The ring-shaped spatial distribution of the argon excimer, $$\hbox {Ar}_2^{*}$$, in the effluent of the kINPen-sci

Spatially resolved measurements of the argon excimer, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mtext>Ar</mml:mtext> <mml:mn>2</mml:mn> <mml:mrow><mml:mrow></mml:mrow> <mml:mo>∗</mml:mo></mml:mrow> </mml:mmultiscripts> </mml:math> , were performed in the effluent of a cold atmospheric plasma jet (CAPJ), the so called kINPen-sci, using cavity ringdown spectroscopy. The spatial distribution of the density of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mtext>Ar</mml:mtext> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mrow></mml:mrow> <mml:mo>∗</mml:mo></mml:mrow> </mml:mmultiscripts> </mml:math> could be distinguished into two distinct populations: a Gaussian and a toroidally shaped distribution. The production mechanisms of these populations seem to differ. On the one hand, a strong correlation was found between the Gaussian <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mtext>Ar</mml:mtext> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mrow></mml:mrow> <mml:mo>∗</mml:mo></mml:mrow> </mml:mmultiscripts> </mml:math> population and the spatial distribution of the filaments produced in the effluent of the kINPen-sci. On the other hand, measurements performed while varying the experimental conditions under which the kINPen-sci was operated indicate that the gas flow velocity must play a major role in the formation of the toroidal <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mtext>Ar</mml:mtext> <mml:mn>2</mml:mn> <mml:mrow><mml:mrow></mml:mrow> <mml:mo>∗</mml:mo></mml:mrow> </mml:mmultiscripts> </mml:math> population. However, the mechanism of formation of the toroid <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mtext>Ar</mml:mtext> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> <mml:mrow><mml:mrow></mml:mrow> <mml:mo>∗</mml:mo></mml:mrow> </mml:mmultiscripts> </mml:math> population remains unclear.