Strong Rf Field Research Articles

Rotary echo NMR spectroscopy in three-level systems

A pulse sequence is introduced for eliminating the enfluence of the rf field inhomogeneity on the nutation NMR powder line-shape in three-level systems. Powder patterns obtained by this method are theoretically analyzed and experimentally demonstrated with 1H-NMR spectra of polycrystalline gypsum CaSO4·0.5H2O. Under strong rf field irradiation, they are found to be very close to the well-known Pake pattern.

Parametric and non-linear behaviour of a YBa 2Cu 3O 7−δ RF squid at 77K

We describe SQUID behaviour and the excitation of harmonics of the drive frequency (up to the 15th) in bulk pieces of YBa 2Cu 3O 7−δ. We have studied these effects under the influence of strong RF fields, (1–21MHz), microwave fields (9 and 90GHz), static magnetic fields (up to 40mT), between 77K and the critical temperature of the material. We give a tentative explanation of the observed effects in terms of both the highly non-linear and parametric behaviour of unbiased Josephson inductances and of non-hysteretic SQUID loops.

Observation of spatially localized atomic electron wave packets.

We have excited and detected an atomic electron wave packet that is localized in the polar and azimuthal angles. The wave packet is formed through the coherent superposition of Rydberg states of atomic sodium. The superposition is achieved by short-pulse optical excitation of the atom in the presence of a strong rf field. The wave packet is detected by dc field ionization. The behavior of this wave packet in a strong dc field is much different from that of an eigenstate. This behavior agrees well with a simple classical model.

Computer-aided permittivity measurements of moistened and pyrolized materials in strong RF fields (part II)

In a previous paper [1], we proposed a computer-aided permittivity measurements (CAPM) system and presented the results on moistened paper samples, exposed to strong RF fields inside a microwave cavity. In this paper, we further develop the proposed CAPM system and present the results of permittivity measurements on moistened wood and ceramic samples. A method to correct the measured sample weight for the effects of the microwave leakage radiation pressure is described. The achievement of optimum heating and drying rates is discussed in detail. A block diagram of overall computational procedure for permittivity measurements by using the CAPM system is illustrated. In addition, the measurements error analysis is pursued according to the computational procedure.

Rare spin polarization by the nuclear solid effect

The development of the cross-polarization method has led to numerous applications of high-resolution solid state NMR spectroscopy by making it possible to obtain the spectra of magnetically dilute-“rare spin” -nuclei (I). Using the CP scheme, the polarization of a rare spin can be enhanced by as much as the abundant to rare spin gyromagnetic ratio (e.g., 4 for H-C experiments) (2). This maximum enhancement is rarely achieved in practice, and a rapid decay of the spin-locked proton magnetization (short proton T,,) is often the cause of this problem. The CP scheme also requires that the strength of the proton (and, because of matching requirements, the carbon) spin-lock field be greater than the width of the proton line, a criterion that can be difficult to satisfy. In this communication, we discuss a method for polarizing rare spins (e.g., carbon13) which neither requires a long Tr, for the abundant spin (e.g., proton) nor very strong matching rf fields at both Larmor frequencies. We also report results which illustrate characteristics of the method that are complementary, and under certain conditions superior, to the cross-polarization technique. The nuclear solid effect (NSE) is analogous to the solid effect associated with dynamic nuclear polarization (DNP) (3). In DNP, the nuclei are polarized by saturation of a forbidden transition involving simultaneous flips of an electron and nuclear spin. This transition is at the sum or difference of the electron and nuclear Larmor frequencies. In the nuclear solid effect, however, the abundant nuclear spins assume the role of the: electrons, so that the forbidden transition is at the sum or difference of the Larmor frequencies of the abundant and rare nuclei (4). For example, this would be either 45 or 75 MHz for a proton-carbon system in a field of 1.4 T. ‘The polarization step merely requires irradiation of the forbidden transition. The time dependence of the carbon polarization during this irradiation is given by

