Emission SpectroscopyEmission Spectroscopy (ES) is the spectrally resolved natural emission of photons from excited species in the gas phase. ES is a line-of-sight method like absoprtion and probes electronically excited species in contrast to LIF which probes ground state species.
The spectral analysis of the spontaneous emission, i. e., the detection of the signal as a function of wavelength, is called emission spectrum. The emission spectrum is a spectral fingerprint of the gas probe under study and reveals information of the gas dynamical process itself like reaction intensity, stability, species concentration and temperature.
Excited states of molecules or atoms are generated by thermal excitation, absorption of radiation or chemical reactions (chemiluminescence).
The major field for emission spectroscopy is the investigation of combustion and plasma processes. The species observed are usually intermediate reaction products like OH*, CH*, C2*, CN* etc. (the asteriks indicates electronically excited states). These free radicals appear mostly in the reaction zone, where they are produced in low quantities.
In comparison with laser based diagnostic techniques ES is handicapped by limited temporal and spatial resolution (line-of-sight method) and by the fact, that the excited species are often probed under non-equilibrium conditions where their internal energy distribution can not be described by the Boltzmann law, which -on the other hand- is the general definition of temperature.
Nevertheless, ES is the first step to investigate unknown light emitting gas reactions. In a second step, based on ES results, more sophisticated laser techniques may be applied to gain more insight of the process under investigation.