Time-resolved vacuum-ultraviolet emission (λ = 60-120 nm) from a high pressure DBD-excited helium plasma: formation mechanisms of the fast component

We report time and wavelength resolved studies of the vacuum-ultraviolet (VUV) emission from a windowless dielectric barrier discharge (DBD) in helium. Short-pulse voltage excitation is utilised to clearly resolve the fast and slow temporal components of the Hopfield continuum between λ = 60-120 nm. Experimental results and theoretical modelling of the spectral distributions indicate that the two components of the VUV emission must originate from the same radiating molecular state - , and that two distinct pumping mechanisms populate this state. The time evolution of the fast component is found to correlate with that from the (0,0) molecular transition (λ = 513.4 nm). Thus the state is initially rapidly pumped via radiative cascade from higher molecular states. In addition, the observed band emissions from the molecular _v=0 and _v=0 states and the line emissions from the atomic He^∗(n = 3) states all exhibit similar temporal behaviour during the discharge excitation period. Our results are consistent with the recent report of Frost et al (J. Phys. B 34 1569 2001) concerning the existence of a so-called 'neglected channel' to fast production from He^∗(n = 3) atomic state precursors.