Electron spin resonance investigation of o-chloranil alkali metal ion pairs

The electron spin resonance spectra of the ion pairs of ortho-tetrachloro benzosemiquinone with the whole series of the alkali metals have been recorded in different solvents in a range of temperatures. ¹³C hyperfine splittings from nuclei in all the positions in natural isotopic abundance were measured; they are discussed together with the g factor and the alkali metal coupling constants. The sign of the coupling constants is obtained from the analysis of the linewidths of the different hyperfine components. The experimental data are consistent with a structure of the ion pairs with the metal ion chelated by the two oxygens of the semiquinone molecule. With Rb and Cs in tetrahydrofuran a different structure is proposed with the metal ion bonded to only one oxygen.     

Relation between internal conversion coefficient and electron density at the nucleus for spin polarized Fe electrons

An ICC computation for the 14.4 keV transition in⁵⁷Fe has been made using relativistic wavefunctions derived from spin polarized UHF wave functions of atomic iron. It is shown that ICC and electron density at the nucleus are proportional for polarized s electrons as has been found already for unpolarized electrons.     

The magnetic field dependence of the electron spin polarization in consecutive spin correlated radical pairs in type I photosynthetic reaction centres

The magnetic field/microwave frequency dependence of the spin polarized EPR spectra of the sequential spin correlated radical pairs P⁺A^? ₁ and P⁺F^? _x in type I photosynthetic reaction centres is investigated. Experimental data are presented for photosystem (PS) I and reaction centres of heliobacteria at × band (9.7 GHz) and K band (24 GHz). In photosystem I at ambient temperatures the lifetime of A ^? ₁ is ~290 ns and both states are observable by transient EPR. In heliobacteria, electron transfer to F_x occurs within ~600 ps and only the state P⁺F^? _x is observed. The experimental data show a net polarization of P⁺ in the state P⁺F^? _x, which displays a clear dependence on the strength of the external field. The net polarization generated in sequential radical pairs is expected to pass through a maximum as a function of the Zeeman energy when the characteristic time of singlet-triplet mixing is comparable with the lifetime of the precursor. In PS I, the precursor lifetime (290ns) is much longer than the characteristic time of singlet-triplet mixing at × band (9 GHz, 3 kG) and K band (24 GHz, 8 kG). As a result, the observable net polarization decreases with the field strength in this region. In contrast, in heliobacteria, the precursor lifetime (600 ps) is much shorter than the characteristic time of singlet-triplet mixing, and the net polarization increases in the same range of Zeeman energies. The polarization patterns in these two systems can be described using the specific limiting cases of a short lived and long lived precursor radical pair and written as a sum of several contributions. The spectra are simulated on this basis using parameters derived entirely from independent experimental data, and good agreement between the experimental polarization patterns is obtained. The calculated polarization patterns are sensitive to spin dynamics on a timescale much shorter than the spectrometer response time, and the expected influence of a 10 ns component in the electron transfer, as observed optically in some PS I, preparations is discussed. No significant influence from such a component is found in the spin polarization patterns of PS I from the cyanobacterium Synechocystis 6803.     

Determination of the electron density at the Cu nucleus in Cu-Au alloys

The half-life of⁶⁴Cu in Cu–Au solid solutions has been measured as a function of the Cu concentration. Relative changes of electron densities (0)/(0) at the Cu nucleus are deduced. The observed nonlinear concentration dependence of (0)/(0) is discussed in terms of volume and charge transfer effects.     

The electron spin resonance spectrum of the trichloromethyl radical in solution

The equation of state PV = - a/V + RT(1 - y)^-4 is shown to be considerably better at high densities than van der Waals' equation PV = - a/V + RT(1 - 4y)^-1 and to be equal in accuracy to two other slightly more complicated equations for PV.     

Relative change of the electron density at the Cu nucleus in Cu-Ag alloys

The decay constant of⁶⁴Cu in Cu–Ag solid solutions has been measured at various Cu concentrations. Deduced values of the relative changes of electron densities (0)/(0) at the Cu nucleus are given. The observed concentration dependence of (0)/(0) is discussed in terms of a volume effect and charge transfer from Cu to Ag.     

