A zero-field electron spin resonance spectrometer for the study of transient radical ion pairs

A zero-field electron spin resonance spectrometer is described for measuring the spectra of spin-correlated radical ion pairs in solution in the frequency range 1-80 MHz. The radical ions are created by continuous optical irradiation and rapidly recombine to form an exciplex whose fluorescence is monitored as the radiofrequency is swept, also continuously. The probability of recombination depends upon the singlet character of the radical pair at the time that the radicals re-encounter after an initial separation in the formation step. This is affected directly by resonant state mixing induced within the radical pair energy manifold by the applied radiofrequency field. The method yields the spectrum of the radical pair and also allows direct observation of the effects of radiofrequency radiation on the yields of products formed from radical combination reactions.