Double step nuclear polarization decay under level anticrossing conditions.

Proton spin polarization decay has been measured in paranitrotoluene crystalline powder after a nonadiabatic magnetic field switch from a high polarization field into a low field. At certain magnetic fields, when 14N-1H level anticrossing conditions are satisfied mediated by dipole-dipole interactions, a double step proton polarization decay has been observed. A first step taking place typically in the millisecond range due to polarization transfer is followed by relaxation in the range of tens of seconds. The experiment has been carried out using a homebuilt field cycling relaxometer allowing for sufficiently fast field switching. The observed two step decay is analyzed analytically in terms of the Liouville-von Neumann equation.