COUNTING SPINS WITH A NEW SPIN-ECHO DOUBLE-RESONANCE

In traditional spin echo double resonance (SEDOR), the echo amplitude M is decreased when the observed spins S are flipped by pi together wi th the pi refocusing pulse on the observed spins I; the dependence on tau is then determined. In the new version of SEDOR, the echo amplitud e is measured as a function of the S spin flip angle theta at a consta nt pulse spacing tau. The analysis is simple and powerful for long tau , where the strong collision limit applies. There, the variation of M with theta can be fit, yielding the number n of spins S to which each spin I is coupled. Data from amorphous silicon with H-1 and D-2 show t he described effect. A MAS version of the new method is used on multip ly labeled alanine and urea, with results in good agreement with the p redictions for n = 2, as expected. By Fourier transforming M with resp ect to the flip angle theta, a stick spectrum results; the largest num bered non-vanishing stick yields the number n of spins S coupled to ea ch spin I. Simulations are presented for an n = 2 system. The present technique is compared to the multiple-quantum spin-counting method, (C ) 1998 Academic Press.