Detecting the Transient ENDOR Response

Free radicals at low temperature are sometimes difficult to study by electron-nuclear double-resonance spectroscopy with the commonly available detection schemes. In some cases, slow nuclear spin relaxation limits the steady-state ENDOR response. A detection scheme optimized for transient signals is described that circumvents this limitation on signal amplitude. The apparatus employs pulsed radiofrequency fields with advance of the radiofrequency between pulses. Some spectra of semiquinone and tyrosine radicals are presented to illustrate the utility of the technique.