Anomalous pulse-angle and phase dependence of Hahn’s electron spin echo and multiple-quantum echoes of the spin correlated radical pair P700+A 1 − in photosystem I

In a spin-correlated radical pair system, anomalous pulse-angle and phase dependence of electron spin echo and multiple-quantum echoes were theoretically calculated by Tanget al. (J. Chem. Phys.106, 7471 (1997)). The maximum intensity of the out of phase signal at 45 degree of spin rotation angle was experimentally verified in two-pulse echoes of the light-induced P700⁺A ₁ ^? radical pair in Photosystem I. The values,D = 1.64 G andJ = 0.00 G, fit well with the experimental ESEEM spectra. Single and double quantum echoes were observed at the value oft = τ andT = 2τ with the laser flash-t-P1_70,ζ1-τ-P2_{140, ζ2}-T pulse sequence, which led to determination of the relaxation time T₂₃ between the singlet and triplet ¦T₀〉 states. The relaxation times of the zero and single quantum transitions were determinedT ₂₃ ≈ 100 ns andT ₂ = 1000 ns, respectively. The field sweep ESE signal shape can be fitted with the hyperfine inhomogeneities of 7 G for P700⁺ and of 10 G for A ₁ ^? .     

Four possible types of pulses for self-induced transparency

We have found four types of possible steady-state solutions to the coupled Maxwell-Bloch equations which describe extremely intense pulse propagation in a resonant medium. This is possible due to the replacement of the usual slowly varying envelope approximation by the assumption that (c/V-1)/ωτ « 1, where V is the pulse speed and τ is the pulse duration. We ignore possible non-resonant losses and set T₁ = T₂ = ∞.     

Multistage radiation-stimulated changes in the microhardness of silicon single crystals exposed to low-intensity β irradiation

Radiation-stimulated and postradiation changes in the microhardness of silicon single crystals exposed to irradiation with a low-intensity flux of β particles (I = 9 × 10⁵ cm^?2 s^?1, W = 0.20 + 0.93 MeV) are studied. It is established that the inversion of the radiation-induced plastic effect occurs at a characteristic irradiation time τ_c = 75 min; i.e., irradiation of silicon single crystals for a time τ < τ_c leads to nonmonotonic reversible hardening, whereas nonmonotonic reversible softening is observed under irradiation for a time τ > τ_c. It is demonstrated that there exists a correlation between the nonmonotonic dependences of the microhardness and the concentration of electrically active defects at acceptor levels with energies E _c ? 0.11 eV, E _c ? 0.13 eV, and E _c ? 0.18 eV on the irradiation time.     

Transient optical absorption induced by an electron pulse in KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystals

The efficiency of formation and time evolution of radiation-induced structural defects and pulsed luminescence in KPb₂Cl₅ crystals under the action of a single electron pulse (E = 250 keV, τ = 20 ns) have been investigated. The spectra (1.1–3.8 eV) and relaxation kinetics (time interval 5 × 10^?8?5 s) of transient optical absorption and the pulsed cathodoluminescence spectra and decay kinetics (1.4–3.1 eV) have been measured in the temperature range 80–300 K. It is revealed that the induced optical density and its time evolution depend strongly on temperature, and the absorption relaxation time contains several components and reaches several seconds at T = 300 K. The decay kinetics of transient absorption and pulsed cathodoluminescence kinetics have different orders and are controlled by different relaxation processes.     

Anomalous pulse angle dependence of the single and double quantum echoes in a photoinduced spin-correlated coupled radical pair

