Mapping the B1 field distribution with nonideal gradients in a high-resolution NMR spectrometer

To understand the behavior of many NMR experiments, it is important to determine the spatial distribution of the B1 field. In this paper, we show how this distribution can be mapped independently of spin density, coil responsiveness, and nonlinearities of the B0 field gradients. As a by-product we obtain a map of the (possibly nonlinear) spatial variation of the B0 field gradients used in the imaging procedure.     

Signal selection in high-resolution NMR by pulsed field gradients

We describe a new and powerful computer program called TRIPLE GRADIENT which calculates optimized pulsed field gradient sequences for specific coherence pathway selection or rejection. Sequences can be computed for gradient coils acting along one, two, or three perpendicular axes. The program is based on the computational minimization of a penalty function formed from the summed amplitudes of the unwanted signals. The underlying mathematical analysis makes use of a vectorial representation of the way in which a gradient sequence suppresses different signals. It is argued that experiments using well-calculated gradient sequences are quicker and generally perform better than those using extensive phase cycling, especially when suppressing extremely strong solvent signals, and it is shown that in many cases gradient experiments of optimal signal-to-noise ratio can be performed. These claims are illustrated by spectra obtained from an HQQC experiment.     

Sample restriction using magnetic field gradients in high-resolution solid-state NMR

Many solid-state NMR experiments are sensitive to inhomogeneity in the radiofrequency field. We propose a method to restrict the sample volume, in magic angle spinning experiments, using a static magnetic field gradient and a selective pulse. The position of the gradient is calculated for our experimental configuration and we have simulated the effects of selective pulses to determine the excited volume. The resulting sequences are applied to a sample of sodium acetate using frequency-switched Lee–Goldburg proton–proton homonuclear dipolar decoupling. A gain of a factor of 2 on the carbon resolution is experimentally observed.     

Pulsed field gradients in simulations of one- and two-dimensional NMR spectra

A method for the inclusion of the effects of z-axis pulsed field gradients in computer simulations of an arbitrary pulsed NMR experiment with spin (1/2) nuclei is described. Recognizing that the phase acquired by a coherence following the application of a z-axis pulsed field gradient bears a fixed relation to its order and the spatial position of the spins in the sample tube, the sample is regarded as a collection of volume elements, each phase-encoded by a characteristic, spatially dependent precession frequency. The evolution of the sample's density matrix is thus obtained by computing the evolution of the density matrix for each volume element. Following the last gradient pulse, these density matrices are combined to form a composite density matrix which evolves through the rest of the experiment to yield the observable signal. This approach is implemented in a program which includes capabilities for rigorous inclusion of spin relaxation by dipole-dipole, chemical shift anisotropy, and random field mechanisms, plus the effects of arbitrary RF fields. Mathematical procedures for accelerating these calculations are described. The approach is illustrated by simulations of representative one- and two-dimensional NMR experiments.     

Artifacts in T1 rho-weighted imaging: compensation for B(1) and B(0) field imperfections

The origin of spin locking image artifacts in the presence of B(0) and B(1) magnetic field imperfections is shown theoretically using the Bloch equations and experimentally at low (omega(1) < Delta omega(0)), intermediate (omega(1) approximately Delta omega(0)) and high (omega(1) > Delta omega(0)) spin locking field strengths. At low spin locking fields, the magnetization is shown to oscillate about an effective field in the rotating frame causing signature banding artifacts in the image. At high spin lock fields, the effect of the resonance offset Deltao mega(0) is quenched, but imperfections in the flip angle cause oscillations about the omega(1) field. A new pulse sequence is presented that consists of an integrated spin echo and spin lock experiment followed by magnetization storage along the -z-axis. It is shown that this sequence almost entirely eliminates banding artifacts from both types of field inhomogeneities at all spin locking field strengths. The sequence was used to obtain artifact free images of agarose in inhomogeneous B(0) and B(1) fields, off-resonance spins in fat and in vivo human brain images at 3 T. The new pulse sequence can be used to probe very low frequency (0-400 Hz) dynamic and static interactions in tissues without contaminating B(0) and B(1) field artifacts.     

