Influence of High Orientational Order on the Shape of the Echo Response from a Hahn Pulse Sequence

Author： van der Struijf C. Nienhuys H.K. Harryvan D. Kothe G. Levine Y.K.

ISSN： 1090-7807

Source： Journal of Magnetic Resonance, Vol.130, Iss.2, 1998-02, pp. : 253-261

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Abstract

The amplitude modulations in the simulations of the Hahn echo responses from cholestane spin labels in samples characterized by a high degree of orientational order are shown to arise from the use of “soft” pulses. Soft pulses have a limited spectral range and cover only a small portion of the CW-ESR spectra, so that not all the spins are on-resonance. The magnetization vectors of the off-resonance spins only partially tilted away from the laboratoryzaxis, the direction of the applied static magnetic field. They thus contribute oscillating components to the magnetization in thexyplane. The contribution from the off-resonance spins to the Hahn echo formation is significant in highly oriented samples, but cancels out in samples exhibiting a small degree of order. Experimental echo responses obtained from CSL molecules embedded in rigid matrices of eggPC bilayers and the liquid crystalline materials ZLI and MBBA confirm the theoretical predictions.