Effect of chemical structure on the electron density at the iodine nucleus

Theoretical estimates of the electron density at the I nucleus in some compounds provide a basis for a systematic consideration of pertinent properties (EC, IC, Mössbauer IS). This paper presents an advanced version of such a systematics which has been extended by including a qualitatively new case (ICl ₄ ^– ) and by remeasuring the chemically induced EC variation of¹²⁵I. The corrections such as for binding-energy variations and for the effect of exchange and overlap were found to be less important here than one would expect from literature.     

Effect of chemical structure on the electron density at the tellurium nucleus

This paper reports results of a study in which the Xα-SW method is employed to deduce estimates of the differences in the electron densities at the Te nucleus in some compounds. On the basis of these estimates some assertions are deduced to the calibration problem of the Mössbauer nuclide¹²⁵Te and to the chemical influence on the IC of the 35.5 keV M1+E2 transition in¹²⁵Te.     

Manifestations in the EPR spectrum shape of memory effects in the electron spin polarization in the case of two consecutive radical pairs

Electron paramagnetic resonance (EPR) spectra of two consecutive radical pairs (RPs) diffusing inside micelles are numerically calculated. Calculations are carried out for various values of the micelle radius, exchange integral, and mutual diffusion coefficient. In the simple case when the hyperfine interaction with magnetic nuclei can be neglected, it is demonstrated that the spin dynamics in the primary radical pair (RP1) manifests itself in the EPR spectrum of the secondary radical pair (RP2) in a characteristic way: the oddness of the EPR spectrum with respect to its center is violated, and the EPR line intensities and widths for the two partners in the RP2 differ. These features of the RP2 EPR spectrum shape are interpreted as follows: the spin dynamics in the RP1 produces a longitudinal spin polarization and a transverse spin polarization (i.e., spin coherence). Both polarizations are transferred from RP1 to RP2. This spin polarization transfer causes the above features of the RP2 EPR spectrum shape. It is shown that the RP2 EPR spectrum in a sequence of RPs cannot be simulated as a spectrum of a single RP. The features of the RP2 EPR spectrum shape may be, in principle, exploited to reveal the existence of the short-lived RP1.     

The spin density matrix for slowly reorienting molecules in zero field

The time evolution of the spin density matrix of a spin embedded in a slowly moving molecule is investigated using Gordon's extended diffusion model for molecular reorientation and allowing the spin to follow the molecular motion. The results for this model are identical with those previously obtained using a jump diffusional model and do not give the adiabatic limit in slow motion. This model is valid provided the molecular angular velocity correlation time is short compared to the spin precession period. The perturbed angular correlation of rays or the spin resonance line shape can be calculated for any value of the rotational diffusion constant by diagonalizing a finite matrix. The necessary matrix elements are given for the 247 keV state of ¹¹¹Cd (I = 5/2).     

Control of the sign of exchange interactions in solvent-separated radical ion pairs

The chemically induced dynamic electron polarization spectra generated by the laser photolysis of several donor-acceptor systems have been studied to verify the solvent effects on the exchange interaction in solvent-separated radical ion pairs. It has been shown that the sign of the exchange interaction depends on the polarity of solvents. The individual reorganization energies have been determined for several solvent-separated radical ion pair systems.     

Theory of electron spin resonance measurements of chalcogen pairs in Si

Calculations are presented which support the identification of two sulfur-related centers in Si (lying 0.37 and 0.19 eV below the conduction band minima) with (S, S)⁺ and (S, S)⁰ nearest-neighbor substitutional S pairs. Explanations in terms of $\text{meso-bonding}$ are given of the following facts: (i) Although S is much more electronegative than Si, the (S, S)⁺ pair level lies at higher energy than the S⁺ level by ? 0.2 eV; (ii) The hyperfine interaction for (S, S)⁺ is considerably smaller than for the isolated S defect; and (iii) The (S, S)⁺ molecular defect has a hyperfine tensor that is virtually isotropic.     

