H/D isotope effects in water radiolysis. 3. Atomic hydrogen in acidic H sub 2 O/D sub 2 O mixtures. [Accelerated electrons]

The authors present measurements of the EPR spectra of H and D atoms immediately (30-50 ns) following pulse radiolysis of acidic H{sub 2}O/D{sub 2}O mixtures with 3-MeV electrons. The spectra exhibit a low-field emission/high-field absorption (E/A) polarization pattern of chemically induced dynamic electron polarization (CIDEP), which is characteristic of random (uncorrelated spin) recombination reactions of free radicals in the radiolysis spurs. As in previous works, the CIDEP amplitudes are analyzed to deduce the isotope effect {alpha} = (H/D){sub product}/(H/D){sub water} which characterizes the formation of the H and D atoms. The relative H and D signal intensities can be accounted for in terms of two strong isotope effects demonstrated in previous work: (a) reaction of hydrated electrons with hydronium ions to give H (or D) and (b) the dissociation of the electronically excited HDO* molecule to give preferentially H. It is estimated that the direct dissociation channel accounts for G{congruent} 0.3 of the atomic hydrogen yield in radiolysis of both acidic and alkaline water.