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 lambda = 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-He-2(*)(A(1)Sigma(+)(u)), 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 He-2(*)(E-1 Pi(g) - A(1)Sigma(+)(u)) (lambda = 513.4 nm). Thus the He-2(*)(A(1)Sigma(+)(u)) state is initially rapidly pumped via radiative cascade from higher He-2(*)(n =3) molecular states. In addition, the observed band emissions from the molecular He-2(*)(E-1 Pi(g))(nu=0) and He-2(*)(F-1 Sigma(+)(u))(nu=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 He-2(*) production from He*(n = 3) atomic state precursors.