Electron Detachment and Relaxation of OH-(aq)

Abstract

Femtosecond transient absorption spectroscopy is used to study the primary reaction dynamics of photoinduced electron detachment of the hydroxide ion in water, OH- (aq). The electron is detached by excitation of OH- (aq) to the charge-transfer-to-solvent (CTTS) state at 200 nm. The subsequent relaxation processes are probed in the spectral range from 193 to 800 nm with femtosecond time resolution. We determine both the time-dependent quantum yields of OH- (aq), OH(aq), and e-(aq), and we observe a transient spectral signature which is assigned to relaxation of hot (OH-)* ions formed via solvent-assisted conversion of the excited CTTS state to OH-. The primary quantum yield of OH(aq) is 65 +/- 5%, while recombination with e-(aq) reduces the yield to 34% after 5 ps and 12% after 200 ps. The yield of hot (OH-)* ions is 35 +/- 5%. Rotational anisotropy measurements of OH- (aq) and OH(aq) indicate a reorientation time for OH- (aq) of 1.9 ps, while no rotational anisotropy is resolved for the OH(aq) radical within our time resolution of 0.3 ps. This is consistent with the notion that OH(aq) radicals formed after electron detachment are only weakly bound to the hydrogen bond network of water. The assignment of the experimental data is supported by a series of electronic structure calculations of simple complexes of OH- (H(2)O)(n).