Measurement of H and H2 populations in a low-temperature plasma by vacuum ultraviolet laser absorption spectroscopy

A new technique, vacuum ultraviolet laser absorption spectroscopy, has been developed to quantitatively determine the absolute density of H and H2 within a plasma. The technique is particularly well suited to measurement in a plasma, where high charged particle and photon backgrounds complicate other methods of detection. The high selectivity and sensitivity of the technique allows for the measurement of the rotational-vibrational state distribution of H2 as well as the translational temperature of the atoms and molecules. The technique has been used to study both pulsed and continuous plasma discharges. H2 state distributions show a high degree of internal excitation, with levels up to v=5 and J=8 being observed. Hydrogen atom measurements indicate that, even for modest discharge currents, the fraction of H2 molecules dissociated can be greater than 0.15