Analysis of the Movement of Line-like Samples of Variable Length along theX-Axis of a Double TE104and a Single TE102Rectangular Cavity

Movement of line-like samples with lengths from 5 to 50 mm along thex-axis of the double TE₁₀₄rectangular cavity has been analyzed. The observed dependencies of the EPR signal intensity versus sample position showed: (i) a sharp maximum for sample lengths from 5 to 20 mm; (ii) a plateau, over which the EPR signal intensity remained constant within experimental errors of 0.26–1.07%, for lengths from 30 to 40 mm; and (iii) a “sloping plateau,” which could be approximated by the linear function (correlation,r= 0.98) for sample length 50 mm. Theoretical values of the experimentally observed dependencies of the intensity versus sample position were calculated using the modified sine-squared function and the correlation between observed and theoretically predicted dependencies is very good. The experimental dependence of the EPR signal intensity versus the sample length for samples situated at the same point in the cavity was nonlinear with a maximum for the 40-mm sample. The dependence of the EPR signal intensity upon the movement of a large cylindrical sample (o.d. 4 mm and length 100 mm) along thex-axis of the cavity was similar to that found for the 50-mm sample. However, an additional oscillating signal superimposed on the sloping plateau was observed. The presence of a large sample fixed in the complementary cavity of the double TE₁₀₄cavity caused an additional deformation of the signal intensity for a 30-mm sample which was moving in the first cavity. The primary effect was that the plateau was replaced by a region in which the intensity increased linearly with sample position,r= 0.99. Each of the above phenomena may be a source of significant errors in quantitative EPR spectroscopy. Cylindrical samples to be compared should be of identical length and internal diameter. Accurate and precise positioning of each sample in the microwave cavity is essential.     

Influence of the diameter and wall thickness of a quartz pipe inserted in the EPR cavity on the signal intensity

Some dependences between the EPR signal intensity of point samples and the diameter and wall thickness of the quartz tubes with which a standard rectangular TE₁₀₂ EPR cavity is loaded are reported. It is found that the EPR signal intensity linearly depends on the wall thickness of the container and can be increased approximately twice when thick sample tubes are used. The microwave power in the location of the sample increases similarly whereas the normalized bell-shape distribution of the signal intensity along thez-axis of the cavity remains unchanged. The effective microwave power necessary to saturate the sample decreases and the cavity filling factor increases.     

Analysis of the radial and longitudinal effect in a double TE(104) and a single TE(102) rectangular cavity

The response of the cavity to the rotation of a point-like sample in the horizontal (y-z) plane passing through the center of the Bruker double TE(104) and single TE(102) rectangular cavities in concentric circles of radii rho = 0, 1, 2, 3, 4, and 5 mm from the cavity center (radial effect) has been analyzed. The experimentally observed dependencies of the EPR signal intensity, I(pp), showed the following: (i) for rho = 0 mm (a sample position in the cavity center), I(pp) is independent of the angle of rotation; (ii) for rho = 1, 2, and 3 mm, the I(pp) dependence progressively changes from circular to oval; (iii) when the radius is further increased to rho = 4 and 5 mm, the I(pp) dependence changes dramatically, giving a figure eight shape. These experimental observations are in very good agreement with the theoretical calculations, in which the response is modeled using modified Cassinian curves, K(rho, phi). Similar trends were observed for any position of the horizontal (y-z) plane at which the sample is situated along the vertical x axis of the cavity; however, the amplitude of the signal decreases with increase in the absolute value of the x coordinate, ||x ||. The variation in the signal amplitude along the cavity x axis (longitudinal effect) can be calculated theoretically using a modified sine-squared curve, G(x). In general, the response of the cavity to a point-like sample situated at any position, P(rho, phi, x), can be represented as a product of the mentioned Cassinian curve, K(rho, phi), and sine-squared curve, G(x), giving for the signal intensity I(pp)(rho, phi, x) approximately K(rho, phi)G(x). The response to a large cylindrical sample which is concentrically situated on the cavity x axis can then be obtained by integrating the above product, K(rho, phi)G(x), over the sample volume. The nonlinear radial effect may give rise to a serious source of systematic error in quantitative EPR spectroscopy and shows that accurate and precise positioning of the sample in the microwave cavity is essential.     

