Detection of OH radicals in atmospheric-pressure plasma jet by evanescent-wave laser-induced fluorescence spectroscopy

This paper reports the signal and noise in the detection of OH radicals by evanescent-wave laser-induced fluorescence (EW-LIF) spectroscopy. We adopted this method to detect OH radicals, which were produced within an atmospheric-pressure argon plasma jet, in the vicinity of a quartz surface. The dominant noise was the stray laser light which could not be eliminated by using a monochromator and an interference filter. This was because the detection optics looked at the total reflection point of the laser beam in the EW-LIF spectroscopy. It was impossible to detect the LIF signal in the duration of the laser pulse because of the intense stray light. On the other hand, thanks to the slow decay of the LIF intensity, we succeeded in detecting the LIF signal after the laser pulse. The intensity ratio between the LIF signal originated from the bulk plasma and the one induced by EW near the surface was 5000-10000, which can be explained by the difference in the observation volumes. The proportionality between the LIF intensities from the bulk plasma and the surface vicinity, when changing the discharge conditions, suggests that the surface loss probability of OH on the quartz surface was not affected by these changes.