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Nuclear magnetic resonance saturation and rapid passage experiments in nonmetalic solids Janzen, Wayne Roger

Abstract

Nuclear magnetic resonance lock-in absorption mode and dispersion mode spectra of polycrystalline samples of CaF₂ potassium caproate (KC₆), and lithium stearate (LiC₁₈) have been obtained at various levels of saturation. The line widths narrow and the line shapes change in both the absorption and dispersion mode spectra on saturation. This behaviour is not predicted by previous theories of saturation, but is predicted by the new magnetic resonance saturation theory of Provotorov. The effects of modulation saturation have also been demonstrated. They are in agreement with Goldman's extension of Provotorov theory to include the audio modulation field. An Important prediction of Provotorov-Goldman theory is that saturation narrowing and modulation saturation do not affect the signal at the centre of resonance (within certain limiting conditions) and so the signals at this point are expected to saturate with the normal saturation factor: Z(O) = [ 1 + ɤ²H₁²T₁f(O)/2]⁻¹, where H₁ is the rf field amplitude, f(Δ) is the absorption line shape function normalized to 2π, and Δ is in rad/sec. Therefore the progressive saturation of the lock-in dispersion signal, u₁(0), has been studied in the CaF₂ KC₆, and LiC₁₈ samples at room temperature. The results verify the above prediction and yield the spin lattice relaxation times (T₁) of the samples. The CaF₂ result of 0.385 ±0.03 sec compares well with 0.45 ± 0.05 sec, the value found by adiabatic rapid passage. A modified Linder signal decay technique has also been used to measure T₁ values in KC₆ and LiC₁₈. The innovation being that the signal u₁(0) was used instead of the lock-in maximum absorption signal. The results are in good agreement with the progressive saturation results. It is concluded that one is finally in a position to measure correct T₁ values in solids by CW techniques. A technique for recording the true shapes of rapid passage signals has been developed. Using the shape of the rapid passage signal as a criterion of whether or not the passage was also adiabatic, it was found that the Bloch adiabatic condition, dH₀/dt « ɤH₁² , is also applicable to solids. The inequality, however, must be larger for solids than for liquids. The width at half its peak height of an adiabatic rapid passage (Arp) signal in a solid was shown to be [12(H₁²+HL²)]⅛, where HL² = /3, is called the local magnetic field, and is the Van Vleck second moment. ARP signals were used to find local field and second moment values in polycrystalline and single crystal forms of CaF₂ and also in polycrystalline LiC₁₈, all at room temperature. The results are in excellent agreement with theory and CW measurements. It is believed that this is the first time this method has been used. The ARP technique was also used to measure T₁ values. A symmetric sweep method was used for the above samples and a two pass method (equivalent to the π- π/2 sequence used in pulse spectrometry) was used for a very pure crystal of maleic anhydride. A value of 76 min was found for this sample at room temperature. This is a particularly good example of the usefulness of the ARP technique since it is difficult to measure such a long T₁ by the usual pulse method. Normal and saturation narrowed lock-in absorption spectra of LiC₁₈ have been obtained over the temperature range 25° to 193°C. There are two phase transitions in this region. They were revealed by both the normal and saturation narrowed spectra.

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