Exchange-Ordering and Observation of Forbidden Spin Resonance Transitions in Crystalline Organic Radicals

Abstract

IN EXAMINING THE E. S. R. (electron spin resonance) spectrum of polycrystalline samples of the organic free radicals picryl-n-amino carbazyl (PAC), 1-3-bis diphenylene-2-phenyl allyl (BDPA), and Wurster's blue perchlorate (WB) at low temperatures and low frequencies, a resonance was observed at g = 4. This line may have arisen from a forbidden transition made possible by the dipolar interaction between spins. In the frequency range from 20 mc to 200 mc the gvalue is 4 to within 3% independent of temperature. At fixed temperature the ratio of the peak intensity of the g = 4 resonance absorption to that of the g = 2 absorption decreases with the increasing frequency. At fixed frequency the intensity ratio decreases with increasing temperature and vanishes above a temperature T characteristic of the particular free radical. The intensity ratio decreases with increasing values of the radio-frequency power. The width of the g = 4 resonance appears to be essentially independent of temperature and is of the same order of magnitude as that of the g = 2 resonance. In some cases there is evidence for a g = 6 resonance in the form of a weak bump on the low field side of the g = 4 resonance. (Author)