FAR-ULTRAVIOLET SPECTRA OF HYDROGEN AND HYDROXYL RADICALS FROM PULSE RADIOLYSIS OF AQUEOUS SOLUTIONS . DIRECT MEASUREMENT OF RATE OF H + H

Pulse radiolytic absorption transients have been observed in aqueous solutions between 200 and 300 nm using an 11-MeV Linac and an optical detection system that allowed accurate measurements (a) down to 200 nm and (b) 0.2 μsec after the electron pulse. With 10-3 M HClO4 + 0.027 M H2 (p(H2) = 35 atm) transients with second-order decay were observed which had amplitudes that decayed monotonically in the region from 200 to 240 nm. Assigning these transients to free H atoms, the molar decadic absorptivities ∈ at 200, 210, and 240 nm of H were found to be 900, 560, and 0 M-1 cm-1, respectively, and 2kH+H = (1.55 ± 0.10) × 1010 M-1 sec-1 from measurements at 200 and 210 nm. The transients could be completely quenched by addition of O2 resulting in a species with the absorption spectrum of HO2. Furthermore, the transient at 210 nm was not affected when HClO4 was left out of the H2-saturated solution and N2O (>2 × 10-3 M) was added instead. The apparent OH transient in 2 × 10-3 M N2O (no H2) decayed according to second-order kinetics with a calculated rate constant that after correction for the reaction of H with OH was found to be (1.04 ± 0.10) × 1010 M-1 sec-1 independent of the wavelength used. The calculated ∈ for OH showed, after correction for the absorbance of H2O2, H, and OH-, one broad absorption maximum near 230 nm with ∈ 530 M-1 cm-1 and one below 200 nm. The measurements at 200 nm had to be corrected for a substantial contribution from OH- to the observed optical absorption. The calculated values of ∈H and ∈OH account quantitatively at all wavelengths used for the initial absorption of the transients in 10-3 M HClO4 (no H2) if it is assumed that H3O, if formed, decomposes to yield H + H2O after no longer than 0.2 μsec. The light absorption of aqueous solutions of H and to some extent of OH at 200 nm is attributed to a red shift of the water absorption continuum beginning at 186 nm, caused by a partial electron transfer from the first excited singlet state of water to a neighboring H or OH free radical in analogy with the optical transition associated with the β bands in alkali halide crystals.