Electron Paramagnetic Resonance Theory

In 1921, Gerlach and Stern observed that a beam of silver atoms splits into two lines when it is subjected to a magnetic field [1–3]. While the line splitting in optical spectra, first found by Zeeman in 1896 [4, 5], could be explained by the angular momentum of the electrons, the s-electron of silver could not be subject to such a momentum, not to mention that an azimuthal quantum number l = 1/2 cannot be explained by classical physics. At that time, quantum mechanics was still an emerging field in physics and it took another three years until this anormal Zeeman effect was correctly interpreted by the joint research of Uhlenbeck, a classical physicist, and Goudsmit, a fellow of Paul Ehrenfest [6, 7]. They postulated a so-called ‘spin’, a quantized angular momentum, as an intrinsic property of the electron. This research marks the cornerstone of electron paramagnetic resonance (EPR) spectroscopy which is based on the transitions between quantized states of the resulting magnetic moment. Cynically, the worst event in the twentieth century boosted the development of EPR spectroscopy as after World War II suitable microwave instrumentation was readily available from existing radar equipment. This lead to the observation of the first EPR spectrum by the Russian physicist Zavoisky in 1944 [8, 9] already one year before the first nuclear magnetic resonance (NMR) spectrum was recorded [10, 11]. The development of EPR and NMR went in the same pace during the first decade though NMR was by far more widely used. But in 1965, NMR spectroscopy experienced its final boost with the development of the much faster Fourier transform (FT) NMR technique which also opened the development of completely new methodologies in this field [12]. The corresponding pulse EPR spectroscopy suffered from expensive instrumentation, the lack of microwave components, sufficiently fast digital electronics and intrinsic problems of limited microwave power. Although the first pulse EPR experiment was reported by Blume already in 1958 [13], pulse EPR was conducted by only a small number of research groups over several decades.