In this work the response of a spin-correlated coupled radical pair to the sequence flash-t-P _ζ-τ-P _2ζ-T is investigated. For the theoretical analysis, the density operator formalism is used. Analytical expressions are derived for the electron spin single (SQ ESE) and double-quantum echoes (DQ ESE) as a function of pulse flip angle and singlet-triplet mixing angle. To illustrate the theoretical results, computer simulations are presented. In the limit of weak coupling, the “out-of-phase” SQ ESE is shown to be of a pure two-spin order having the maximal amplitude for the flip angle of 65.9°. The echo following the Hahn sequence vanishes in the same limit. This confirms the theoretical result already presented in the literature. However, the more general analysis shows that outside the weak coupling approximation the Hahn echo is of purely one-spin order, whereas the echo following the flash-t-P _ζ-τ-P _2ζ-t sequence has its maximal amplitude for the flip angle of 75° and the singlet-triplet mixing angle of 27°. The “in-phase” single- and double-quantum echoes are shown to vanish due to averaging out, within the electron spin resonance spectrometer deadtime, of contributions modulated with the sum and difference of the zero-quantum beat frequency and the frequency due to the spin-spin interaction within the pair. The calculated out-of-phase DQ ESE signal is inverted with respect to the out-of-phase SQ ESE and has only the half of its amplitude. The DQ ESE vanishes for the Hahn sequence. The echo has maximal amplitude in the weak-coupling limit for the flip angle of 65.9°. In contradiction to the analytical result previously published, the out-of-phase DQ ESE does not vanish for long τ and large zero-quantum-beat frequency.     

The influence of stimulated echoes on contrast in fast spin-echo imaging

Tissue contrast obtained using fast spin-echo (FSE) and conventional spin-echo (SE) sequences is not identical and a number of mechanisms are thought to contribute to these contrast differences. The effect of stimulated echoes has previously been apparently ruled out as a contributing mechanism. Signal-to-noise ratios of single-slice matched FSE and conventional SE sequences were compared in aqueous solutions of CuSO₄, Cr₂(SO₄)₃ and MnSO₄ with various T₁ and T₂ values. Enhancement of the FSE signal was observed in short T₂ solutions and the effect was greater in samples where the T₁ to T₂ ratio was high. Reducing the refocusing pulse flip angle to increase the contribution from stimulated echoes also resulted in slightly increased enhancement. Experimental results were verified by computer simulations. Our results show that stimulated echoes do contribute to the contrast differences between FSE and conventional SE images and may be significant in the imaging of brain hemorrhage.     

Quasi-spin locking and simultaneous averaging of signal and baseline in dipolar solids

An alternating train of solid echoes and zero signals obtained by a 90° pulse sequence could be prolonged for a time of the order of T_1?. This quasi-spin locking effect enables one to average simultaneously the signal of the transverse magnetization and the baseline at a high data rate.     

Time of a semiconductor-metal phase transition induced by an ultrashort light pulse in vanadium dioxide

A relationship is obtained for a time τ of a photoinduced semiconductor-metal phase transition of the Peierls type under exposure to an ultrashort laser pulse. It is demonstrated that (i) the time τ increases as the pulse energy density W decreases, and (ii) the phase transition is initiated when the pulse energy density W exceeds a critical value W _c . The results obtained are compared with the experiment on irradiation of a vanadium dioxide film in a light field.     

Coherent optical pulse evolution in a CO2 amplifier

The coherent reshaping of short duration (2–5 nsec) CO₂ laser pulses in a low-pressure (∽ 5 torr), longitudinal discharge CO₂ amplifier is experimentally studied in the linear regime for a variable number of gain lengths (αL?7). Single pulses grow considerably in duration as well as amplitude in agreement with theoretical considerations. Analysis of the observed pulse evolution is used to obtain the transverse relaxation parameter T₂. Zero-degree pulses {∫^+∞_-∞E ( z, t) dt = 0} are observed to terminate much of the long tail which occurs in single-pulse amplification. Off-resonant amplification of short-duration pulses is shown to lead to dramatic changes in the zero-degree pulse evolution. Numerical calculations relating to pulse amplification in the nonlinear regime for high-pressure CO₂ amplifiers are also presented.     

Sublevel echoes induced by resonant light pulses: Quantum beat echoes

We predict and observe a new class of echoes of atomic sublevel coherences induced by two impact excitations with resonant light pulses which may be either coherent or incoherent. The Zeeman-sublevel echoes in 3 ²S_{$\text{1}\text{2}$} of sodium were observed by using polarization selective detection technique under the excitation condition that the optical T₂ (5 × 10^-11 s) was much shorter than the pulse width (5 ns). Theoretical analysis also predicts the echoes involving coherence transfer between ground and excited states.     