Two-dimensional NMR spectroscopy in Earth's magnetic field

We demonstrate the first two-dimensional correlation NMR (COSY) spectra obtained at ultra low frequencies (ULF) using the Earth's magnetic field. Using a specially developed spectrometer with multiple audio-frequency pulses under controlled pulse phase, we observe magnetisation transfer arising from heteronuclear J-couplings in trifluoroethanol and para-difluorobenzene. The 2D COSY spectra exhibit all diagonal and off-diagonal multiplets consistent with known J-couplings in these molecules.     

Development of NMR instrumentation to achieve excitation of large bandwidths in high-resolution spectra at high field

A prototype 2.5-mm (1)H high-resolution probe for an 18.8-T (800 MHz) nuclear magnetic resonance spectrometer has been designed, together with a dedicated amplifier capable of delivering up to 1 kW of power. This probe permits a 90 degrees pulse length of 2 mus to be achieved at 300 W, corresponding to an excitation bandwidth of +/-125 kHz. Probe performances were tested on samples commonly used for this purpose as well as on protein and paramagnetic model compound samples. It is shown that this probe is useful for a wide range of applications at high magnetic field, especially in the study of systems characterized by very broad and far-shifted resonances and in experiments that require high-power radiofrequency irradiation.     

Momentum resolved bremsstrahlung isochromat spectra from Ni(0 0 1)

Momentum resolved inverse photoemission spectra from Ni(0 0 1) are presented. The experimental dispersion of direct transitions are in good agreement with predictions from a recent band structure calculation and with results from spectra calculated on the basis of a one step theory. Comparison of the present data with earlier normal incidence spectra collected with a different experimental setup reveals a very strong polarization effect.     

PFG NMR and internal magnetic field gradients in plant-based materials

In this contribution, it is demonstrated that inner magnetic field gradients can seriously affect the results of stimulated echo PFG NMR experiments on plant-based materials even if there is no notable content of paramagnetic substances. Such effects could be observed both in experiments on water in pharmaceutical grade cellulose powder materials and on eggplant fruit tissue. In both cases, it was observed that the effects of internal magnetic field gradients led to different relative values of the diffusion coefficient compared to values obtained with a gradient-compensating pulse sequence.     

NMR spectroscopy in inhomogeneous B0 and B1 fields with non-linear correlation

Resolved NMR spectra from samples in inhomogeneous B0 and B1 fields can be obtained with the so-called "ex situ" methodology, employing a train of composite or adiabatic z-rotation RF pulses to periodically refocus the inhomogeneous broadening during the detection of the time-domain signal. Earlier schemes relied on a linear correlation between the inhomogeneous B0 and B1 fields. Here the pulse length, bandwidth, and amplitude of the adiabatic pulses of the hyperbolic secant type are adjusted to improve the refocusing for a setup with non-linear correlation. The field correlation is measured using a two-dimensional nutation experiment augmented with a third dimension with varying RF carrier frequency accounting for off-resonance effects. The pulse optimization is performed with a computer algorithm using the experimentally determined field correlation and a standard adiabatic z-rotation pulse as a starting point for the iterative optimization procedure. The shape of the z-rotation RF pulse is manipulated to provide refocusing for the conditions given by the sample-, magnet-, and RF-coil geometry.     

Contrasting ion neutralisation spectra of Ni(1 1 1) and W(1 1 0) surfaces

We present the results of an appropriate calculation of the ion neutralisation spectrum of W(1 1 0) exposed to a beam of very low energy He⁺ ions. The calculated spectrum shows considerable structure in contrast to the spectrum of Ni(1 1 1). It appears, therefore, that experimental data on W(1 1 0), presently non available, would be most useful in understanding the ion neutralisation process.     

