H/D ISOTOPE EFFECTS IN WATER RADIOLYSIS .2. DISSOCIATION OF ELECTRONICALLY EXCITED WATER

We present pulsed EPR measurements of the H• and D• atom magnetization signals present immediately (within 30-50 ns) after radiolysis of alkaline H2O/D2O mixtures with short pulses of 3-MeV electrons. The H• and D• spectra are initially polarized by radical pair chemically induced dynamic electron polarization (CIDEP) from spin-dependent recombination reactions in the spurs. The relative amplitudes of the H• and D• spectra are analyzed to deduce the relative isotope effect α = (H/D)product/(H/D)water in the formation of atomic hydrogen. The atomic hydrogen (deuterium) formed in water radiolysis spurs arises from both direct dissociation of electronically excited water and from the recombination of solvated electrons with hydronium ions. In sufficiently alkaline solution, the hydronium ion can be scavenged to prevent H• formation by cross-recombination in spurs. The value of a determined in this limit is characteristic of the excited water dissociation. We find α = 2.4 ± 0.2 in a 20% H2O/80% D2O mixture containing 10-2-10-1 M hydroxide. Values of α in other mixtures all fall between 2 and 3, with a weak trend toward larger a as the water proton fraction decreases. The magnitude and trend of the measured α are shown to be consistent with a combination of two previously established large isotope effects: (a) preferential dissociation of electronically excited HDO to give H•, and (b) preferential formation of H• in fast geminate recombination of electrons with Haq+ (Daq+) ions. We suggest that fragmentation/autoionization of excited water molecules is a more important radiolysis process than has been generally supposed. © 1990 American Chemical Society.