An improved TM110 resonator for continuous-wave ENDOR studies at X-band

An improved TM110 resonator for continuous-wave and time-resolved electron-nuclear double resonance (ENDOR) studies at X-band frequencies is described that has been designed with small samples and/or light excitation in mind. The filling factor is increased by reducing the resonator length to only 16 mm. The radio-frequency field is generated by either a conventional solenoid for stable samples or by a pair of coils for samples which require photoexcitation. This arrangement leaves the central volume of the resonator free for optimal sample illumination, which is achieved through a slit in the wall of the resonator. Microwave coupling is achieved by the incorporation of an iris in the top of the resonator, and magnetic field modulation is applied by external coils. The resonator's high sensitivity is illustrated by a study of the temperature dependence of the continuous-wave ENDOR signal of the stable neutral flavin radical in DNA photolyase and an investigation of the photoexcited triplet state of free-base tetraphenylbacteriochlorin by time-resolved ENDOR.