Time and wavelength resolved measurements of the VUV excimer emission generated in a windowless dielectric barrier discharge (DBD) in helium

The vacuum-ultraviolet (VUV) emission band between λ=60-120nm resulting from the radiative decay of the low-lying energy levels of the helium excimer He₂* is of current interest for applications in surface treatment requiring high-energy photons (10-20eV). Previous time-resolved spectroscopic studies of the He₂* VUV emission bands in medium pressure (100-1000 torr) pulsed discharge excited plasmas [1][2] have suggested the existence of a relatively weak, but rapidly evolving fast component superimposed on a slower pulse. Whereas the slower pulse typically decays with a duration of several microseconds, the fast component’s characteristics have not been extensively studied to date. Moreover, its precise origin its not fully understood, and in particular, its apparent prompt appearance (within a few 10’s of nanoseconds) following plasma excitation. This appears to suggest a new pumping mechanism that is substantially faster than that attributed to known processes responsible for the formation of the low-lying He₂* molecular states e.g. three-body conversion of He*(n=2) species. Interestingly, laser-induced fluorescence studies of a low pressure (~10mb) helium plasma by Frost et-al [3] yielded unexplained molecular emissions from higher-lying helium molecular states He₂** which 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 the origin of the “fast” VUV component, a comprehensive set of time and wavelength resolved VUV emission curves from a short-pulse excited dielectric barrier discharge (DBD) in helium have been measured as a function of gas pressure to clearly resolve the fast and slow temporal components over the wavelength range λ=60-120nm. A “windowless” DBD geometry has been used since this wavelength range is strongly absorbed by standard VUV optics (e.g. LiF, MgF¬₂). The experimental system and methodology are described in detail in [4]. Experimental results, together with results from computer modeling of the excimer emissions He₂*(A1∑u+) => He₂+ hV, suggest that both temporal components of the VUV emission must originate from the same molecular state He₂*(A1∑u+), 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*(A1∑u+) molecules from He* atomic excited states, and involving a rapid radiative cascade process from higher He₂** molecular states.