Experimental investigation of a ns-pulsed argon plasma jet for the fast desorption of weakly volatile organic compounds deposited on glass substrates at variable electric potential

Abstract

A ns-pulsed argon atmospheric pressure plasma jet (APPJ) was studied for the fast desorption of weakly volatile organic molecules (bibenzyl) deposited on glass substrates at variable electric potential (floating potential or grounded). The experiments focused particularly on thin resistant bibenzyl films (a large thick bibenzyl deposit was also studied), which are more difficult to be desorbed when using a substrate at a floating-potential. The APPJ was probed by means of high-resolution laser absorption spectroscopy to map the Ar(1s₅) metastable absolute density (spatially and temporally resolved) at the close vicinity of the glass plate where bibenzyl was deposited. Furthermore, the electrical, optical and thermal features of the APPJ were investigated systematically. In this way, the plasma desorption efficacy on thin resistant bibenzyl deposits was evaluated for the envisaged application, by varying the exposure time of the molecules to the APPJ (t_exp , from 10 s up to 180 s). The obtained results confirm the relatively low desorption efficacy in the case of a floating-potential substrate, which improves to some extent with increasing t_exp . However, when the substrate is grounded, the effect of the plasma becomes much more significant (i.e. much higher desorption efficacy). Besides, contrary to the case of a floating-potential substrate, an almost complete desorption of bibenzyl is achieved for t_exp =180 s. Similar effects of the APPJ were recorded on a thick bibenzyl deposit, validating the previous results. For both operating conditions (floating-potential and grounded substrate), the plasma action should be due to the production close to the glass surface of relatively high densities of Ar(1s₅) (up to 2×10¹³ cm^-3) and of oxidative species, such as atomic oxygen, hydroxyl radical and ozone. Thermal effects might play a synergistic role only when the substrate is grounded, since relatively high gas and glass-surface temperatures (>60 ^oC) are reached only in this case. The present results are of interest for public-security applications related to the fast detection of resistant prohibited substances, such as narcotics and explosives.