The decay of the hydrated electron solvated by D20 molecules, eaq" (D20), and the OD radical as well as the reactions of the precursor of the hydrated electron have been investigated in D20 solution by using the Argonne stroboscopic pulse radiolysis system. These processes have been compared to the analogous reactions in H20. The decay of the electron is slower in D20 relative to the corresponding species in H20, while the decay of the hydroxyl radical and the efficiencies of the electron scavengers in reducing the initial yield of the hydrated electron are comparable in both solvents. The OD radical absorbance decays to 0.75 ± 0.06 of its initial value from 200 ps to 3.0 ns. This parallels that for the OH radical (0.74 ± 0.06). The similarity of the electron scavenging efficiencies indicate a hydrated electron precursor that exists in a distinct, localized state for both D20 and H20. The slower decay of the electron in D20 has been interpreted as a greater thermalization distance and a broader initial spatial distribution for the electron in D20 compared to H20. The spurs of the D20 and H20 systems have been modeled by using a spherically symmetric electron distribution around a Gaussian hydroxyl radical core. The computer simulations agree quite favorably with the experimental results.
The yield of the hydrated electron, [Formula: see text], was measured as a function of time in H2O, D2O, and mixtures of the two solvents. The results of these measurements showed that the decay of [Formula: see text] was considerably slower in D2O than in H2O. These results suggest that the electron travels a longer distance in D2O than in H2O. In H2O–D2O mixtures there had to be at least 50% D2O in the solution before the decay was appreciably different from the pure H2O solution. The decay in 80% D2O is still considerably different than the decay in pure D2O solution. From these results we draw the following conclusions. (i) The distance an electron travels before solvation is greater in D2O than in H2O. (ii) Much of the travel of the electron prior to thermalization occurs when the energy of the electron is less than the excitation energy of the solvent. (iii) A major mode of energy loss is due to energy transfer to the stretching vibrational modes of the water molecules.
4099of TMD is electronic quenching by the CH3F buffer gas. The data in Figure 6 were obtained by using 1 1.5 Torr of CH3F. Thus, if quenching is the mechanism for depletion of 3A* population, the quenching rate constant is approximately 5 X cm3/ (molecule s). This value can be compared to previously measured acetone triplet quenching rates in solution and in the gas phase. Rate constants in the range 10-16-10-12 cm3/(molecule s) have been measured for a variety of quenchers in s o l~t i o n .~~-~ In the gas phase, O2 has been shown to quench 3A* with a rate constant of 7.5 X cm3/(molecule s).~~ Further tests of the quenching postulate under our experimental conditions are difficult because variations in the CH3F pressure cause variations in the temperature of the reacting system. We have been unable to perform absorption studies using SF6 as the heating gas due to the technical problem of low reflectivity of quartz at 10.6 pm. Further studies (45) Costela, A,; Crespo, M. T.; Figuera, J. M. J . Photochem. 1986, 34, 165.of electronic quenching of directly excited gas-phase acetone would prove very useful. ConclusionThe high-temperature decomposition of gas-phase TMD has been studied by using LPHP to obtain quantitative values for the chemiexcitation yields. The observed emission is due entirely to 'A*, whose yield is measured to be as = 0.017 f 0.008. This value is somewhat higher than that observed at room temperature in solution. A lower limit to the triplet yield was found to be aT 2 0.10 f 0.03. This yield is comparable to that found previously in solution. The main difference observed in the present study is the very short lifetime of 3A* under our experimental conditions. This is postulated to be due to electronic quenching by CH3F with an estimated rate constant of 5 X Acknowledgment.Streak camera measurements of the relative yield of the excited state of cyclohexane (CH*) as a function of electron scavenger concentration (perfluoro-n-hexane, COz, and N20) strongly indicate that less than 10% of the CH* is formed by direct excitation. The effects of the scavenging of geminate electrons on the yield of CH*, in terms of the formulationf, = (cY[S])~/(I + (cY[S]~),wheref, is the fraction of geminate electrons scavenged, [SI is the scavenger concentration, and CY and n are constants (parameters), can be described as follows: for perfluoro-n-hexane, n = 1 and a = 35 M-]; for C 0 2 , n = 0.6 and a = 50 M-'; for N20, n = 0.6 and a = 40 M-'. The effects of static quenching of CH* by the electron scavenger are taken into account in the analysis and are estimated from the values measured for the quenching rate constants. We obtain quenching rate constants, corrected for the effects of time-dependent rates, 2.4 X 1O'O M-I s-l for N 2 0 and C 0 2 (3Z20% estimated uncertainty) and 0.71 X 10" M-' s-I for perfluoro-n-hexane ( 3~1 5 % estimated uncertainty). IntroductionWhen liquid hydrocarbons are subjected to ionizing radiation, excited states can be formed instantly by direct excitation and almost instantly (in ca. 100 ps...
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