Deuteron nuclear magnetic resonance spin echoes in solids

The deuteron spin echo responses to resonant 90°-τ-β90° (XY) and 90°-τ-β0° (XX) pulse sequences in powdered molecular crystals (C6D6, (CD3)2CO and CD3CN) are reported. An echo is formed at time τ following the second pulse and the dependence of its amplitude on β and τ is The sin2 β component dominates E XY(β, τ) at short τ, but decays much faster than the sin2 β cos2 β component, so that the maximum shifts to β < 90° with increasing τ. For the 90°-τ-90°90° sequence, E XY(90°, τ) exhibits a gaussian dependence on τ over at least 50 per cent of the decays : for acetone-d 6 and benzene-d 6, the gaussian decay coefficient M 2 E corresponds, within experimental error, to ⅚M 2 vv, where M 2 vv is the van Vleck second moment for the dipolar interactions between the deuterons. For acetonitrile-d 3, M 2 E contains an additional contribution of 2M 2 DN. Theoretically, only the sin2 β component of the XY echo is predicted for isolated spin-1 particles subject to quadrupole interactions. To account for the observed β and τ dependences it is necessary to introduce dipolar interactions. By considering a model of two dipolar coupled spins-1 subject to quadrupole interactions, both the general form of the responses and the decay of the echo amplitudes with increasing τ, to second order in time, can be predicted. In the calculation the dipolar hamiltonian ℋd 0 is truncated so as to commute with the quadrupole hamiltonian ℋQ 0; it is found that the quenching of the dipolar interaction can be investigated by comparing the decay coefficients of E XY(90°, τ) and E XX(54·7°, τ) at short time. The second-order decay coefficient for E XY(90°, τ) is predicted to be M 2 vv and is close to the experimental value. Results are presented which suggest that E XY(90°, τ), measured as a function of τ, maps out a time-domain spectrum characteristic of the dipolar interactions.