Appl. Magn. Reson. 28, 239-249 (2005) Applied Magnetic Resonance 9 Springer-Verlag 2005 Printed in Austria Impact of Resonator on Direct-Detected Rapid-Scan EPR at 9.8 GHz J. P. Joshi, G. R. Eaton, and S. S. Eaton Department of Chemistry and Biochemistry, University of Denver, Denver, Colorado, USA R› August 16, 2004 Abstract. Rapid-scan electron paramagnetic resonance spectra at 9.8 GHz were obtained on a Bruker E580 spectrometer. Spectra of lithium phthalocyanine (LiPc) needles (Ti = 8 p.s, T 2 = 3.4 p.s) and of a 0.2 mM aqueous solution of Nycomed triarylmethyl (trityl-CD3) radical (T~ = 11.5 p.s, T 2 = 8 p.s) were recorded at scan rates between 3.4.10 ~ and 7,5.105 G/s at the center of sinusoidal seans. Sig- nals for LiPc were obtained with a split-ring resonator, a rectangular resonator anda dielectric reso- nator. At faster scan rates the small bandwidth of the high-Q dielectric resonator filters out high- frequency components of the rapid-scan signals. Field inhomogeneities induced by the rapidly changing magnetic field increase with scan rate and are greater with the dielectric and split-ring resonators than with the rectangular resonator. Data for trityl-CD 3 were recorded with the rectangular resonator. The extended trityl sampLe, about 3 mm Iong, shows larger effects of magnetic field inhomogeneities than the small LiPc crystals. 1 lntroduction In conventional slow-scan electron paramagnetic resonance (EPR) the magnetic field passes through the spin packet resonance in a time that is long relative to relaxation times (Tt and T2) and the signal is detected with magnetic fŸ modu- Iation and phase-sensitive detection at the modulation frequency. Typically the microwave magnetic field B 1 is small enough that there is little change in z-axis magnetization during a scan and the line shape is independent of scan rate. On the other extreme, in pulsed spectroscopy the high BI fields cause a large change in the z-axis magnetization, and the signal is recorded with direct detection. In between these two regimes there is a variety of nonlinear responses that ate referred to as passage effects. Bloch first described passage effects for nuclear magnetic resonance (NMR) signals [1]. The initial observation of adiabatic rapid passage EPR by Portis was followed by papers by Weger [2], Hyde [3, 4], Mailer [5], among others, that demonstrated the importance of passage effects in EPR spectra. In the notation of Weger [2] the term "rapid" refers to the regime in which Bt/[(dBo/dt)(T~T2) t/E] < 1, where dBo/dt is the magnetic field scan rate. This