A. Bielecki

Zero field NMR and NQR

Methods are described and demonstrated for detecting the coherent evolution of nuclear spin observables in zero magnetic field with the full sensitivity of high field NMR. The principle motivation is to provide a means of obtaining solid state spectra of the magnetic dipole and electric quadrupole interactions of disordered systems without the line broadening associated with random orientation with respect to the applied magnetic field. Comparison is made to previous frequency domain and high field methods. A general density operator formalism is given for the experiments where the evolution period is initiated by a sudden switching to zero field and is terminated by a sudden restoration of the field. Analytical expressions for the signals are given for a variety of simple dipolar and quadrupolar systems and numerical simulations are reported for up to six coupled spin-1/2 nuclei. Experimental results are reported or reviewed for 1H, 2D, 7Li, 13C, and 27Al nuclei in a variety of polycrystalline materials. The effects of molecular motion and bodily sample rotation are described. Various extensions of the method are discussed, including demagnetized initial conditions and correlation by two-dimensional Fourier transformation of zero field spectra with themselves or with high field spectra.

Zero‐field NMR and NQR spectrometer

In comparison to high‐field NMR, zero‐field techniques offer advantages in terms of spectral interpretability in studies of polycrystalline or amorphous solids. This article describes a technique and apparatus for time‐domain measurements of nuclear magnetism in the absence of applied fields (Fourier transform zero‐field NMR and NQR). Magnetic field cycling and high field detection are employed to enhance sensitivity. The field cycling is accomplished with an air‐driven shuttle system which moves the sample between regions of high and low magnetic field, in combination with switchable electromagnets in the low‐field region. Sudden field steps or pulses are used to initiate coherent nuclear spin evolution in zero field and to monitor such evolution as a function of time. Experimental results are shown and analyzed. Possible variations on the basic method are described and their relative advantages are discussed.

Zero field NMR and NQR with selective pulses and indirect detection

Zero field NMR and NQR spectra are obtained by the application of dc magnetic field pulses to a demagnetized sample. Pulsed dc fields allow for selective excitation of isotopic species and provide a means for coherent manipulation of the spin system in zero field. Using these selective pulses and level crossing techniques, indirect detection of a quadrupolar nucleus may be accomplished via protons without obtaining the proton background signal in the NQR spectrum. Experimental results from a variety of 1H, 2H, and 14N homo‐ and heteronuclear systems are presented as an illustration of these techniques.

Nuclear magnetic resonance with DC SQUID preamplifiers

Describes five experiments illustrating the application of DC SQUID (superconducting quantum interference device) amplifiers to magnetic resonance experiments. The first experiment involved the observation of nuclear spin noise, that is the spontaneous emission of photons from an ensemble of /sup 35/Cl nuclei in the zero polarization state. The second experiment involved the use of the Q-spoiler in conventional NQR (nuclear quadrupole resonance) and NMR measurements in which one applies a large RF pulse to the nuclei to make them precess. The Q-spoiler was then used in an experiment to detect the oscillating electric polarization induced by /sup 35/Cl nuclear quadrupole moments. The fourth experiment involved the extension of the use of the Q-spoiler and SQUID amplifier to NMR, detecting the signal from /sup 119/Sn nuclei at 30 MHz. Finally, a SQUID amplifier was used with an untuned input circuit to detect the low-frequency NMR signal at 55 kHz from /sup 195/Pt nuclei in an applied field of 60 Gauss. >

Heteronuclear zero-field NMR

Abstract The NMR spectra of polycrystalline solids are often best resolved in the absence of applied magnetic fields. Additionally, heteronuclear spin systems in zero field display features not observed in homonuclear systems. In this letter, spectra are presented and analyzed for the cases of heteronuclear spin pairs in 13 C-enriched β-calcium formate (solid) and in diethyl phosphite (liquid).

Method for increasing resolution in two-dimensional solid-state nmr heteronuclear correlation spectra

A new RF pulse sequence is applied to selected nuclei in the evolution period of a 2D NMR heteronuclear correlation experiment to more effectively suppress heteronuclear interactions. The new pulse sequence is designed to be effective with the existing BLEW-12 pulse sequence so that both homonuclear and heteronuclear interactions are suppressed. In addition, the new pulse sequence effectively suppresses homonuclear interactions so that it can be used with a variety of nuclear species.

Time Domain Zero Field NMR and NQR

Field cycling methods are described for the time domain measurement of nuclear quadrupolar and dipolar spectra in zero applied field. Since these techniques do not involve irradiation in zero field, they offer significant advantages in terms of resolution, sensitivity at low frequency, and the accessible range of spin lattice relaxation times. Sample data are shown which illustrate the high sensitivity and resolution attainable. Comparison is made to other field cycling methods, and an outline of basic instrumental requirements is given.