Picosecond pulse radiolysis study of highly concentrated nitric acid solutions: formation mechanism of NO3• radical.

The formation of nitrate radical, NO3 center dot, is observed for the first time directly by picosecond pulse radiolysis of highly concentrated nitric acid solutions. The experimental yield of NO3- ionization is deduced from the pulse-probe transient absorption measurements in the visible region where this radical absorbs. On the basis of the value of the extinction coefficient of nitrate radical at 640 nm equal to 1300 M cm(-1), the experimental yield of NO3 center dot at 20 ps is found to be around 0.36 x 10(-7), 1.33 x 10(-7), and 2.85 x 10(-7) mol J(-1) for 1, 3.5, and 7 M nitric acid solutions, respectively. Relative to the dose absorbed by nitric acid by the direct effect, we find an unexpected high formation yield of the nitrate radical within the electron pulse. Therefore, we suggest that the trapping of the positive hole, H2O center dot+, by NO3- also contributes to the formation of NO3 center dot within the electron pulse. Moreover, after the pulse and within 4 ns, the beginning of the reaction of OH center dot radical with undissociated nitric acid is observed for the most concentrated nitric acid solution.