Optical Emission Spectrometry (OES)
OES exploits the fact that plasmas emit light. Emission lines in a plasma spectrum originate from the decay of electronically excited states of the atoms and molecules, which are present in a reactor. These excited states are most often created through collisions of species with electrons, e.g. electron-impact excitation, ionization, or dissociation reactions. Radiating atomic, ionic, and molecular species reveal information on the species present in the plasma as well as about reactions taking place. Emission lines in the OES spectrum indicate the presence of a particular species in the Plasma, that is the spectrum acts as a “fingerprint” of a process being monitored.
Emission intensity of particular lines is proportional to the density of electronically excited species (in case if self-absorption can be neglected). In most cases emission intensity of lines is also a measure of the ground state species density. This is true when the excited state is predominantly populated through excitations from the ground state, which is often the case in low temperature glow discharges.
Geometrical aspects and specifications of a detection system are important. For example absorption of light by the window material is possible. Position of monitoring assembly and the fact that measurements are a line-of-sight type may have influence on the measurement results too.