Yu. E. Kandrashkin

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.

Electron spin polarization in consecutive spin-correlated radical Pairs: Application to short-lived and long-lived precursors in type 1 photosynthetic reaction centres

An analytical treatment of the spin dynamics in sequential photoinduced correlated coupled radical pairs is presented and applied to the spectra of the states P+A1− and P+Fx− in type 1 photo-synthetic reaction centres. Expressions for the spin polarized spectra are derived for the specific limiting cases of a very short-lived and very long-lived primary radical pair which correspond to the situation found in heliobacteria and photosystem I (PSI), respectively. The inhomogeneous line-broadening due to the unresolved hyperfine couplings is taken explicitly into account. It is shown that the density matrix of the secondary pair ρ2 can be written as the sum of two terms corresponding to (i) the part which is independent of the spin dynamics in the precursor, (ii) the additional spin polarization which is generated during the lifetime of the precursor and transferred to the secondary pair. The latter term contains two contributions which arise from the difference of the Zeeman interactions of the radicals in the primary pair and from the inhomogeneous line broadening. The predicted polarization patterns are compared to those established for chemically induced dynamic electron polarization (CIDEP) when uncoupled radicals are generated from a radical pair precursor. The expressions are then used to simulate the experimental spectra of the consecutive pairs P+A1− and P+Fx− in PSI using parameters derived entirely from independent experimental data. Excellent agreement with the experimental results is obtained. The spectra of P+Fx− in heliobacteria at X- and K-band are also simulated and it is shown that the observed polarization patterns can be reproduced assuming direct electron transfer from A0 to Fx with a time constant ofτ = 600 ps.

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 Fx 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.

Spin dynamics and EPR spectra of consecutive spin-correlated radical pairs. Model calculations

Time-resolved continuous wave EPR signals of two consecutive radical pairs are found in the linear response limit. Numerical simulations of the EPR observables visualize two characteristic features. First, there is a shift of a phase of quantum beats of the EPR line intensities of the secondary pairs. This phase shift originates from a certain time delay in a formation of the secondary pairs (due to time spent by electron spins in the primary radical pair state) and from the difference of the spin dynamics in the secondary and the primary pairs. This phase shift might be detected even in the cases when the primary radical pair has the very short lifetime and, as a result, the EPR spectrum of the primary pair cannot be detected directly. Second, for two consecutive radical pairs, there might be a pronounced non-equality of intensities of EPR lines at the EPR resonance frequencies of the secondary pairs. Indeed, in a case of two consecutive pairs there is the additional mechanism which induces the non-equality of the EPR line intensities: a polarization transfer from the primary to secondary pair and the change of a electron spin quantization axis when a primary radical pair transforms to a secondary radical pair. A possibility to detect experimentally these features of the EPR signals when studying consecutive charge separated states in photosynthetic reaction centers is discussed briefly.

The magnetoelastic effect in permalloy particles

Two independent methods—ferromagnetic resonance and magnetic-force microscopy—have been used to study the magnetoelastic effect in permalloy microparticles. The values of effective magnetic-anisotropy fields that are induced by mechanical compression of microparticles have been obtained from the analysis of ferromagnetic-resonance data. These data have been used to model magnetic-force images of stressed and unstressed particles. The images coincide well with experimentally observed ones.