# Was NMR/ESR spin flip-flop line ever measured?

Let $$a$$,$$b$$ be two unlike spins in a uniform magnetic field and RF excitation, and let $$B_0$$ be the magnetic field for which some transition energies of the spins coincide $$E_a^{i,j}(B_0)=E_b^{k,l}(B_0)$$. This is the classic condition for spin flip-flop due to dipolar interaction. Now we move away to $$B=B_0+\delta$$, and for the flip-flop to occur, an RF excitation of frequency $$f/h=E_a^{i,j}(B)-E_b^{k,l}(B)$$ is required. In an ESR/NMR B-f scan it should appear as an additional line. It all looks like something that should be covered by theory in the 40s, and experimentally proven in the 50s, yet I see no sign of that. For like spins it is indeed covered by double Larmor line literature. What buzzword am I missing in my literature review?