Electron Spin Resonance

When the molecules of a solid exhibit paramagnetism as a result of unpaired electron spins, transitions can be induced between spin states by applying a magnetic field and then supplying electromagnetic energy, usually in the microwave range of frequencies. The resulting absorption spectra are described as electron spin resonance (ESR) or electron paramagnetic resonance (EPR). Electron spin resonance has been used as an investigative tool for the study of radicals formed in solid materials, since the radicals typically produce an unpaired spin on the molecule from which an electron is removed. Particularly fruitful has been the study of the ESR spectra of radicals produced as radiation damage from ionizing radiation. Study of the radicals produced by such radiation gives information about the locations and mechanisms of radiation damage.

当固体中的分子由于未配对的电子自旋而表现出顺磁性时，可以通过施加磁场并随后提供电磁能量（通常在微波频率范围内）来诱导分子间自旋态的跃迁。所产生的吸收光谱被描述为电子自旋共振（ESR）或电子顺磁共振（EPR）。电子自旋共振已被用作研究固体材料中自由基的工具，因为这些自由基通常会在分子中产生未配对的自旋。特别是，研究由辐射损伤产生的自由基的ESR谱取得了显著成果。对这类辐射产生的自由基的研究提供了关于辐射损伤位置和机制的信息。

The interaction of an external magnetic field with an electron spin depends upon the magnetic moment associated with the spin, and the nature of an isolated electron spin is such that two and only two orientations are possible. The application of the magnetic field then provides a magnetic potential energy which splits the spin states by an amount proportional to the magnetic field (Zeeman effect), and then radio frequency radiation of the appropriate frequency can cause a transition from one spin state to the other. The energy associated with the transition is expressed in terms of the applied magnetic field B, the electron spin g-factor g, and the constant μ_B which is called the Bohr magneton.

外部磁场与电子自旋的相互作用取决于与自旋相关的磁矩，以及孤立电子自旋的性质，即只有两种可能的取向。当施加磁场时，它提供了一个磁势能，该势能将自旋态分裂，分裂量与磁场成正比（塞曼效应）。然后，适当频率的无线电波辐射可以引起一个自旋态到另一个自旋态的跃迁。与跃迁相关的能量可以表示为施加的磁场B、电子自旋g因子g以及常数μ_B的函数，μ_B被称为玻尔磁子。

If the radio frequency excitation was supplied by a klystron at 20 GHz, the magnetic field required for resonance would be 0.71 Tesla, a sizable magnetic field typically supplied by a large laboratory magnet.

如果无线电频率激发是由20 GHz的克利斯特龙（klystron）提供的，那么为了实现共振所需的磁场为0.71特斯拉，这是一个通常由大型实验室磁体提供的较大磁场。

If you were always dealing with systems with a single spin like this example, then ESR would always consist of just one line, and would have little value as an investigative tool, but several factors influence the effective value of g in different settings. Much of the information obtainable from ESR comes from the splittings caused by interactions with nuclear spins in the vicinity of the unpaired spin, splittings called nuclear hyperfine structure.

如果你一直处理的是像这种只有一个自旋的系统，那么电子自旋共振（ESR）将始终只产生一条线，作为研究工具价值不大，但多种因素会影响不同环境下的有效g值。从ESR获得的大部分信息来自未配对自旋附近核自旋的相互作用所引起的分裂，这种分裂被称为核超精细结构。

Electron Spin Resonance

Nuclear Hyperfine Structure in ESR Spectra