Strongly positive semigroups and faithful invariant states

Let (?, τ, ω) denote aW*-algebra ?, a semigroupt>0?τ _t of linear maps of ? into ?, and a faithful τ-invariant normal state ω over ?. We assume that τ is strongly positive in the sense that $$\tau _t (A^ * A) \geqq \tau _t (A)^ * \tau _t (A)$$ for allA∈? andt>0. Therefore one can define a contraction semigroupT on ?= \(\overline {\mathcal{M}\Omega } \) by $$T_t A\Omega = \tau _t (A)\Omega ,{\rm A} \in \mathcal{M},$$ where Ω is the cyclic and separating vector associated with ω. We prove 1. the fixed points ?(τ) of τ are given by ?(τ)=?∩T′=?∩E′, whereE is the orthogonal projection onto the subspace ofT-invariant vectors, 2. the state ω has a unique decomposition into τ-ergodic states if, and only if, ?(τ) or {?υE}′ is abelian or, equivalently, if (?, τ, ω) is ?-abelian, 3. the state ω is τ-ergodic if, and only if, ?υE is irreducible or if $$\mathop {\inf }\limits_{\omega '' \in Co\omega 'o\tau } \left\| {\omega '' - \omega '} \right\| = 0$$ for all normal states ω′ where Coω′°τ denotes the convex hull of {ω′°τ _t } _t>0. Subsequently we assume that τ is 2-positive,T is normal, andT* _t ?₊Ω \( \subseteqq \overline {\mathcal{M}_ + \Omega } \) , and then prove 4. there exists a strongly positive semigroup |τ| which commutes with τ and is determined by $$\left| \tau \right|_t \left( A \right)\Omega = \left| {T_t } \right|A\Omega ,$$ 5. results similar to 1 and 2 apply to |τ| but the τ-invariant state ω is |τ|-ergodic if, and only if, $$\mathop {\lim }\limits_{t \to \infty } \left\| {\omega 'o\tau _t - \omega } \right\| = 0$$ for all normal states ω′.     

Measurements of scintillation efficiency and pulse shape for low energy recoils in liquid xenon

Results of observations of low energy nuclear and electron recoil events in liquid xenon scintillator detectors are given. The relative scintillation efficiency for nuclear recoils is 0.22±0.01 in the recoil energy range 40–70 keV. Under the assumption of a single dominant decay component to the scintillation pulse shape the log-normal mean parameter T₀ of the maximum likelihood estimator of the decay time constant for 6 keV <E_ee<30 keV nuclear recoil events is equal to 21.0±0.5 ns. It is observed that for electron recoils T₀ rises slowly with energy, having a value ∼30 ns at E_ee∼15 keV. Electron and nuclear recoil pulse shapes are found to be well fitted by single exponential functions although some evidence is found for a double exponential form for the nuclear recoil pulse shape.     

CPMG echo amplitudes with arbitrary refocusing angle: explicit expressions, asymptotic behavior, approximations

Exact explicit analytical expression for echoes in the Carr–Purcell–Meiboom–Gill sequence with arbitrary excitation and refocusing angles and resonance offset of RF pulses was obtained, employing the generating functions formalism developed earlier by authors. Asymptotic form and analytical approximation for echoes were derived in an elegant way and analyzed in details. In particular, it was shown that depending on T₁, T₂ and parameters of the pulse sequence, oscillatory behavior of echoes can take place. Accuracy of asymptotic forms and approximations were tested by comparison with exactly calculated echo amplitudes. Besides, it was shown, that the generating function approach can be applied to the consideration of terminated pulse sequences, when after-pulses echoes are registered.     

