OH rotational temperature measurements via a two temperature distribution analysis in plasma with water microdroplets

We study plasma processing with water/solution microdroplets for a new nanoparticle synthesis method. In the process, it is important to know gas temperature (Tg) for understanding the mechanism of the particle growth and controlling its properties. Since OH emissions are naturally observed in such plasma, the rotational temperature (Tr) of OH (A-X) is estimated and compared with Tr from N$_{\mathrm{2\thinspace }}$(C-B). The plasma is generated by dielectric barrier discharges in He with N$_{\mathrm{2}}$ (2.6 {\%}) gas flow, and microdroplets are generated by an ultrasonic atomizer and carried into He/N$_{\mathrm{2}}$ plasma. Optical emission spectroscopy revealed that with the increase of voltage and frequency of plasma generation, the Tr of N$_{\mathrm{2}}$ increases. While the good theoretical spectrum fit on N$_{\mathrm{2}}$ experimental spectrum could be achieved, it was hard to obtain a reasonable fit for the OH spectrum with a single rotational energy distribution. On the other hand, two rotational distribution analysis could reproduce the experimental spectrum of OH and the lower Tr agrees to Tr by N$_{\mathrm{2}}$. The results suggest that the lower Tr obtained with the two rotational temperature analysis of OH spectrum represents Tg of the environment.