TIME-REVERSAL OF CROSS-POLARIZATION IN NUCLEAR-MAGNETIC-RESONANCE

It is demonstrated that the time evolution of the heteronuclear polari zation-transfer process in a dipolar-coupled nuclear spin system can b e reversed, leading to the observation of cross polarization echoes. T he cross-polarization echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in sign, Cross-polarization echoes have been recorded for a powder sa mple of alanine. The time evolution of the spin;system is consistent w ith both unitary quantum dynamics and with spin thermodynamics of two systems approaching a common spin temperature. (C) 1998 American Insti tute of Physics.