A SLOW-TURNING METHOD FOR MEASURING LARGE ANISOTROPIC INTERACTIONS ININHOMOGENEOUSLY BROADENED NUCLEAR-MAGNETIC-RESONANCE SPECTRA

A new NMR method is proposed for measuring anisotropic spin-Hamiltonia n parameters, STEAMER (slow turning echo amplitude modulation and echo reduction). It involves slow turning of a powdered sample about an ax is perpendicular to the external magnetic field. We have theoretically analyzed and experimentally demonstrated this technique for the speci fic case of an axially symmetric second-rank tensor interaction. The m ethod relies upon a slow continuous rotation of the sample that change s the orientation of the principal-axis systems of interaction tensors with respect to the external magnetic field and renders the spin Hami ltonian time dependent. As a consequence a conventional Hahn spin-echo pulse sequence yields imperfect refocusing and altered echo amplitude s. Two principal advantages of STEAMER are that only a small portion o f the entire powder pattern need be observed at a single frequency and that anisotropic interactions can be distinguished from distributions of isotropic interactions. The N-14 (I=1) selectively excited pi/2-pi spin-echo NMR signal in KNO3 provides a good test case for the effect s of slow turning (rotation at 1 rpm) since it has a first-order quadr upole interaction from a N-14 nuclear quadrupole coupling constant (NQ CC) of 0.75 MHz and a small asymmetry parameter (0.022), Theoretical s imulations of the echo amplitude as a function of the pulse interval f or an axially symmetric second-rank tensor agree well with the experim ental results at a chosen frequency, These numerical simulations are c ompared to an analytical approximation derived in terms of the zeroth- order Bessel function. In addition, a simple graphical method for obta ining NQCC values from STEAMER data is developed. Other aspects and po ssible extensions of the STEAMER experiment are discussed.