Comparison of temporal variation in emission intensity of OH(A) in after-glow period of Ar/H$_2$O and He/H$_2$O gas-mixture plasmas in water*

Previously, we have reported quite long duration (approx. 500 ns) of optical emission intensity of OH(A) in an after glow period of Ar plasma in water.\footnote{T. Shirafuji, Y. Oguda and Y. Himeno: Jpn. J. Appl. Phys. \textbf{53}, 010211 (2014).} Numerical simulation has revealed that this phenomenon can be explained in terms of production of OH(A) through the reaction of H$_3$O$^+$ and low temperature electrons. We can perform similar plasma processing using He plasma in water with almost the same process performance in the case of decomposition of methylene blue molecules in aqueous solution. Thus, we have expected that the long duration of OH(A) optical emission can be observed also in He plasma in water. However, such long duration of OH(A) optical emission has not been observed in the case of He plasma in water. To understand this difference, we have performed numerical simulation of Ar/H$_2$O and He/H$_2$O plasmas, and discuss differences in major reaction pathways to produce OH(A) in Ar/H$_2$O and He/H$_2$O plasmas.