Spin dynamics and thermodynamics in solid-state NMR cross polarization

We investigate spin thermodynamic processes taking place during Hartmann–Hahn cross-polarization experiments in solids, in which spin polarization is transferred between two nuclear spin species by the application of two strong rf fields. As is well known, optimum cross polarization is achieved for a particular ratio of the two rf field strengths called the Hartmann–Hahn condition; we find experimentally that this condition provides the optimum transfer not only on kinetic grounds, as is usually supposed, but also for thermodynamic reasons. By measurement of the evolution of the spin observables in a ferrocene single crystal we demonstrate the existence of a quasi-equilibrium state with nonuniform spin temperature and less than maximum entropy. A modified spin thermodynamic theory is developed whose main features are the important role of the dipolar energy for the quasi-equilibrium state and the existence of constants of the motion other than the total energy. A new cross-polarization experiment is suggested which is shown to provide efficient cross polarization even for mismatched Hartmann–Hahn conditions.

Off-axis sample spinning in the slow-spinning regime

The past few years have witnessed an increasing interest in the study of solids by means of high-resolution NMR techniques (I, 2). Of the various approaches that have been developed, the combination of polarization transfer and dipolar decoupling (3) together with magic-angle sample spinning (MASS) (4, 5) has become a popular tool in the study of a wide variety of amorphous and polycrystalline samples (6). In this case, heteronuclear dipolar broadening is attenuated by irradiation of the proton spectrum by a strong resonant rf field, and residual broadening due to the chemicalshift anisotropy of the dilute spins is removed by spinning the sample around the magic angle. When the frequency, UR, is fast compared to the shift anisotropy (in hertz), then a high-resolution “liquid-like” spectrum, devoid of all information other than the isotropic chemical shifts, is observed. Because these experiments lead to a suppression of the chemical-shift anisotropy, they have stimulated a search for methods to reintroduce this information in a manner consistent with the goal of high resolution. The two methods most frequently discussed for this purpose involve application of a train of pulses synchronized with the sample spinning (7-11) or analysis of the sideband intensities in the slowspinning regime (12, 13). The former approach results in a distorted (but calculatable) powder lineshape or sideband pattern which must be resolved in some cases by twodimensional techniques. In the latter case, the spinning speed is reduced so that the condition VR AC this approach leads to a centerband consisting of a powder pattern of much reduced breadth and which changes sign as the spinning angle moves from one side of 54.7” to the other. However, in the limit vR < AU we have observed some unexpected results. In particular, the centerband and sidebands consist of

Practical aspects of two-dimensional transverse NOE spectroscopy

Recently, Bothner-By and co-workers proposed a new method for measuring homonuclear NOE effects under spin-locked conditions, which they named the CAMELSPIN experiment (I). We prefer to refer to this experiment as rotatingframe Overhauser enhancement spectroscopy (ROESY) since this name suggests a similarity with the closely related 2D NOESY experiment (2, 3). The ROESY method is particularly suitable for molecules that have a motional correlation time, TV, near the condition UT, = 1, where w is the angular Larmor frequency. In this case the laboratory-frame NOE effect is near zero, whereas the rotating-frame NOE (under spin-locked conditions) is always positive and monotonically increases for increasing value of T,, and can be significantly large under those conditions. The pulse scheme of the ROESY experiment is sketched in Fig. 1. At the end of the evolution period, a strong rf field is switched on. for a duration 7,. During this mixing time, T,, projections of the magnetization vectors present at the end of the evolution period, t,, onto the vector of the effective rf field remain spin locked along this effective t-f field vector. Spin exchange among spin-locked magnetization components of different nuclei can then occur, analogously to spin exchange in the regular NOE experiments, where magnetizations are aligned along the -tz axis. The spectral density functions describing spin exchange in the ROESY experiment differ from those in the NOESY experiment and cause the effect to be positive for all values of 7,. We will analyze some experimental considerations that are relevant to this new technique. In particular, we will consider the effect of limited rf field strength of the spin-locking field as well as artifacts that can occur in this type of experiment. First, as is the case for the NOESY experiment, it is very advantageous to record the ROESY spectrum in the 2D absorption mode. .4s will be shown below, there is an additional advantage in this type of experiment to the use of absorption-mode spectra: distinction between positive and negative peaks facilitates the identification of certain types of artifacts. A 2D hypercomplex Fourier transformation, as outlined by Miiller and Ernst (4) and States et al. (5) is conveniently used in this type of experiment. A simple description of this procedure is presented elsewhere (6). A minimum of four experiments with phase cycling according to Table 1 is required

