Transient nutations of an electron spin 1/2 with dipolar coupling to   neighbouring nuclear spins
  A model cluster, with a resonant electron spin surrounded with identical nuclear spins occupying the sites of a cubic lattice, and with dipolar coupling between the spins, is used for a theoretical investigation of the transient nutations induced by a strong oscillating field but disturbed by these local dipolar fields. An effective Hamiltonian valid at the time scale of the nutations is first established. An analytical expression for the Dipolar Coherence time tDC is derived by adapting Van Vleck's perturbative approach to the present situation, with weak rather than intermediate dipolar coupling. It is found that the dipolar coupling does not affect tDC at exact ESR resonance (Delta=0), a result reminiscent of a behaviour found in transient NMR in liquids, in an inhomogeneous static field. The expression for tDC also presents a qualitative similarity with a time introduced in that liquid-state NMR context. Introducing a collection of independent clusters, a bridge is built between tDC and the half-width delta of the usual steady state unsaturated ESR line. Assuming a simple cubic lattice, numerical values for (tDC)Delta=delta are given, for both undiluted and dilute samples. It is shown that the TN do not obey the Bloch equations. Precautions to be taken when trying a qualitative comparison between the present results and existing experimental data, and some thermodynamic aspects, are discussed. 
Alain Deville, Anne-Marie Dar\'e, Julien Azema
quant-ph