Spin-Hamiltonian parameters measurements

Comparing spin-Hamiltonian parameters measured for NO centers in different inorganic matrixes (Table 8.5) one can conclude that for all of them Ay = 30 2.5 G, Az < 10 G and A < 5 G although g-tensor values are varying in a rather wide range depending on the local crystal fields in the lattice. 

In Fig. 3.32, one can see the room temperature EPR spectra of Fe + paramagnetic centers substituted for Ti + ions in BaTiOs nanopowders. The mean size of the particles decreased from several hundreds nm to several tens nm at annealing temperature decrease from 1,350 to 900 °C [101]. It has been established, that in some samples the size distribution function has one peak while in the other ones it has two peaks In particular, fc the sample annealed at 900 °C, these two peaks positions were R 40 nm and R 140 nm. In Fig. 3.32, the calculated powder spectrum is reported. The spectrum has been calculated with the help of spin-hamiltonian parameters measured earlier for Fe + in BaTiOs single crystals [106]. It follows from Fig. 3.32, that the spectrum for the largest particles (about pm) contains all the transitions inherent in bulk samples so that the powders consisting of micron size particles can be considered as bulk ones. The cubic symmetry line... 

It should be apparent from the above that considerable information about the magnetic properties of triplet states can be obtained from a zf experiment. Table 6 summarizes the spin Hamiltonian parameters measured for 1-halonaphthalenes (IXN X = Cl, Br, I) in zero field additional parameters are known fi-om the high-field experiments (Kothandaraman et al., 1974b, 1975,1979) and are included for discussion purposes. We also list the parameters of the lowest triplet states of the parent molecule (Hutchison and Mangum, 1961) and its 1-fluoro derivative (Mispelter et al., 1971) for comparison. Measurements of many of the relevant optical properties of these species have been reported (Schwoerer and Sixl, 1969 Sixl and Schwoerer, 1970a Kothandaraman et al., 1975, 1979 Saigusa and Azumi, 1978) and some of these are listed in Table 7. 

The ESR measurements were made at RT or 77 K on a Varian E-9 spectrometer (X-band), equipped with an on-line computer for data analysis. Spin-Hamiltonian parameters (g and A values) were obtained from calculated spectra using the program SIM14 A [26]. The absolute concentration of the paramagnetic species was determined from the integrated area of the spectra. Values of g were determined using as reference the sharp peak at g = 2.0008 of the E i center (marked with an asterisk in Fig. 3) the center was formed by UV irradiation of the silica dewar used as sample holder. 

Section 3, the main section of this paper, deals with the NMR of bulk semiconductors. Section 3.1 lists the various relevant terms in the NMR spin Hamiltonian. The NMR techniques and strategies that can be employed to obtain the individual NMR parameters of the spin Hamiltonian and theoretical calculations of NMR parameters will be discussed in Sect. 3.2. The remaining subsections will provide examples from the important classes of semiconductors that illustrate the measurement and interpretation of each of the spin Hamiltonian parameters, with an emphasis on what information about semiconductors the parameters convey. 

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

These values are in reasonable agreement with the values reported on the basis of far IR work (67a, 67b). In general the anisotropy measurements establish that the ligand field in the XFe(R2Dtc)2 complexes is rhombic and provide a method for estimating spin-Hamiltonian parameters. 

VTVH MCD data for non-Kramers ions allowing the measurement of spin Hamiltonian parameters of EPR inactive centers.29,34... 

ESR is known to be a very sensitive tool and can therefore be used in studying structural features of nanosized semiconductor particles doped with paramagnetic metal ions. In many studies vanadium impurities inside the Ti02 matrix or on the particle s surface were used as dopants. Moreover, V4+ ions are very convenient ESR probes since the 51V nuclei have a large magnetic moment leading to informative hyperfine structures (S = 1/2 / = 7/2). At low vanadium concentration, the EPR spectrum has well resolved sharp lines (Fig. 8.10) allowing precise measurement of spin-Hamiltonian parameters. 

The piperidone pL and bispidone L are air-stable, colorless crystalline compounds. As for bispidones in general, the IR spectrum of L has characteristic Bohlmann bands (2795 and 2848 cm ), which disappear upon complexation. The blue crystalline copper(II) complex [Cu(L)(Cl)]+ is stable in air. In the UV-vis spectrum (aqueous solution), it shows a d-d transition at 2max = 656nm (15244cm x) with 656 = 90cm 1 M 1. The spin Hamiltonian parameters (obtained by simulation of a measured frozen solution EPR spectmm [116K, DMF/ CH3OH = 2 l]) are gz= 2.224, gx = gy = 2.Q5-, 175 x 10 4cm, Ay 22... 

