Spectroscopy is the branch of science that is concerned with the measurement of spectra that is produced when matter interacts with or emits electromagnetic radiation. In short, spectroscopy deals with measuring and interpreting spectra that arise from the interaction of electromagnetic radiation (a form of energy propagated in the form of electromagnetic waves) with matter. It is concerned with the absorption, emission, or scattering of electromagnetic radiation by atoms or molecules.
Electron Spin Resonance 