Contrasting characteristics of sub-microsecond pulsed atmospheric air and atmospheric pressure helium–oxygen glow discharges

Abstract

Glow discharges in air are often considered to be the ultimate low-temperature atmospheric pressure plasmas for numerous chamber-free applications. This is due to the ubiquitous presence of air and the perceived abundance of reactive oxygen and nitrogen species in air plasmas. In this paper, sub-microsecond pulsed atmospheric air plasmas are shown to produce a low concentration of excited oxygen atoms but an abundance of excited nitrogen species, UV photons and ozone molecules. This contrasts sharply with efficient production of excited oxygen atoms in comparable helium-oxygen discharges. Relevant reaction chemistry analyzed with a global model suggests that collisional excitation of O 2 by helium metastables is significantly more efficient than electron dissociative excitation of O 2 , electron excitation of O, and ion-ion recombination. These results suggest different practical uses of the two oxygen-containing atmospheric discharges, with air plasmas being well suited for nitrogen and UV based chemistry and He-O 2 plasmas for excited atomic oxygen based chemistry.

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“…28 Similar results have also been reported by other researchers 28,29 and can be explained by the fact that the quenching processes of excited oxygen atoms with oxygen and nitrogen molecules are two orders of magnitude faster than the radiative processes, making it very difficult to observe atomic oxygen in the spectra, although it is certainly present in the different air plasma discharges.…”

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“…The small emission lines noted in the area 750 and 780 nm were attributed to O atoms [22]. The relatively lower intensities of peaks associated with oxygen is due to quenching of O( 3 P) and O( 5 P) in the air plasma, as has been explained by Walsh, Liu, Iza, Rong and Kong [20]. The OH peak around 300 nm was also noticeable.…”

“…Literature reveals that most DBD treatments for food application operate in single diatomic gases or noble gases [6]. However, for practical applications, it is attractive to use less costly molecular gases, for example air [20]. The atmospheric air conditions at the time of packaging and treatment was 42±1% relative humidity (RH) and 25±2 °C, as measured using a humidity-temperature probe connected to a data logger (Testo 176 T2, Testo Ltd., UK).…”

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“…In a study comparing an atmospheric air plasma with its counterpart in helium/oxygen both sustained in submicrosecond voltage pulses, it is observed that the air plasma is dominated by N 2 * species with their emission in the UVC region whereas the helium/oxygen plasma is dominated by oxygen atoms [89]. It is of particular interest to contrast these against bacterial inactivation kinetics data and establish a hierarchical list of bactericidal plasma agents (and their threshold bactericidal doses).…”

“…The long-standing attempt to understand plasma inactivation mechanisms has very often suffered a common lack of an appreciation that even the same plasma source can have very different plasma chemistries as it transits, often abruptly and rapidly, from one plasma mode to another [89] [111]. Similarly under-appreciated is the paucity that the resistance of microorganisms to external stresses is influenced by their micro-environment [112], an obvious example being their interaction with the surface of their supporting substrate [113].…”