Direct-detected rapid-scan EPR at 250 MHz

EPR spectra at 250 MHz for a single crystal of lithium phthalocyanine (LiPc) in the absence of oxygen and for a deoxygenated aqueous solution of a Nycomed triarylmethyl (trityl-CD3) radical were obtained at scan rates between 1.3 x 10(3) and 3.4 x 10(5) G/s, These scan rates are rapid relative to the reciprocals of the electron spin relaxation times (LiPc: T-1 = 3.5 mus and T-2 = 2.5 mus; trityl: T-1 = 12 mus and T-2 = 11.5 mus) and cause characteristic oscillations in the direct-detected absorption spectra. For a given scan rate, shorter values of T-2 and increased inhomogeneous broadening cause less deep oscillations that damp out more quickly than for longer T-2. There is excellent agreement between experimental and calculated lineshapes and signal amplitudes as a function of radiolfrequency magnetic field (B-1) and scan rate. When B-1 is adjusted for maximum signal amplitude as a function of scan rate, signal intensity for constant number of scans is enhanced by up to a factor of three relative to slow scans. The number of scans that can be averaged in a defined period of time is proportional to the scan rate, which further enhances signal amplitude per unit time. Longer relaxation times cause the maximum signal intensity to occur at slower scan rates. These experiments provide the first systematic characterization of direct-detected rapid-scan EPR signals. (C) 2004 Elsevier Inc. All rights reserved.