Thomson, Rayleigh and Raman scattering
If a laser beam is passing through a plasma, the laser photons can be scattered on free electrons in the plasma (Thomson Scattering), atoms and molecules (Rayleigh scattering) and possibly even accompanied by a rotational or vibrational transition in the molecule (Raman Scattering). The detection of the scattered light typically occurs at 90 degree angle with the laser bundle.
Thomson scattering can be used to determine the electron density (through the number of scattered photons) and temperature (Doppler broadening of the Thomson spectrum).
In the same way as Thomson scattering can give information on free electrons in the plasma (density and temperature), Rayleigh scattering can reveal the density and temperature of atoms and ions (heavy particles).
Also molecules, if present, can scatter photons, just like the atoms. However, with molecules, there is a possibility of an additional transition in rotational or vibrational energy. Therefore, the scattering spectrum will yield a great number of peaks, each corresponding with a certain ro-vibrational transition. Raman Scattering can be used for the determination of rotational or vibrational temperature and the heavy particle temperature. These techniques have been applied to a large number of different plasmas at EPG.