Photochemistry plays an important role in many chemical processes. Irradiating a sample with a suitable wavelength of light often results in free radicals, radical pairs, excited states, and changes of oxidation state.

Electron Paramagnetic Resonance (EPR) is the best technique for shedding light on these reactions and processes because of its unique ability to unambiguously detect paramagnetic species in a direct and non-intrusive manner.

EPR identifies the reaction intermediates as well as quantifies their concentrations, thus elucidating the reaction mechanisms and kinetics of photochemical reactions. It can be applied to samples in gaseous, liquid or sold states over a wide range of temperatures.