engleski

Non-linearity, frequency shifts and other surprises with nuclear spin noise

We have investigated spin noise spectra of liquids and solids under a variety of conditions to put some theoretical predictions to experimental test. It was found that the observed line shapes of the proton nuclear spin noise spectra depend in complex ways on the properties of the resonance circuit, the spin density, transverse relaxation, inhomogeneous broadening, radiation damping and temperature. Although spin noise spectra by their nature are power or magnitude spectra, they generally exhibit dispersion-like line shapes. According to theory spin noise signals should appear as negative deviations from the otherwise flat positive (thermal) noise power baseline, if the tuning frequency of the circuit is equal or close to the Larmor frequency. However, we find this predicted symmetrical "dip" line shape at a considerable tuning offset, the "spin noise tuning optimum" (SNTO). At the conventional tuning optimum, which can be several kilohertz away from the SNTO, a dispersion like line shape is usually observed. Systematic investigations revealed that the peak areas are not proportional to the number of nuclei. For instance broad OH proton signals appear much larger than narrow signals of mathyl group. We have found semi-quantitative explanations of these experimental observations, which also include frequency shifts at high spin densities and dependence of line shapes on sample and coil temperatures.There is a complex interplay of transverse relaxation, radiation damping, and inhomogeneous broadening which can be tracked by numerical simulations.

spin noise spectra; NMR spectroscopy

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