High frequency EPR on dilute solutions of the single molecule magnet Ni(4).

Dilute frozen solutions of the single molecule magnet Ni(4) (S=4) have been studied using 130 GHz electron paramagnetic resonance (EPR). Despite the random orientation of the molecules, well defined EPR absorption peaks are observed due to the strong variation of the splittings between the different spin states on magnetic field. Temperature dependent studies above 4 K and comparison with simulations enable identification of the spin transitions and determination of the Hamiltonian parameters. The latter are found to be close to those of Ni(4) single crystals. No echo was detected from Ni(4) in pulsed experiments, which sets an upper bound of about 50 ns on the spin coherence time.

Experiments on free decay of quasi-two-dimensional turbulent flows.

Decaying quasi-two-dimensional turbulence in a thin-layer flow is explored in laboratory experiments. We report the presence of power-law interval in the enstrophy decay law, in agreement with earlier experiments by Cardoso et al. [Phys. Rev. E 49, 454 (1994)] and Hansen et al. [Phys. Rev. E 58, 7261 (1998)]. The decay exponent proves sensitive to the way in which the energy decay is compensated. For the range of initial microscale Reynolds numbers between 35 and 95, the decay exponent is close to -0.4 for the ratio of enstrophy to energy, and to -0.75 for the enstrophy multiplied with a compensating factor of exp(-2lambda(t)), where lambda is the bottom-drag coefficient and t the decay time. The vorticity behavior does not comply with the theory of Carnevale et al. [Phys. Rev. Lett. 66, 2735 (1991)]: robust vortices are not observed in the vorticity field and the vorticity kurtosis is less than the Gaussian value.

A W-band pulsed ENDOR spectrometer: setup and application to transition metal centers.

The design and performance of a 95 GHz pulsed W-band EPR/ENDOR spectrometer is described with emphasis on the ENDOR part. Its unique feature is the easy and fast sample exchange at 4.2 K for frozen solution and single crystal samples. In addition, the microwave bridge power output is relatively high (maximum 267 mW), which allows the application of short microwave pulses. This increases the sensitivity in echo experiments because of the broader excitation bandwidth and the possibility of employing short pulse intervals, as long as the dead time does not increase significantly with the power. The spectrometer features two microwave and radiofrequency (0.1-220 MHz, 3 kW pulse power) channels and a 6 T superconducting magnet in a solenoid configuration. The magnet is equipped with cryogenic sweep coils providing a sweep range of +/-0. 4 and +/-0.2 T for a center field of 0-4 and 4-6 T, respectively. The spectrometer performance is demonstrated on Cu(II) centers in single crystals, a zeolite polycrystalline sample, and a protein frozen solution.

An advanced investigation of interaction of allocated quasi-two-dimensional vortices.

This work should be regarded as a natural development of the investigations by Dolzhanskii, Krymov and Manin [Sov. Phys. Usp. 33, 495-520 (1990); J. Fluid Mech. 241, 705-722 (1992); Russ. J. Comput. Model. 1, 107-118 (1993)] of quasi-two-dimensional (Q2D) flows in which the linear and weakly nonlinear stability theory based on the 2D hydrodynamic equations with the Rayleigh (Ekman) friction term imitating the influence of the bottom on the motion of upper fluid layers was corroborated with laboratory and observational data. The applicability of the Q2D approach to describe self-oscillating supercritical regimes was even more vague as Batchaev's experiments [Izv. AN SSSR Fiz. Atmos. Okeana 25, 434-439 (1989); Z. Prikl. Mech. Tech. Fiz., No. 4, 85-91 (1990)] on modeling the four vortex self-oscillations in a thin fluid layer by the magnetohydrodynamics method (the so called hydrodynamical clock [Obukhov, Dolzhanskii and Batchaev, Topological Fluid Mechanics, Proceedings of the IUTAM Symposium, Cambridge, 13-18 August 1989 (Cambridge University Press, Cambridge, 1989), pp. 304-314]) did not find an appropriate theoretical explanation. To remove earlier uncontrolled effects the supplementary detailed measurements of the experimental flow characteristics were implemented, including the spatial spectral composition of the external vorticity sources and free surface 2D velocity fields. Satisfactory agreement is found between experimentally measured flow characteristics and the results of numerical simulations. The frequency of self-oscillations was found to be greatly susceptible to the spectral composition of the external vorticity sources and fluid layer thickness, which should be taken into account in designing laboratory experiments to simulate the natural Q2D processes observed in the ocean and atmosphere. Applicability conditions of the Q2D approach and the influence of geometrical parameters of vortices on their nonlinear interplay are also discussed. (c) 1996 American Institute of Physics.

Quasi-two-dimensional hydrodynamics and problems of two-dimensional turbulence.

The principal problems of quasi-two-dimensional (Q2-D) hydrodynamics are discussed. Accounting for Q2-D flow vertical structure is shown to eliminate "genetic" defects of the formal 2-D idealization of 3-D Navier-Stokes equations and allows under certain conditions to formulate corrected 2-D motion equations which adequately describe real hydrodynamic processes. The applicability of the approach is directly verified in laboratory experiments. Special attention is paid to the problem of 2-D turbulence. Its simulation on the basis of ordinary 2-D equations is unjustified because of the absence of the external Kolmogorov dissipation scale and reverse spectral energy flux. An alternative approach allows one to introduce the natural external scale of 2-D turbulence which depends only on physical properties of the system under consideration and to formulate the conditions under which the large scale vortex dynamics is expected to be universal at large Reynolds number, i.e., to be independent on the size and form of integration domain and lateral boundary conditions.

[Analysis of existing models of spin label movement during spin labeling of biological macromolecules]. / Analiz sushchestvuiushchikh modelei dvizheniia spinovoi metki pri spin-mechenii biologicheskikh makromolekul.

The spin-labeled bovine serum albumin and IgG were studied in search of an experimental approach for comparison of different models of rotational mobility of spin label. These models are: the model of isotropic motion of spin label together with the macromolecule (IM); the model of highly anisotropic motion of spin label (HAM); and the model of slow isotropic motion of label around the binding site (SIML). The experimental spectra were measured on a common X-band ESR spectrometer and on the unique 140 GHZ (lambda = 2 mm) ESR spectrometer under the same conditions. Theoretical spectra were computer-calculated according to Freed's theory. We have found, that the results of temperature-viscosity experiments in X-band are contradictory to the model of IM both for the BSA and IgG species. The models of HAM and SIML for the BSA give identical X-band spectra. The bovine serum albumin spectra in the 2 mm region strongly contradict to the assumptions of the HAM model. Also, the SIML model fails to describe the experimental spectra in terms of isotropic motion of the spin label around the binding site. X-band spectra of IgG can not be explained by the SIML model, while the same spectra in the 2 mm region can not be explained by the HAM model.