Assessment of the thermal stability of a polymer liquid by the controlled pulsed heating method

A method of controlled pulsed heating of a quick-response probe placed in the experimental material is developed. It is shown that the “temperature plateau” regime can be used to estimate the average lifetime of a polymer liquid before its ebullition. A correlation was found between the slope of the curve of the lifetime versus the temperature on the plateau and the thermal stability of the material. Zh. Tekh. Fiz. 69, 92–94 (December 1999)     

Droplet suction on porous media

We study the forced aspiration of small ( mm) and large ( cm) liquid drops, deposited on prewetted porous membranes, and pumped mechanically with a constant current J. Two kinds of membranes are used where the pores are i) disconnected, cylindrical and calibrated or ii) interconnected “sponge-like”. Whatever the size of the drops and the intensity J of the current, two suction regimes are observed versus time: 1) a “locked” regime, when the drop is pinned, with a dynamic contact angle decreasing from advancing () to finite receding () contact angle; 2) an “unlocked” regime, where the contour line recedes with a constant contact angle closed to . In both regimes, the shape of the drop remains quasistatic, during the suction process, i.e. a spherical cap for small drops and a flat “gravity pancake” for large ones. Received 19 January 2000     

Emission spectra of terahertz quantum cascade laser

We calculated energy levels, wave functions, and energies of radiative transitions in terahertz quantum cascade lasers based on GaAs/Al_0.15Ga_0.85As heterostructures. Current-voltage characteristics and current dependences of laser radiation intensity were measured, and the maximum operating temperatures reaching 85 K were determined. Radiation spectra of quantum cascade lasers were measured for different temperatures, and the effect of intensity “pumping” from low-frequency modes to high-frequency modes was found to happen in the case of an increase in the current and time delay of the signal capture, which is explained by heating of the sample during a pulse of the current. Application of the lasers for registration of impurity photoconductivity signals in semiconductor heterostructures was demonstrated.     

In Vivo EPR Imaging of a Perfluoroalkyl Radical in Mouse Abdomen

Although it is thought that perfluoro-2,4-dimethyl-3-isopropyl-3-pentyl (PFR-2) is a candidate for electron paramagnetic resonance (EPR) imaging agents because of its high stability, no study has been made yet on the EPR imaging of PFR-2. In this study, EPR imaging of a phantom including PFR-2 and mice that had received PFR-2 was performed by an in vivo EPR imaging system operating at an EPR frequency of 700 MHz equipped with a bridged loop-gap resonator (inner diameter, 41 mm; axial length, 10 mm). Because PFR-2 is insoluble in water, it was dissolved in perfluorocarbon. The PFR-2 solution was put in cylindrical sample tubes with various inner diameters, and these sample tubes were placed together in a larger cylindrical sample tube filled with a physiological saline solution, which was used as a phantom. The spatial resolution was estimated to be about 3 mm on the basis of EPR imaging of the phantom. EPR images of mice that had received a PFR-2 injection via the intraperitoneal route indicated that PFR-2 remained in the peritoneal cavity even 2 days after the injection. This finding suggests that it is possible to perform EPR imaging of experimental animals using PFR-2 as an imaging agent which persists in a biological system. Authors' address: Hidekatsu Yokoyama, National Institute of Advanced Industrial Science and Technology, 2266-98 Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Aichi 463-8560, Japan     

A Fast Algorithm for Wave Propagation from a Plane or a Cylindrical Surface

The newly developed Taylor-Interpolation-FFT (TI-FFT) algorithm dramatically increases the computational speeds for millimeter wave propagation from a planar (cylindrical) surface onto a “quasi-planar” (“quasi-cylindrical”) surface. Two different scenarios are considered in this article: the planar TI-FFT is for the computation of the wave propagation from a plane onto a “quasi-planar” surface and the cylindrical TI-FFT is for the computation of wave propagation from a cylindrical surface onto a “quasi-cylindrical” surface. Due to the use of the FFT, the TI-FFT algorithm has a computational complexity of O(N ² log₂  N ²) for an N × N computational grid, instead of N ⁴ for the direct integration method. The TI-FFT algorithm has a low sampling rate according to the Nyquist sampling theorem. The algorithm has accuracy down to −80 dB and it works particularly well for narrow-band fields and “quasi-planar” (“quasi-cylindrical”) surfaces.     

