Multiple echoes, multiple quantum coherence, and the dipolar field: Demonstrating the significance of higher order terms in the equilibrium density matrix

It is well known that dipolar field effects lead to multiple spin echoes in a simple two-RF pulse experiment (the MSE experiment). We show here that c oherence transfer echoes (which identify the existence of multiple quantum coherences in liquid NMR) and multiple spin echoes have a common origin. Us ing density matrix theory we have calculated the phase and timing of multip le spin echoes from all quadrature phase combinations of RF pulses. We show for the MSE experiment that there is a one-to-one correspondence between t he time domain echo order and the multiple quantum coherence order. The exp erimental confirmation of these phase predictions shows that multiple spin echoes provide independent evidence for the breakdown of the high temperatu re approximation as proposed by Warren et al. (Science 262, 2005 (1993)). ( C) 1999 Academic Press.