A mathematical model for time-resolved radiofrequency-induced optical nuclear polarization

Abstract

A mathematical model for a novel type of optical nuclear polarization (ONP) experiment is presented. Our calculations show that a polarized excited molecular triplet state, prepared by pulsed laser excitation, and immediately subjected to a radiofrequency (rf) pulse of variable duration τ and amplitude B 1, can create a ground state proton spin polarization, or ON P, which as a function of τ has the appearance of a damped oscillation with beat frequency, ω r, equal to √2 g eβ e B 1/ h. The effects of triplet decay, dephasing, and spin-lattice relaxation rates are examined. It was found that: (1) with certain limitations, the amplitude of the ONP increases with decreasing triplet lifetime; (2) triplet dephasing damps out oscillations without markedly diminishing the ONP amplitude; and (3) spin-lattice relaxation diminishes only the overall amplitude of the ONP. We will examine two mechanisms for the creation of rf-induced ONP, one carried out near and one removed from the level crossing point of the triplet sublevels. As with all ONP phenomena hf coupling is essential for the creation of ONP. Additionally, for the rf-induced ONP mechanisms investigated, it is found that allowed, not forbidden, rf transitions determine the shape of the ONP curves. Experimental evidence for rf ONP beats, with frequency ω r, has been mentioned in the literature.