On the origin of the vacuum-ultraviolet molecular band emission from high pressure helium plasmas

R. J. Carman, R. Ganesan

Research output: Contribution to conferenceAbstract


The vacuum-ultraviolet (VUV) emission band from the He₂* excimer between 60-120nm (the well-known Hopfield continuum) is of topical interest as potential source of high energy photons (10-20eV) for many emerging applications in materials processing and surface treatment. Previous time-resolved spectroscopic studies of the He₂* excimer emission bands in medium pressure pulsed-excited plasmas reported by Bartell et-al [1] have indicated the existence of a relatively weak but rapidly evolving fast component superimposed on a slower pulse. The slower pulse typically decays with a duration of several microseconds. However, the fast component has not been extensively studied to date, and its characteristics and precise origin are not well understood. In particular, its prompt appearance (within a few 10’s of nanoseconds) following plasma excitation appears to suggest a new production mechanism that is substantially faster than that attributed to known processes for forming the low-lying He₂* molecular states e.g. three-body conversion of He*(n=2) species. Interestingly, in LIF studies of a low pressure (~10mb) helium plasma by Frost et-al [2], unexplained He₂** molecular emissions were observed as a direct result of He(31,3P) + He(1¹S) collisions. Based on their results, they proposed the existence of a “neglected channel” to He₂** production.

To further investigate these observations, a comprehensive set of time and wavelength resolved VUV emission curves from a windowless dielectric barrier discharge (DBD) in helium (100mb-1000mb) utilising short voltage pulse excitation have been measured to clearly resolve the fast and slow temporal components of the Hopfield continuum over the wavelength range 60-120nm. Experimental results, together with results from computer modeling, suggest that both temporal components of the VUV emission must originate from the same molecular state He₂*(A¹Su+), and that two distinct pumping mechanisms must populate this state. The time signature of the fast component suggests a new channel for producing low-lying He₂*(A¹Su+) molecules from He* atomic excited states, and involving a rapid radiative cascade process from higher He₂** molecular states.
Original languageEnglish
Number of pages1
Publication statusPublished - 12 Feb 2014
EventGaseous Electronics Meeting - McCracken Country Club, Victor Harbor, Adelaide, Australia
Duration: 9 Feb 201412 Feb 2014
Conference number: XVIII


ConferenceGaseous Electronics Meeting
Abbreviated titleGEM


  • plasma physics
  • Atomic processes


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