark splittings, hyperfine structures and A doublings in molecules can be measured with high precision, even if the observed level splittings are far less than the optical dopp-ler width. From the width of the rf resonance and from the time response of the pumped systems, orientation relaxation rates can be evaluated for individual v J") levels. Other possible applications of this promising technique have been outlined by Zare 30) Experiments to test some of these proposals are currently under investigation and their results will be reported elsewhere. 

ESR spectra of La Cs2/ Y Cs2, Ho Cs2, and Tm C82 taken from the solid soot extract were reported by Bartl et al. (1994, 1995a,b, 1996) and showed low resolved but split hyperfine structure, indicating that the metal atoms exist in ionic form in the fullerene cage also in the solid state. The research group also reported (Knorr et al., 1998) the principal values of the hyperfine tensor A and the relative orientation of g and A tensors of M C82 (M = Sc, Y, La) applying three- and four-pulse electron spin-echo envelope modulation techniques (ESEEM). 

Figure Bl.4.9. Top rotation-tunnelling hyperfine structure in one of the flipping inodes of (020)3 near 3 THz. The small splittings seen in the Q-branch transitions are induced by the bound-free hydrogen atom tiiimelling by the water monomers. Bottom the low-frequency torsional mode structure of the water duner spectrum, includmg a detailed comparison of theoretical calculations of the dynamics with those observed experimentally [ ]. The symbols next to the arrows depict the parallel (A k= 0) versus perpendicular (A = 1) nature of the selection rules in the pseudorotation manifold.

Most ESR studies of organic radicals were carried out in the 1950s and 1960s. They provided important tests of early developments in valence theory. The results of these early studies are nicely summarized in a review by Bowers.11 Applications of hyperfine splittings to structure determination are discussed in many of the texts and monographs referenced in Chapter 1. 

Hyperfine splitting. As was discussed above, one consequence of placing a free electron onto a molecule is to alter its 0-value. Another is that the electron spin comes under the influence of any magnetic nuclei present in the radical, with the result that the spectrum is split into a number of lines centred on the position of the single resonance expected for the simple /transition discussed above. This hyperfine structure is the most useful characteristic ofepr spectra in the identification of an unknown radical species. 

In the ESR spectra of adsorbed oxovanadium(IV) ions on minerals, Information on the nature of the adsorbed species is obtained from the g-values and V hyperfine coupling constants, but ligand hyperfine structure is seldom, if ever, observed. With ENDOR much smaller hyperfine splittings can be observed than with ESR and it is possible to measure hyperfine coupling from nuclear spins in... 

Hyperfine coupling constant, 22 267, 269 Hyperfine interaction, ESR data for, 22 274 Hyperfine parameters for O, 32 128-130 Hyperfine splitting, 31 81 Hyperfine structure, trimer species, 31 98-99 Hyperfine tensor, 22 267, 273-279, 336, 340 constants, 32 20-21 dioxygen species, 32 18-25 equivalent oxygen nuclei, 32 18-21 ionic oxides, 32 40... 

With this technique the Doppler width could be reduced by two orders of magnitude below the natural linewidth, and spectral structures within the Doppler width could be resolved. Examples are the resolution of hyperfine structure components in an 12-beam using a single-mode argon laser (tunable within a few gigahertz) or the investigation of the upper state hfs-splitting in the atomic... 

This method is specially suited for measurements of closely spaced Zeeman or Stark splitting and fine and hyperfine structures, which are separated only within their doppler linewidth 5 ). 

A breakthrough was achieved a few years ago when it was realized that an anal dic calculation of the deuterium recoil, structure and polarizability corrections is possible in the zero range approximation [76, 77]. An analytic result for the difference in (12.29), obtained as a result of a nice calculation in [77], is numerically equal 44 kHz, and within the accuracy of the zero range approximation perfectly explains the difference between the experimental result and the sum of the nonrecoil corrections. More accurate calculations of the nuclear effects in the deuterium hyperfine structure beyond the zero range approximation are feasible, and the theory of recoil and nuclear corrections was later improved in a number of papers [78, 79, 80, 81, 82]. Comparison of the results of these works with the experimental data on the deuterium hyperfine splitting may be used as a test of the deuteron models and state of the art of the nuclear calculations. 

The monohydrate displayed an ESR spectrum associated with a thiyl (RS ) radical with g-tensor (1.997, 2.043, 2.004). The low field component, which probably contained some unresolved hyperfine structure, disappeared in hours at 180 K. The dihydrate spectrum, resulting in g = (1.998,2.007,2.087), displayed two low field singlets, one of much higher intensity than the other the line splitting present was not analyzed. Both lines disappear below 200 K with no successor radical. It is concluded that both hydrated crystals give rise to a ir-type sulfur-centred electron-loss radical, which were both expected to be de-protonated at 77 K. 

The multiplet hyperfine pattern of ESR lines in organic radicals is most frequently due to electron-proton interactions, but other nuclei with nonzero spin may also cause hyperfine structure. Weak satellite lines arising from interactions between the unpaired electron and C13 nuclei are sometimes observed C13 has and the analysis is straightforward. Nitrogen-containing radicals may show hyperfine splittings due to N14, which has 7 1. The possible Mj values are 1, 0, and -1, so that an electron... 

The hyperfine structure (splitting) of energy levels is mainly caused by electric and magnetic multipole interactions between the atomic nucleus and electronic shells. From the known data on hyperfine structure we can determine the electric and magnetic multipole momenta of the nuclei, their spins and other parameters. 

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