ABSTRACT
Spectroscopy requires a source of radiation, a sample, and a detector; magnetic spectroscopy additionally requires an external magnetic field. The term spectroscopy implies that at least one of these four elements is variable, or tunable, in some way or other, and that one measures the amount of radiation absorbed by the sample as a function of this variable. For example, the source generates radiation with energy
in which h is Planck’s constant, and v is the frequency (in units of Hertz or cycles per second) with its corresponding wavelength λ (in meters) according to the conversion
in which c is the speed of light (2.99792 × 108 meters per second). Tuning the frequency over a limited range of the electromagnetic spectrum is the most common approach taken in the majority of spectroscopies. Dealing with different ranges of the EM spectrum requires different technologies, and therefore each range has its own spectroscopy (or spectroscopies), from very low-frequency (i.e., very low energy and very long wavelengths) radio waves in NMR spectroscopy to very high-frequency (i.e., very high energy and very short wavelengths) gamma rays in x-ray spectroscopy. In magnetic spectroscopy, one has the alternative possibility to vary the magnetic field while keeping the frequency at a constant value. This is the approach usually taken in EPR spectroscopy. On the contrary, in other magnetic spectroscopies-for example, NMR, MCD (magnetic circular dichroism), and MS (Mössbauer spectroscopy)—the magnetic field is kept constant and the frequency is varied. In principle, one can, of course, also vary both the field and the frequency at the same time, but this is rarely done. The choice of what to vary is always based on practical considerations of technical limitations, which we will discuss for EPR later.
