Active actinometry on a cold hydrogen afterglow

Summary form only given. A new method of actinometry has developed to characterize the cold afterglow of an expanding thermal plasma source in hydrogen. A small electrode is placed in the afterglow to generate a local low-frequency (100-500 kHz) plasma. In this plasma fast electrons are created that can excite particles from the ground state to visible light emitting levels. The atomic Balmer /spl alpha/ line and the molecular Fulcher band are used to determine the atomic and molecular abundances of the plasma. The power input from the low frequency discharge is kept low enough to assure that the plasma composition and the gas temperature are not significantly influenced. Active actinometry thus offers a method to sample the composition and the ground state molecular populations of the flowing afterglow plasma. The method has been successfully applied under plasma conditions with a low electron temperature (< 0.2 eV) and a low electron density (< 10/sup 17/ m/sup -3/).