Systematization of intermediate-mass fragments

We consider the power law dependence of cross-sections of formation of intermediate-mass fragments on the mass number and atomic number, σ (A _f ) ～ A _f ^?τ and σ (Z _f ) ～ Z _f ^?τ . The values of parameter τ are determined for fragments detected by the ΔE ? E and the induced activity methods. It is shown that in “liquid-gas” phase transition τ varies within the limits of 2–3. A conclusion was drawn that at high beam energies parameter τ does not depend on the detection angle of the fragment. At the increase in the energy τ passes through a minimum at E _i ～ 6–7 GeV which is typical “critical behavior” predicted by models.     

N.M.R. solid echoes in systems of dipolar-coupled inequivalent spins-1 or dipolar-coupled inequivalent pairs of spins-1/2

Proton solid-echo transverse relaxation functions for many thermotropic and lyotropic mesophases, mapped by measurement of the echo amplitude S_yx (t′ = τ) as a function of τ using a P_y (90°)-τ-P_x (90°)-t′ sequence, yield gaussian behaviour of the form exp [-½M ^E ₂τ²] for decays up to 20–30 per cent of the value at τ = 0. M ^E ₂, the second moment for the dipolar interactions between the spin-½ pairs, is related to the van Vleck second moment M ^VV ₂ through a factor f. Whilst experiments suggested a value of 0·70–0·72 for f = M ^E ₂/M ^VV ₂, simple models that ignored the non-equivalence of the dipolar-coupled spin-pairs had predicted f = 0·65. In this paper we derive an exact analytic expression for the spin response of a model of two dipolar-coupled inequivalent spins-1 to the pulse sequence P_y (90°)-τ-P _α(β)-t′, and show that the present model, with the quenching of the spin-flip terms of the dipolar hamiltonian, resolves the afore-mentioned discrepancy. We also reconcile the differences between the experimental and the earlier predicted values of f for deuteron N.M.R. spin echoes in perdeuterated solids.     

Competition between different mechanisms of stability loss for the (C20)2 cluster dimer

The thermal stability of the (C₂₀)₂ cluster dimer consisting of two C₂₀ fullerenes has been numerically examined using the tight-binding method. The simulation of the dynamics of the (C₂₀)₂ dimer at temperatures of T = 2000–3500 K shows that the finite lifetime τ of this metastable system is determined by two fundamentally different processes: the decay of one of the C₂₀ fullerenes and the coalescence of two C₂₀ fullerenes to the C₄₀ cluster. The activation energies for these processes. E _a ≈ 3.4 and 2.7 eV, respectively, as well as their frequency factors, have been determined by analyzing the τ(T) dependence.     

Critical phenomena in the β-(2×4) → α-(2×4) reconstruction transition on the (001) GaAs surface

The critical exponents of the β-(2×4) → α-(2×4) reconstruction phase transition on the (001) GaAs surface are determined experimentally. It is found that the phase transition is analogous to a van der Waals transition. The critical parameters T _c , P _c , and Θ_c have been measured experimentally. The mean field theory is applied, and three-parameter isotherms are obtained that agree with the experimental results at the following values of the parameters: E_st = 0.36 eV, ΔE = 0.18 eV, and E _i = 0.134 eV. Precision measurements of the critical exponents β and δ are carried out. Their values β = 1/8 and δ = 15 indicate that the phase transition is truly two-dimensional.     

Renormalization of the axial anomaly operators

An expressionE(d ³ σ/dp ³)=Kp _T ^?n exp(?E/τ, wheren, K and τ are parameters to be fitted, has been used to represent the large-p _T hadron invariant single-particle inclusive cross sections. In most cases the fitting is within the quoted experimental uncertainties. An interpretation of the expression in terms of fireballs is given. Scaling is seen to be violated in large-p _T phenomena.     

Temperature dependent time delay of the voltage response to supercritical current pulses in superconducting aluminium

Current pulses larger than the critical current I_c are passed through narrow superconducting aluminium strips. The time delay between the start of the pulse and the onset of the voltage response is measured as a function of current amplitude for various temperatures (0.84<T/T_c<0.98). We find the delay time to be dependent on the temperature as (1 - T/T_c)^α with α changing from a value of - 0.5 for I/I_c close to 1 to a value of - 1.0 for I/I_c larger than 3.