NANOSECOND TIME RESOLUTION OF ELECTRON-NUCLEAR CROSS POLARIZATION WITHIN THE OPTICAL NUCLEAR-POLARIZATION (ONP) PROCESS

The spin dynamics of the generation of high nuclear spin polarization as a result of optical excitation (ONP) is studied with time resolution extending into the nanosecond range with the help of synchronized light and RF pulses. The well-characterized system - acridine doped into a crystalline fluorene matrix - has been used for the present dynamics study. With a short laser pulse, a selective sublevel population of the lowest acridine triplet state is generated. Resonant RF pulses of variable length initiate transfer of the electronic to nuclear spin polarization; the ONP-amplitude as a function of the RF pulse-length shows oscillating behaviour closely related to nutations of the electronic spins due to the resonant RF field. The ONP amplitude at the oscillation maximum is more than twice as high as the ONP level obtainable by CW RF-irradiation, which reflects an improved efficiency of the polarization transfer process. In contrast to the wide range of ONP experiments employing electron spins of stable paramagnetic systems the pulsed version of the ONP experiment permits direct time resolution of the electron nuclear cross-polarization process. In order to explain the results a two-step process is proposed and tested against the experimental data. In the primary step, the RF changes the spin order in the highly polarized electronic non-Zeeman reservoir in a similar way to hole-burning in a wide EPR line. This leads to a state which is far off equilibrium. In a second step, on a slower time-scale, the non-Zeeman reservoir re-equilibrates, mediated by electron nuclear spin-coupling causing the nuclear spins to be polarized. The process is compared with the thermal mixing concept as developed for dynamic nuclear polarization in solids with inhomogeneously broadened EPR lines.