Computer-Aided Permittivity Measurements of Moistened and Pyrolized Materials in Strong RF Fields (Part I)

This paper presents the results of computer-aided permittivity measurements (CAPM) on moistened paper samples, exposed to strong RF fields inside a microwave cavity. Permittivity measurements, sample weight, and RF power absorbed by the sample are monitored in real time by a microcomputer. From the CAPM, the permittivity values and the moisture content of the material are calculated. The effects of paper pyrolization on the permittivity measurements are examined, and a reversal in the measured dielectric parameters is observed at the 0-percent moisture level. This CAPM method is useful to examine the dielectric properties of materials critically dependent on the moisture content, for example, in food products and in the telephone wire industry where the insulating material is obtained from dried paper pulp. In addition, this CAPM technique can be applied to study the effects of thermal and RF shocks in dielectric materials such as used in high-power microwave windows or in studying the biological effects of microwave radiation using both short and long exposure cycles.

Aligned atoms in a strong oscillating RF field. A simple treatment by a continued fraction of 2×2 matrices

Continued fractions of matrices solve the spin J-RF-field interaction completely. However, the general method requires (2J+1)2*(2J+1)2 matrices. In this work the authors develop a treatment for the interaction of an aligned system with arbitrary spin J and an oscillating RF field requiring 2*2 matrices only. A numerical evaluation of the Bloch-Siegert shift and a comparison with recent experimental results are presented.

Computer experiments on the formation and dynamics of electric double layers (in plasma)

Electric double layers provide a mechanism for the acceleration of particles. The results of computer simulations of double layers are presented. Simulations are carried out for the case of two interpenetrating plasmas with different plasma potentials. It is found that when the two plasmas satisfy the Langmuir condition for the existence of a double layer, stable double layers eventually form. During formation the initial constant electric field evolved in a solitary electric pulse of a constant shape. This pulse makes few oscillations before it becomes stationary. Scaling laws giving the dependence of the amplitude and width of the electric pulse on the potential difference between the two plasmas are given. When the Langmuir condition is not satisfied the double layer forms very swiftly and beam-plasma interactions on the high potential side of the layer cause strong instabilities, giving rise to strong RF fields which create an additional double layer through the action of a ponderomotive force.

A Tem RF Test Cell For Consumer Products Emission and Suspectability Measurements

The transverse electromagnetic RF test cell provides a new means of measuring consumer eletronic products for both RFI susceptibility and RF radiation without interference from ambient RF energy. This test cell is an enlarged section of a 50 ohm coaxial transmission line with two spaces for samples under test. Each space has a volume of four cubic meters which is large enough for most consumer products. The sample in either space can be subjected to very strong RF field for RFI susceptibility measurements, or conversely, the RF radiation from the sample may be measured.

The resonance fluorescence of a P1state under the influence of a rotating magnetic field

The Hanle effect of the resonance line 326.1 nm (51S0-53P1) of Cd has been studied under the influence of a magnetic RF field rotating with frequency omega 0 in a plane perpendicular to the static magnetic field H. This non-resonant RF field causes a shift of the Hanle signal that is large compared with the natural linewidth for strong RF fields. The measured line shifts are in good agreement with the semiclassical theory of Dodd and Series (1961). As predicted by the theory a coherence resonance could be observed near H approximately=-H0=- omega 0/ gamma .