Spin-Hamiltonian parameters were measured at the temperature given in the How produced column unless otherwise noted. Consult original reference for relative orientation of spin-Hamiltonian coordinate system and crystallographic axes. Only one g value or A value quoted indicates parameter is isotropic. Absolute magnitudes are given for ), and A unless indicated by (+) or (-). 

E.P.R. measurements were carried out specifically to answer the question whether different interactions with the solvent were present in the series of these copper(II) complexes. The spin-Hamiltonian parameters reported in Table VI are characteristic of tetragonally elongated octahedral copper(U) complexes with a dx2-y2 ground state and do not show significant differences. On the basis of these EPR results we can exclude that the observed stereoselectivity is due to the varying degree of interaction of the metal ion with the solvent in the L- and D-enantiomer complexes. 

The temperature dependence of the Gd + spin Hamiltonian parameters in PrV04 and of the resonance line width was investigated by Mehran et al. (1980) and Andronenko et al. (1981). A measurement of the Tm + ion spectrum in the VV paramagnet HoND (holmium nicotinate dihydrate) should also be noted (Baker et al. 1986b). The EPR in TbND has shown a spectrum from a relatively rare species of a paramagnetic ion (defect sites) in an undiluted compound of the same paramagnetic ion (Baker et al. 1987). 

TABLE 17.1 Experimental Spin Hamiltonian Parameters for the Prepared Homc eneous (Measured at 77 K) and Heterogeneous V 0-Compounds at Room Temperature)... 

The least-squares fitting (LSF) procedme that is used here is similar to that applied to the evaluation of spin-Hamiltonian parameters fi om cw-EPR line positions cf. Misra, 1976, 1999). Relaxation times and T2 are treated as components of parameter vector, to be estimated from the values of the measured signal, Sf, for various values of modrdation frequency 2, (z = 1, 2,. .., n). In the present case, the value required in the LSF procedme, a function of parameters and Ti, is expressed as... 

Mn(II) EPR spectra in biological systems are very much like those in glasses — e.g., that in lithium-borate glass (Griscom Griscom, 1967) matches closely that in kinase oxalate ternary complex (Reed Markham, 1984 referred to hereafter as RM, and references therein). Table 1 lists the measured values of the spin-Hamiltonian parameters parameters (g, D, E) in some proteins, as taken from RM. 

Fig. 326. Cu(C44Hj8N4) sc, p. Dependence of Xm versus 1/T. Magnetic temperatures were measured using powdered cerrous magnesium nitrate in which 90% of the Ce sites were replaced by La. Full lines indicate zero-field susceptibilities xii and calculated using the spin Hamiltonian parameters g = 2.179, gx=2.033,. 4=212.2 10 cm", 5 = 30.10" cm", S = i, I=, where r is parallel to c axis. Also shown is the Curie law behavior of Xn obtained for 5 = 0. Broken lines give first-order correction to Xn and Xj. obtained by the Van Vleck moment-expansion method [7413].

The ESR measurements on lanthanide ions in Lap3 indicate six magnetically inequivalent sites having Cj/, symmetry or lower [Baker and Rubins (1961)]. This result is not in agreement with the NMR measurements of Andersson and Proctor (1968) which indicate only three inequivalent sites (see section 2.3.2). In any event, each of the six ESR sites is described by identical spin Hamiltonian parameters, the only difference being in the orientation of the principal axes of the g-tensor with respect to the crystalline c axis. Because of the low symmetry of the CEF, one expects that all degeneracy in the ground state multiplet will be completely lifted in the case of non-Kramers ions so that in these cases ESR will not be detectable [Schulz and Jeffries (1966)]. The measured g-tensor components are listed in table 18.28. 

Cr(III) was doped into CsAl(S04) to a level of 1% and multifrequency EPR spectra recorded at X-, Q-, and W-band frequencies (Fig. 46). Computer simulation of all three spectra was achieved with a single set of spin Flamiltonian parameters, giso = 1.975, D = -0.078 cm and E D = 0, and the spin Flamiltonian given in Eq. (11). Measurement of the perpendicular B L B ) and parallel (Ri Ro) mode EPR spectra (Fig. 47) allows the observation and identification of the allowed and forbidden transitions. Computer simulation of each of these spectra with the above spin Hamiltonian parameters reproduces the spectral features accurately (Fig. 47). 

Automatic fitting of EPR spectra is possible, although most techniques have problems with false minima and some degree of manual steering is necessary. One of the frequently encountered problems is that the least-squares difference is rarely an adequate measure of goodness of fit. In practice it is usually necessary to obtain a reasonable fit manually before automatic fitting is capable of further optimizing the spin Hamiltonian parameters. 

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