Temperature dependence of the electron spin resonance in colloidal sodium

Electron spin resonance line width of colloidal sodium conduction electrons in sodium azide has been investigated as a function of temperature. The width varies approximately linearly with temperature from 77°K to the melting point of sodium (371°K) with a slope of 0·027 G/deg. An abrupt change in the resonance width occurs at the melting point of sodium. From the melting point to 475 °K the curve is almost linear with a slope of 0·041 G/deg. It can be shown that the data is in good agreement with the idea that the width of the resonance is proportional to the resistivity. This work extends the range of previous data to higher temperatures.     

Electron spin resonance and structure of the CO- 2 radical ion

Analysis of the electron spin resonance spectrum shows that CO^? ₂ radical ions are trapped in sodium formate at room temperature after irradiation of the crystal with high energy γ-rays. This radical is bent with an angle of about 134° and has a ² A ₁ ground state. From the experimental values of the ¹³C coupling coefficients and the g values, coefficients are derived for the mixing of atomic orbitals in the half-filled 4a ₁ molecular orbital which appears to be 14 per cent carbon 2s, 66 per cent carbon 2p _z and 11 per cent of each oxygen 2p _z . There is also coupling to the sodium nucleus in an adjacent cation.     

Formation of annihilating pairs from molecules of aromatic compounds in n-paraffin matrices

The formation of annihilating pairs from molecules of aromatic compounds incorporated into the crystal lattice of n-paraffins has been studied. It has been established that annihilating pairs can be formed both from the molecules incorporated into adjacent unit cells according to the replacement principle and from the molecules incorporated with intervals of one or two unit cells of the crystal lattice of the matrix.. It has been shown that the influence of the solvent on the annihilation rate of triplet excitations of aromatic compounds is associated with the dependence of the degree of overlapping of electron clouds of interacting molecules and the distances between them on the solvent.     

Anisotropy of the electron density and effective charge in alkali halide monocrystals

We discuss the use of the positron annihilation method to study the electron structure of alkali halide monocrystals, particularly the electron density and the effective charge of the ions. The distribution of electron density and the values of the effective ion charge are found experimentally for two crystallographic directions, 110 and 100, in a KBr monocrystal. On the basis of the data obtained it is concluded that this is an effective method for obtaining information about the anisotropy of electron states in the crystal.Translated from Izvestiya Uchebnykh Zavedenii, Fizika, No. 5, pp. 34–37, May, 1972.     

Hyperfine parameters from nonresonant luminescence effects and spin memory of F center pairs in alkali halides

In a static (100) magnetic field, the luminescence of F centers in KCl shows peaks at 1134, 1516, (790) G, attributed to 4th (3rd) nearest neighbouring nuclei with equal Zeeman and hyperfine energies. The electronic spin memory losses for KCl, NaCl and KBr are in the ratio 1: (4.5 ± 1): (11 ± 3).     

Peculiarities of free induction and primary spin echo signals for spin-correlated radical pairs

Keeping in mind ion-radical pairs in a photosynthesis reaction centre first of all, we calculated free induction and spin echo (ESE) signals for an ensemble of radical pairs which initially start in a singlet state. It was shown that the intensity of signals should oscillate depending on the time interval τ between the start of a pair and a microwave pulse forming free induction (FI) or between the start of a pair and the first of two microwave pulses forming primary ESE signal. ESE phase of spin-correlated pairs does not coincide with the corresponding ESE phase of radical pairs in thermal equilibrium. One should also note an interesting feature of FI: immediately after the microwave pulse free induction signal equals zero, and non-zero free induction signal appears only due to spin evolution. This behaviour formally resembles the situation occurring when the primary ESE is formed: a light pulse which creates spin-correlated radical pairs acts as the first microwave pulse in conventional spin echo experiments. Analysis of FI and ESE in experiments on pulse photolysis or radiolysis may provide useful information about the contribution of spin-correlated radical pairs.     

Relative change of the electron density at the Cu nucleus in CuSCN and Cu(SCN)2

The change of the decay constant of⁶⁴Cu in CuSCN and Cu(SCN)₂ compared to metallic Cu has been measured. Values of the relative change of the electron density at the Cu nucleus are deduced taking condensed matter effects on overlap and exchange corrections into account.