Observation of Bulk Susceptibility Effects in High-Resolution Nuclear Magnetic Resonance

The phenomenon of multiple echoes following a two-pulse stimulated-echo experiment in highly polarized systems is reinvestigated and explained using the Bloch equations. A semiquantitative formalism based on the nuclear contributions to the magnetic susceptibility throughout a multiple-pulse sequence is outlined and used to understand the formation of the echoes. This theory is used to direct the design of phase-cycled and pulsed-field-gradient experiments to control the echo formation. The theory also suggests a means of indirectly detecting a heteronucleus contained in a different molecule. This kind of detection is accomplished by modifying the static magnetic field with a stimulated-echo experiment on one set of the spins. Heteronuclear experiments show that this classical semiquantitative formalism is a useful tool in the understanding of spin dynamics in highly polarized systems.