Production of a quiescent plasma by a rf discharge

Quiescent argon plasmas of densities of 109–1011 electrons/cm3 with a symmetric radial density profile and a Maxwellian electron velocity distribution (Te≃5 eV) have been produced by a rf discharge for neutral gas pressures of 1×10−4 to 2×10−3 mm Hg and magnetic fields in the range 0.2–0.5 kG. The main sources of density inhomogeneities and noise, which can arise from the formation of a plasma in regions of strong rf field inhomogeneities, have been eliminated by appropriate construction of the rf wave structure. The rms density fluctuation level δn/n is in the range 2×10−3 to 5×10−2, and decreases with increasing neutral gas pressure.

Erratum: Interaction between a spin-½ atom and a strong rf field

Erratum: Interaction between a spin-½ atom and a strong rf field

Fourier transform NMR in the rotating frame

A density matrix description of the Fourier transform (FT) NMR experiment on a coupled spin system interacting with a strong rf field (maintained throughout the experiment) is presented. The strong irradiation causes a mixing of the nuclear spin states giving rise to additional transitions, as well as polulation redistribution among the various energy levels. Both these features manifest themselves in the FT NMR spectrum in the form of intensity and linewidth changes that are dependent upon the details of the nuclear spin relaxation as well as the parameters of the irradiating rf field. Thus this experiment provides an interesting alternative to the study of the details of nuclear spin relaxation in coupled spin systems using the FT NMR technique. The problem is solved in a frame rotating at the frequency of irradiation. The resulting expressions indicate that the FT NMR spectrum is dependent upon the flip angle as well as the relative phase angle ψ (at the time of pulse) between the irradiating rf field and the pulse rf field. The conditions under which this experiment can be analyzed by the much simpler theory of the slow passage continuous wave (cw) Nuclear Magnetic Double Resonance (NMDR) experiment are discussed. General expressions for the FT NMR line shapes are derived for the case of a single nucleus of spin 1/2 and the theory is tested on the single spin 1/2 system in a sample of neat undegassed CHCl3. The experiment is in good agreement with theory.

Level crossing mechanism in new coherence resonance

Abstract The mechanism of level crossing in the new coherence resonance for the case that a spin- 1 2 atom interacts with an oscillating rf field in optical pumping is theoretically analyzed considering the higher-order effects of a strong rf field.

Interaction between a spin-1/2 atom and a strong rf field

Energies of the coupled system of a spin-1/2 atom in a static magnetic field and an oscillating rf field are analyzed with the resolvent formalism and numerically calculated by quantizing the rf field, for the general case that the strong rf field is oriented in an arbitrary direction to the static magnetic field. It is shown that, if the component of the static field along the direction of the rf field has particular values, level crossings take place and multiphoton transitions by even or odd numbers of rf photons become forbidden, even when the polarization of the rf field satisfies the parity condition for the multiphoton transitions. Particular attention is given to the direction and amount of the shifts of the level crossings by the rf field, together with the shifts of the anticrossings or of the multiphoton transitions. Theoretical results are verified by the optical-pumping experiments with cesium vapor.

Saturation effects in magnetic resonances for a general magnetic field configuration

Various magnetic resonances appearing in optical pumping experiments have been analysed theoretically and experimentally. However, there is little work on the saturation effects caused by strong RF fields. Recently, The saturation effects of the multiple quantum resonances and of the Haroche resonances have been investigated by Stenholm (1972) and by Tsukada and Ogawa (1973). The saturation effects are treated for the more general case of a static magnetic field in an arbitrary direction with respect to the oscillating RF field. The experimental results are compared with the exact solutions computed by the numerical integration of the Bloch equation. It is shown experimentally and theoretically that the resonances similar to the Haroche resonance appear at omega /sub ///=n omega , where omega /sub ///= gamma H/sub ///, n is an integer, H/sub /// is the parallel component of the static magnetic field with respect to the oscillating RF field, and gamma is the gyromagnetic ratio.

Sizeable pure tensor polarization of deuterons in a solid

A new effect of dynamic nuclear orientation has been observed in deuterated 1, 2-ethanediol, (CD 2OH) 2, at 0.12 K in a magnetic field of 25 kG: a strong RF field applied at a frequency slightly different from the proton Larmor frequency changes the deuteron tensor polarization. Mechanisms for this effect are proposed.