Evolution of a pair of classical ions in a cavity with elastic walls,crosspieces, and implanted charges

Using the method of classical trajectories, we have simulated the motion of a pair of oppositely charged ions in spherical and ellipsoidal cavities, including cavities that contain cylindrical “crosspieces” and positively charged “nuclei.” We supposed that each of the ions reflected off the cavity wall, crosspieces, and nuclei according to the elastic impact law. Most of attention is focused on the statistics of the events of ion recombination and those of dissociation of the corresponding molecule with an ionic bond. When nuclei are absent, recombination and dissociation events are possible only on collisions of the ions with the cavity wall or crosspieces. In the presence of nuclei, on the other hand, the major part of the events occur in time intervals between the collisions of the ions with the cavity wall, crosspieces, or nuclei.     

Aqueous sample in an EPR cavity: sensitivity considerations

The radial mode matching (RMM) method has been used to calculate accurately the microwave field distribution of the TE(011) mode in a spherical EPR cavity containing a linear aqueous sample, in order to understand in detail the factors affecting sensitivity in EPR measurements at X band. Specific details of the experiment were included in the calculations, such as the cavity geometry, the presence of a quartz dewar, the size of the aqueous sample, and the sample's dielectric properties. From the field distribution, several key physical parameters were calculated, including cavity Q, filling factor, mean microwave magnetic field at the sample, and cavity efficiency parameter Lambda. The dependence of EPR signal intensity on sample diameter for a cylindrical aqueous sample was calculated and measured experimentally for non-saturated and half-saturated samples. The optimal aqueous sample diameter was determined for both cases. The impact of sample temperature, conductivity, and cavity Q on sensitivity of EPR is discussed.     

Influence of the lens effect in a sample with large dielectric constant in a loop-gap resonator on the EPR signal intensity at 700 MHz

The influence of the lens effect on the electron paramagnetic resonance (EPR) signal intensity was investigated in a loop-gap resonator (LGR) with an inner diameter of 41 mm. TheQ- value and EPR signal intensity were measured when the phantoms containing 3-carbamoyl-2,2,5,5-tetramethyl-pyrrolidin-l-yloxy dissolved in sodium chloride aqueous solutions were put in the LGR. TheQ- value and signal intensity reduced with increasing concentrations of sodium chloride in the phantom, indicating that the imaginary part of the dielectric constant is larger in the phantom with the higher concentration of sodium chloride. However, relationships betweenQ-values of the resonator and EPR signal intensities were not proportional and signal intensities were relatively higher compared with theQ-values. These findings suggest that the signal reduction due to lowQ is slightly compensated by the lens effect in the sample with the large real part of the dielectric constant. In the distribution of the signal intensities of a pinpoint sample made of diphenylpicrylhydrazyl in the agar medium containing sodium chloride in the LGR, it was found that the signal intensity decreased according to the distance from the center and the difference in the signal intensity within 10 mm from the center was about 20%, indicating the inhomogeneity of the alternating magnetic field at the center and marginal region in the sample with the large dielectric constant caused by the lens effect.     

Radiation-dose dependence of E′1 centers in samples of quartz containing uncharged oxygen vacancies

EPR is used to study the generation of E′₁ centers (oxygen vacancies that have trapped one electron) in quartz samples containing uncharged oxygen vacancies as a function of irradiation dose. It is found experimentally that an irradiation dose of order 400 Gy is sufficient to allow every oxygen vacancy to trap two electrons apiece in essentially all such quartz samples. The linear segment of the dose dependences of E′₁ centers in samples annealed at 300 °C for 15 minutes can be used to reconstruct prior radiation doses up to 60–70 Gy. If the concentration of oxygen vacancies in the original sample is larger than 10¹⁸ cm⁻³, the signal intensity from E′₁ centers in the sample can be used to detect radiation doses as low as 1–3 Gy, which is significantly lower than the minimum radiation dose detectable by other paramagnetic centers in quartz. Fiz. Tverd. Tela (St. Petersburg) 40, 651–652 (April 1998)     

