Hyperfine lines

Fe which have full width 2r at 0.2 mm s . Other isotopes are less demanding, e.g., Au, for which the lines are ten times wider. Most spectrometers are equipped with electromechanical Mossbauer velocity transducers of the loudspeaker type. This technique is suitable for velocity variations ranging from less than 1 mm s full scale up to several cm s and covers the whole reach of hyperfine splitting for most of the common isotopes. Kalvius, Kankeleit, Cranshaw, and others [1-5] have been pioneers in the field, who laid foundations for the development of high-precision drives with feedback amplifiers for proper linear velocity scales with high stability and low hum. Other techniques for Doppler modulation have been developed for isotopes with extremely narrow hyperfine lines, e.g., Zn. For such isotopes, piezoelectric transducers are mostly used [6, 7], more details of which are found in Sect. 7.2.1. 

The nuclear y-resonance effect in ° Ni was first observed in 1960 by Obenshain and Wegener [2]. However, the practical application to the study of nickel compounds was hampered for several years by (1) the lack of a suitable single-line source, (2) the poor resolution of the overlapping broad hyperfine lines due to the short excited state lifetime, and (3) the difficulties in producing and handling the short-lived Mossbauer sources containing the Co and Cu parent nuclides, respectively. 

Under ideal conditions, a commercial X-band spectrometer can detect about 1012 spins (ca. 10-12 moles) at room temperature. This number of spins in a 1 cm3 sample corresponds to a concentration of about 1(U9 m. By ideal conditions, we mean a single line, on the order of 0.1 G wide, with sensitivity going down roughly as the reciprocal square of the line width. When the resonance is split into two or more hyperfine lines, sensitivity decreases still further. Nonetheless, ESR is a remarkably sensitive technique, especially compared with NMR. 

Things get a little more complicated when a spin 1 nucleus like 14N is added to the picture, but the same technique works again for the determination of the relative intensities of the ESR lines. Consider, for example, the relative intensities of the hyperfine lines arising from the pyrazine anion radical, whose spectrum is shown in Figure 2.3. Like that of the naphthalene anion radical, the spectrum observed for the pyrazine anion radical2 consists of 25 well-resolved... 

Clearly, the eight hyperfine lines (7 = 7/2 for 51V) have different widths but careful examination also shows that the line spacing varies, increasing with increasing B. To understand the origin of this effect we must take a closer look at the solutions to Eqn. (3.1) for the case of an unpaired electron interacting with a single nucleus. This will lead us to a derivation of eqns (2.5) and (2.11) of Chapter 2. 

Two corollaries stem from this generalization. Since a spin-1/2 nucleus gives only two hyperfine lines, there can be no variation in spacings. Thus powder spectra cannot be analyzed to extract the orientations of hyperfine matrix axes for such important nuclei as 3H, 13C, 19F, 31P, 57Fe, and 103Rh. Secondly, since the observable effects in powder spectra depend on the magnitude of the matrix... 

Because the d5 configuration is spherically symmetric, high-spin Mn(ii) and Fe(m) usually have nearly isotropic -matrices and Mn(ii) usually has a nearly isotropic -matrix. This means that there usually is not much information in the ESR spectrum of these high-spin species. Indeed, high-spin Mn(n) is usually an unwanted interference for those interested in low-spin Mn(n) the ESR spectrum is very characteristic with six hyperfine lines with a coupling constant of 80-100 G. Because the g- and -matrices are nearly isotropic, the six-line spectrum persists in frozen solutions. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

FIGURE 5.1 Isotropic hyperfine pattern for 51VIV in S-band. The spectrum is from V0S04 in aqueous solution. Use of the low frequency enhances the second-order effect of unequal splitting between the eight hyperfine lines. 

FIGURE 10.4 Anisotropy averaging in the EPR of TEMPO as a function of temperature. The spectra are from a solution of 1 mM TEMPO in water/glycerol (10/90). The blow-up of the middle 14N (/ = 1) hyperfine line in the 90°C spectrum has been separately recorded on a more dilute sample (100 pM) to minimize dipolar broadening and, using a reduced modulation amplitude of 0.05 gauss, to minimize overmodulation. The multiline structure results from hyperfine interaction with several protons. 

The hyperfine interaction is shown in Figure 21 of reference 16. The Ms = 1/2 states of an S = 1/2 paramagnet interact with an I = 1/2 nuclear moment to create the hyperfine interaction. Interactions from ms = — 1/2 to Mi = - 1/2 and ms = - 1/2 to Mi = + 1/2, for instance, create the magnetic field specified as the hyperfine interaction A. Figure 21 of reference 16 describes the behavior for an 1=1/2 nuclear moment. The number of hyperfine lines will be equal to 21 + 1 for nuclear moments greater than 1/2. Each hyperfine line will be of equal intensity when the electron is interacting with its own nucleus. For instance the Cu2+, / = 3/ 2 nucleus will produce four hyperfine lines as described in the next section. 

As described in the previous section, four hyperfine lines will be found in the EPR spectrum of Cu,Zn-superoxide dismutase (CuZnSOD) because of the 1 = 312... 

ESR spectra of Cu + in dry smectite powders are similar to that shown in Figure 14b, but the 3/2 nuclear spin of Cu splits each g and g resonance into four peaks (6). The individual hyperfine lines of the gy resonance are easily resolved because Ay is large, but the gA resonances are not always resolved because A is small. Thus, A, g, and g can be measured from the powder spectrum. 

Magnetic Hyperfine Structure. The magnetic fields in most magnetic iodine and tellurium compounds are not sufficient to separate the 18 magnetic hyperfine lines of the two iodine isotopes. Several measurements (13, 21, 34, 36) have indicated that fields of only about 100 kilo-gauss can be expected from such compounds. Therefore, other methods must be utilized. 

The spectrum of Mn++ adsorbed on the substrates consisted of 6 hyperfine lines due to the Mn nuclear spin with 7 =. In addition, smearing of hyperfine lines was observed. The spin-Hamiltonian of Mn++ in axial symmetry is fairly well represented by the following equation ... 

On cation exchangers (sulfonic acid-type and carboxylic acid-type), a value of = 0.0089 cm. i was observed, corresponding to the hydrated Mn(H20)6++. The effect of axial symmetry (D 9 0) was observed as evidenced by a fine structure in the hyperfine lines. A value of D however was not determined. The EPR spectrum of Mn++ on an anion exchanger studied consisted of a single unresolved broad line. A smaller A value and larger... 

Figure 2 EPR spectra of rhenium(VI) complexes a) liquid solution spectum of [NBu4][ReNCl4] showing exclusively the six hyperfine lines due to the nuclear spin of 5/2 of the nuclei and b) well resolved...

Fig. 7. ESR spectrum of oxobis(0,0 liethyldithiophosphato)vanadium in benzene. The eight vanadium hyperfine lines are indicated...

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