Introduction:

In 1921 Otto Stern and Werner Gerlach laid the foundations for Electron Paramagnetic Resonance (EPR), also known as Electron Spin Resonance (ESR). They demonstrated the spatial quantization associated with orbiting electrons in atoms by passing a collimated beam of silver atoms through an inhomogeneous magnetic field. The magnetic field was used to deflect the silver atoms in a direction dictated by the angle between the moment and the magnetic field. It was believed that a continuous gradient would be observed on their detector plate, but two distinct spots were recorded instead. Although the results of the Stern-Gerlach experiment were not as expected, it revealed that space is indeed quantized because the unpaired electrons oriented themselves such that two states of angular momentum were observed. Building on the results of the Stern-Gerlach experiment, in 1925 Samuel Abraham Gouldsmit and George Eugine Uhlenbeck confirmed the existence of electron magnetic moments and related it to internal electron angular momentum, also known as spin (1). In 1938 Isodor Isaak Rabi first demonstrated magnetic resonance using a molecular beam and resolved the magnetic moments for nuclei to within 0.1% (2). Taking magnetic resonance a step further, in 1944 Evgeny Zavoisky published the first observation of ESR absorption in a copper (II) chloride dihydrate sample submerged in a 4.76 mT field at 133 MHz (3). The procedure presented by Zavoisky had significant implications, which would lead to the development of a spectroscopic method using EPR.

From the Gouldsmit and Uhlenbeck experiment it was proven that electrons have an intrinsic angular momentum called spin and a magnetic dipole moment. As the electrons fill the outer orbital shells of an atom, they will tend to couple together in pairs of spin up and spin down electrons. Substances with uncoupled electrons will have net magnetic moments, these unpaired electrons will allow for us to perform EPR spectroscopy on a paramagnetic sample. Expanding on Zavoisky’s experiment, when a paramagnetic substance is placed in a magnetic field the unpaired electrons will align themselves with the field such that their orientations are said to be spin up s=1/2 or spin down s=-1/2. By subjecting the sample to radio frequency (RF) radiation, transitions between spin states can be induced if the energy level of the RF field is equivalent to that required for the transition to occur. When this transition occurs, energy is absorbed and this absorption can be measured. By sweeping the magnetic field or RF radiation through a small range, it is possible to obtain a highly sensitive spectrum, which reveals the hyperfine structure of the spectrum. Similar to traditional ultraviolet, visible, and infrared spectroscopic methods, the Zeeman splitting is due to the interactions between the electrons and their immediate surroundings when placed in a magnetic field. This will then reveal the hyperfine spectrum. This hyperfine structure allows for the calculation of the g-value of a sample with high precision. The g-value is a proportionality constant that deviates from the accepted value for free electrons due to the magnetic interactions between unpaired electrons and their surroundings. Zavoisky used an RF field of 133 MHz, by increasing the frequency to X-band microwaves, 8.2 GHz to 12.4 GHz, the higher spectral resolution and hyperfine structure is achieved. This paper presents the theory and procedure for obtaining the signal for an EPR spectrum for a sample of DPPH as well as synthetic ruby. It also discusses the significance of this spectroscopic technique in studying the internal interactions within paramagnetic systems.