A study of the temperature dependence of the rate constant for the reaction of O<Subscript>2</Subscript> with surface alkoxy radicals

A method for studying the reactions of surface alkoxy radicals with O₂ at temperatures of 230 to 300 K is described. Alkoxy radicals were generated directly in the cavity of an EPR spectrometer. Surface organic radicals, prepared from paraffin wax ((CH₃)₂(CH₂) _n , n = 16–20), were applied to Aerosil particles from a solution in heptane. The Aerosil sample was placed in the cavity of the EPR spectrometer in a cylindrical cup with a central hole for pumping out gases and exposed to H atoms. In this way, it is possible observe a steady increase in the EPR signal from the surface radicals. To measure the rate constant at tropospheric temperatures, the reaction tube was placed in a Teflon jacket, through which cool nitrogen vapor was pumped. The temperature in the reactor was varied from 230 to 300 K. The recorded EPR spectra belong to the (RO) _s radical. After obtaining a stable EPR signal from the surface radicals, treatment with H atoms was stopped, additional flow of O₂ was introduced ([O₂] = 10¹⁴–10¹⁶ cm⁻³), and the reaction of O₂ with the surface organic radicals was studied by monitoring the EPR signal decay. The temperature dependence of the rate constant for the (RO) _s + O₂ → HO₂ + ketone was obtained within T = 230–300 K. The extrapolation of the data to real tropospheric conditions ([O₂] = 10¹⁸ cm⁻³) was performed.     

Retrieval of the quasi-optimal signal activating the excitable systems using preceding noise samples

We propose a method for obtaining a signal leading to the activation of an excitable dynamic system for a signal energy close to minimal. The efficiency of this technique, which is based on recording and processing of noise samples preceding the activation was tested using the FitzHugh–Nagumo, Hodgkin–Huxley, and Luo–Rudy models as examples. It is shown that the proposed procedure gives good results when the noise intensity is smaller than or close to the system activation energy. The criteria of “low” and “high” intensities of fluctuations are proposed. The method of increasing the stability of the excitable system with respect to “low-intensity” noise by filtering or another way of suppression of the spectral components that make the main contribution to the energetically optimal activation signal is justified. The relation between eigenvalues of the linearized system of the Hamiltonian equations, which describe the optimal trajectories and the activation signal, and eigenvalues of the excitable system linearized near the initial equilibroum state is found.     

Effects of interaction of nanoparticles in a highly anisotropic ferrimagnet

This paper reports on a study of the field and temperature dependences of the parameters of the particle magnetic interaction in a densely packed system of nanocrystals of the highly anisotropic hexagonal ferrite BaFe₁₂O₁₉ with the particles distributed in diameter within the range 10—100 nm and having volumes satisfying the criterion of “small Stoner—Wohlfarth particles.” It is shown that the resultant particle interaction in the temperature range 300 K≤T≤640 K has a negative sign, whereas for T>640 K, it is positive. The maximum values of the parameter Δm allow classification of the interaction as moderate in strength. The temperature dependences of the interaction parameters are found to correlate with manifestation of the size and surface effects in the system, which are characteristic of small particles (transition to the superparamagnetic state, “surface” anisotropy, and reduced exchange interaction in a structurally defective near-surface layer of particles).     

Young’s modulus and internal friction in porous biocarbon white pine wood precursors

This paper reports on a study performed in the temperature range 100–293 K, in air and in vacuum, for the amplitude and time dependences of the Young’s modulus and the internal friction (ultrasound damping) of biocarbon precursors prepared from white pine wood at two pyrolysis (carbonization) temperatures of 1000 and 2400°C. The measurements have been conducted by the resonance technique with a composite vibrator on samples cut along and across the tree growth direction. The desorption of molecules of the external medium at low amplitudes of ultrasonic vibrations has been found to produce the pronounced influence on the effective elastic modulus and elastic vibration decrement. The data obtained from acoustic measurements of the amplitude dependences of the elastic modulus have been used to estimate the microplastic properties of the samples. It has been shown that increasing the carbonization temperature gives rise to noticeable changes in the Young’s modulus and internal friction, as well as to reduction of the microplastic stress σ _y of the biomaterial studied. The stress σ _y of the samples cut across the growth direction has been found to be substantially smaller than that of the “longitudinal” samples. The elastic and microplastic properties of precursors prepared from white pine wood have been compared with those of the white eucalyptus wood.     

