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Abstract

The electronic sublevels of optically excited triplet states in molecular crystals are very selectively populated due to the selection rules of spin-orbit coupling. It was shown previously that this population difference can be transferred to the nuclear spin system by inducing forbidden ESR transitions with the selection rules Δms=±1, ΔmI=±1 by using simultaneous cw irradiation with light and microwaves, a technique which was termed “microwave-induced optical nuclear polarization” (MI ONP). However, it was predicted and confirmed experimentally that with a time-resolved MI ONP where the irradiation is performed with short laser and microwave pulses an even higher polarization can be achieved. In this paper we discuss the parameters which determine the polarization obtained with this technique. The maximum nuclear spin polarization measured was p=0.33 or 33% in excellent agreement with the theoretical prediction.