Methanol adsorption on Pd(1 1 0) and Ag/Pd(1 1 0) studied by high-resolution photoelectron spectroscopy

Adsorption of methanol on clean Pd(1 1 0) and on an alloyed Ag/Pd(1 1 0) surface has been studied by high-resolution photoelectron spectroscopy. On Pd(1 1 0) two different chemisorbed methanol species were observed for temperatures up to 200 K, with the one at lower binding energy remaining at low coverage. These species were attributed to methanol adsorbed in two different adsorption sites on the Pd(1 1 0) surface. As is well established for this system, heating to 250 K resulted in decomposition of methanol into CO. The adsorption and decomposition behaviour of methanol on the Ag/Pd(1 1 0) surface alloy formed by depositing Ag on Pd(1 1 0) at elevated temperature was similar to that of the pure Pd(1 1 0) surface. This suggests that the amount of Ag present in the Pd(1 1 0) surface in this study does not affect the decomposition behaviour of methanol as compared to pure Pd(1 1 0). Complementary density functional theory calculations also show little influence of Ag on the binding of methanol to Pd. These calculations predict an on-top adsorption site for low methanol coverages.     

Comparative study of (1 0 0) and (1 1 1)B InGaAs single quantum well laser diodes

In this work, a series of identical In_xGa_1−xAs/Al_yGa_1−yAs single quantum well laser diodes, grown on (1 0 0) and (1 1 1)B GaAs substrates, have been thoroughly studied. For all samples, clear evidence of reduced threshold current densities in the (1 1 1)B substrate has been observed in electroluminescence spectra at 17 and 300 K. Modelling of the devices, based on a self-consistent solution of Schrödinger–Poisson's equations, was utilised in order to reproduce the experimental results. The model incorporates strain and piezoelectric effects on the quantum well states, free carrier screening, overlap integral computation, and optical gain calculation. The underlying mechanism, that explains the threshold reduction observed in the (1 1 1)B laser diodes, is discussed based on the results of the modelling.     

Observation of B(0) --> D0pi(0) and B(0) -->D(*0)pi(0)

We have studied the color-suppressed hadronic decays of neutral B mesons into the final states D*0pi(0). Using 9.67 x 10(6) BB pairs collected with the CLEO detector, we observe the decays B( 0) --> D0pi(0) and B( 0) -->D(*0)pi(0) with the branching fractions BB( 0) -->D0pi(0)) = (2.74(+0.36)(-0.32) +/- 0.55)x10(-4) and BB( 0) -->D(*0)pi(0)) = (2.20(+0.59)(-0.52) +/- 0.79)x10(-4). The first error is statistical and the second systematic. The statistical significance of the D0pi(0) signal is 12.1sigma ( 5.9sigma for D(*0)pi(0)). Utilizing the B( 0) -->D*0)pi(0) branching fractions we determine the strong phases delta(I,D(*)) between isospin 1/2 and 3/2 amplitudes in the Dpi and D*pi final states to be cosdelta(I,D) = 0.89 +/-0.08 and cosdelta(I,D*) = 0.89 +/- 0.08, respectively.     

The infrared and microwave spectra of ozone for the (0, 0, 0), (1, 0, 0), and (0, 0, 1) states

New measurements of the ozone spectrum in the microwave and 10-μm infrared regions have been made. These new lines have been fit to a three-state Hamiltonian model which includes Coriolis interactions beteen (0, 0, 1) and (1, 0, 0). The model requires additional Coriolis parameters, as well as an appreciation for the parameters which are indeterminate. Transition dipole moments have been derived from the intensities of selected infrared lines.     

Magnet design with high B(0) homogeneity for fast-field-cycling NMR applications

The design, construction, and performance of a low-inductance solenoidal coil with high B(0) homogeneity for fast-field-cycling NMR is presented. It consists of six concentric layers. The conductor width is varied to minimize the B(0) inhomogeneity in the volume of the sample. This is done using an algorithm which takes the real shape of the conductor directly into account. The calculated coil geometry can be manufactured easily using standard computerized numeric control equipment, which keeps the costs low. The coil is liquid cooled and produces a B(0) field of 0.95 T at 800 A. The field inhomogeneity in a cylindrical volume (diameter 5 mm, length 10 mm) is about 10 ppm, and the inductance is 190 microH. Switching times below 200 micros can be achieved. During 6 months of operation the coil has shown good stability and reliability.