Effect of regimes of cooling in a magnetic field on the magnetization of a La0.9Sr0.1MnO3 single crystal

Magnetization measurements were performed on a lanthanum manganite La_0.9Sr_0.1MnO₃ single crystal in the temperature interval 4.2–300 K and magnetic field interval 50 Oe-55 kOe in two sample cooling regimes: 1) cooling down to 4.2 K in a high (55 kOe) magnetic field, and 2) cooling in a “zero” field. It is shown that the temperature dependences of the magnetization M(T) are substantially different in these regimes. Pronounced anomalies of M(T) were observed at temperatures T*=103 K and T _c =145 K. The first anomaly is attributed to a structural transition, while the second one corresponds to a ferromagnet-paramagnet phase transition. The magnetization of a La_0.9Sr_0.1MnO₃ single crystal in the cooling regimes studied shows typical “spin-glass” behavior. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 1, 39–43 (10 July 1998)     

Modeling of time-resolved laser-induced incandescence transients for particle sizing in high-pressure spray combustion environments: a comparative study

In this study experimental single-pulse, time-resolved laser-induced incandescence (TIRE-LII) signal intensity profiles acquired during transient Diesel combustion events at high pressure were processed. Experiments were performed between 0.6 and 7 MPa using a high-temperature high-pressure constant volume cell and a heavy-duty Diesel engine, respectively. Three currently available LII sub-model functions were investigated in their performance for extracting ensemble mean soot particle diameters using a least-squares fitting routine, and a “quick-fit” interpolation approach, respectively. In the calculations a particle size distribution as well as the temporal and spatial intensity profile of the heating laser was taken into account. For the poorly characterized sample environments of this work, some deficiencies in these state-of-the-art data evaluation procedures were revealed. Depending on the implemented model function, significant differences in the extracted particle size parameters are apparent. We also observe that the obtained “best-fit” size parameters in the fitting procedure are biased by the choice of their respective “first-guess” initial values. This behavior may be caused by the smooth temporal profile of the LII cooling curve, giving rise to shallow local minima on the multi-parameter least squares residuals, surface sampled during the regression analysis procedure. Knowledge of the gas phase temperature of the probed medium is considered important for obtaining unbiased size parameter information from TIRE-LII measurements. PACS 42.62.-b; 51.30.+i; 82.20.Wt     

Role of the parameters of external fields acting on a spin system in harmonic-generation processes

Crossed dc and ac magnetic fields are applied to a paramagnet in a continuous regime in an investigation of the induction signal at the doubled and tripled pump frequencies. The experimental object is powdered diphenyl picryl hydrazyl (DPPH), and the pump frequency used is 11 MHz. It is shown that harmonic generation and the orientational dependences of the harmonics arise due to nonlinear variations of the magnitude and the “rocking” angle of the vector sum of the external fields and the precession frequency of the spin magnetization. Anomalies are observed in the Bloch-Siegert effect. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 104–109 (25 July 1997)     

Mobility and structure of human serum albumin powders in water-acetonitrile mixtures by1H NMR relaxation and FTIR spectroscopy

The effect of acetonitrile on protein dynamics was investigated for solid human serum albumin samples at various hydration levels. Temperature dependences of¹H nonselective nuclear magnetic resonanceT ₁ andT ₂ relaxation times at 27 MHz have been measured and data were interpreted in terms of three kinds of internal motions in the protein. Microdynamic parameters of the motions were obtained within a “model-free” approach. It was found that acetonitrile hardly affects the fast motions but noticeably influences the slow motion of side chain groups, shortening the correlation time and increasing the amplitude of the motion. The acetonitrile effect on dynamics is likely based on the appearance of additional free volume as a result of the formation of rigid helical parts in the protein structure. Water, plasticizing the protein structure, promotes the action of organic solvent. A definite part of side chain groups, slowly moving in the same frequency window as the rest of protein side chain groups, performs less constrained “liquidlike” motion. The relative population of these highly movable protons is closely correlated with the increment of the helical structure induced by water and acetonitrile.