Applicability of reaction rate in gas phase data to supercritical water region

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To study reaction rate of OH radical in high temperature water, the most direct method is to observe the transient absorption of OH radical itself. At room temperature there are a several demonstrations for such direct measurements, however, there are few example under high temperature conditions. In the direct UV absorption measurements, the absorption by sapphire windows prevents the measurement of OH signal. The actual absorption intensity due to OH radical was determined by subtraction of the sapphire window signals. The absorption spectra of OH radical in high temperature water shifted to blue. The absorption coefficient of OH radical at 250 nm decreased with increasing the temperature. At room temperature, the absorption coefficient of OH radical is known as to be 500. At 300 C, we found that the absorption coefficients decreased to 200.The temperature dependence of OH radical reaction rate was examined. The rates are followed by the Arrhenius temperature dependence up to 125 C. Above 125 C, however, the reaction rates are independent on the water temperature. The reaction rate at 125 C was 1x10^<10> M^<-1>s^<-1>.