L. Van Gerven

NMR relaxation in adsorbed water

Abstract The NMR relaxation behavior of the proton magnetization in adsorbed water is investigated. Longitudinal as well as transverse relaxation of the adsorbed water as a whole shows a distinct nonexponentiality and the nature of this nonexponentiality is studied. Different methods of fitting distribution of relaxation times are examined and various simulated and experimental decay curves are submitted to these fitting methods. The results of these fittings clearly indicate that both the longitudinal and the transverse relaxation of the proton magnetization in adsorbed water is related to two or three discrete components and that there is certainly no broad distribution of relaxation times present in these water-adsorbent systems.

QUANTUM AND CLASSICAL DYNAMICS OF A METHYL GROUP WITH TUNNELING FREQUENCY 3.4 MHZ STUDIED BY LOW FIELD NMR

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5‐dimethyl‐1,3,4‐thiadiazole are reported. The tunneling frequency (νt = 3.39 MHz) was obtained using low field NMR spectroscopy. By means of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spectral lines in the low field NMR spectra, can be quantified. The results obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with νt = 3.39 MHz. Measurements of the temperature dependence of νt, the spin conversion time τcon and an analysis of the proton spin lattice relaxation time T1, the latter two revealing the correlation time τc, enabled the study of the methyl group dynamics over a temperature range encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering barrier of height V3/kB = 1175 K and are compared with ex...

Methyl Zeeman-tunnel resonance and nuclear spin relaxation in copper acetate

The methyl tunnel spectrum of copper acetate is studied at low temperatures by means of nuclear magnetic resonance level-crossing spectroscopy employing a field-cycling technique. The recovery of the proton magnetisation following saturation is measured as a function of magnetic field. Changing the magnetic field causes energy levels to cross when the methyl tunnel frequency is equal to one or two times the proton Larmor frequency, and abrupt changes in the recovered magnetisation, showing up as peaks or dips in a smooth background, reveal the positions of the level crossings. A variety of possible field cycles and sequences of cycles leads to a series of different experimental procedures which highlight different aspects of the thermodynamic properties of the system. In particular the dependence of the spectra on scanning schemes and on the thermodynamic preparation conditions is investigated. The experiments are understood in a semi-quantitative way in terms of a simple mathematical model for the thermal equilibration of three energy reservoirs, of which two are resonantly coupled at the level crossings. This simulation shows that, while the dominant processes have been clearly identified and incorporated in the model, some important factors remain to be explained. The methyl tunnel frequency in copper acetate is found to be (64+or-5) MHz.

Measurements on the electrical resistivity of thin nickel films at very low temperatures

Synopsis This paper deals with the electrical and magnetic properties of very thin nickel films at liquid helium and liquid hydrogen temperatures. It was found that the electrical resistance of such nickel films is a very sensitive function of temperature and measuring current, thus confirming the Leiden measurements on iron films. An external magnetic field did not change the current-dependence of the resistance. The existence of hysteresis was demonstrated by means of resistance measurements, and the coercive force determined. This was found to be much greater than for the bulk metal, and also than for the Leiden iron films.

Proton nmr-study of molecular-motion in solid sih4

Abstract Proton Zeeman- and dipolar spin—lattice relaxation times and second moments have been measured in polycrystalline SiH 4 at 24 MHz, between 90 and 3 K. Down to 25 K the classical model for molecular applies; below 2S K, quantum effects have to be taken into account.

A cryogenic RE oscillator, the heart of a new NMR dispersion spectrometer

Describes a new CW-NMR dispersion spectrometer based on the detection of the frequency shift of a cryogenic tunnel diode oscillator, in which the inductance of the coil is influenced by the magnetic resonance phenomenon. Besides the design, the details of the construction and the operation of the oscillator, an outline of the performances and the major advantages and disadvantages of this spectrometer are given. The performances are compared with those of conventional NMR absorption (and dispersion) spectrometers.

The spin magnetism of α, α′‐diphenyl‐β‐picrylhydrazyl (DPPH)

Magnetic susceptibility measurements are performed on DPPH–benzene complex and solvent‐free DPPH in the temperature range 300–2.2 K and in a field range 6–20 kOe. The experimental values of the DPPH–benzene complex fit the Brillouin function for noninteracting paramagnetic centers (S=1/2); at temperatures below 4 K; however, a systematic deviation is observed. The values in solvent‐free DPPH obey the same law at temperatures above 150 K. Below this temperature a strong deviation occurs, and below 10 K the susceptibility even decreases with decreasing temperature. This deviation is attributed to electron spin pairing. In solvent‐free DPPH less than 1 % of paramagnetic centra is left at 2.2 K.

Dipolar polarization in a single crystal of lithium acetate measured by NMR dispersion spectroscopy

Abstract Using a new type of NMR dispersion spectrometer we have made an extensive study of the tunnelling induced nuclear dipolar polarization effect or Haupt effect in lithium acetate single crystals. Special attention is paid to the temperature dependence and angular dependence of the Haupt effect.

A NMR study of the spin conversion in solid methane

Abstract The proton magnetization of solid methane is studied during the conversion of the spin modifications, as a function of temperature (4.2 to 2.0 K), oxygen concentration (12 ppm to 4.1 %) and frequency (25.6 and 8.95 MHz). Kinetics of the conversion is discussed and some comparison with the ortho-para conversion in hydrogen is made.

Proton spin lattice relaxation time in solid methane during spin conversion at 4.2 K

Abstract The proton spin-lattice relaxation time of solid methane has been measured during the conversion of the spin modifications at 4.2 K. Conversion is catalyzed by oxygen impurities. The peculiar time dependence of T 1 is examined.