We present measurements of the spectrum (1–80 MHz) of the effect of a weak ( $\sim 500\mu \mathrm{T}$) radio frequency magnetic field on the electron-hole recombination of radical ion pairs in solution. Distinct spectra are observed for the pyrene anion/dimethylaniline cation radical pair in which one or both of the radicals are perdeuterated. The radical pair mechanism is developed theoretically and shown to account satisfactorily for both the magnetic field effect and the associated magnetic isotope effect.

• Received 8 December 2000

DOI:https://doi.org/10.1103/PhysRevLett.87.077602