INFLUENCE OF HIGH ORIENTATIONAL ORDER ON THE SHAPE OF THE ECHO RESPONSE FROM A HAHN PULSE SEQUENCE

The amplitude modulations in the simulations of the Hahn echo response s from cholestane spin labels in samples characterized by a high degre e of orientational order are shown to arise from the use of ''soft'' p ulses, Soft pulses have a limited spectral range and cover only a smal l portion of the CW-ESR spectra, so that not all the spins are on-reso nance. The magnetization vectors of the off-resonance spins only parti ally tilted away from the laboratory z axis, the direction of the appl ied static magnetic field. They thus contribute oscillating components to the magnetization in the xy plane. The contribution from the off-r esonance spins to the Hahn echo formation is significant in highly ori ented samples, but cancels out in samples exhibiting a small degree of order. Experimental echo responses obtained from CSL molecules embedd ed in rigid matrices of eggPC bilayers and the liquid crystalline mate rials ZLI and MBBA confirm the theoretical predictions. (C) 1998 Acade mic Press.