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Abstract

The protons in a single crystal of p-dibromobenzene doped with p-dichlorobenzene placed in an external magnetic field are polarized by simultaneous irradiation with light and microwaves. The selection rules of spin-orbit interaction cause an alignment in the triplet electron spin system of p-DCB. This alignment is transferred to the proton spin system by inducing forbidden transitions Δms = ± 1, ΔmI = ± 1 analogous to the well-known solid-state effect. The sign and the amount of polarization, of about a factor of 10 as compared to the Boltzmann equilibrium, is shown to be in agreement with the expectation for our system. Using proton and chlorine hfs data, the field range with observable proton polarization is calculated and found to be in agreement with the experimental result. Conditions which favor higher nuclear polarizations are discussed.