Reorientation of hydroxide ions in water

We use the charge transfer to solvent (CTTS) transition of OH
⁻
in water to probe the reorientation of OH
⁻
ions in water(1–3). The CTTS transition corresponds to an electronic excitation of OH
⁻
to a quasi-bound state supported by the water molecules in the hydration sphere of the ion. Using a linearly polarized femtosecond pump pulse, with a wavelength of 200 nm, we can efficiently excite the CTTS state of OH
⁻
. As the light pulse preferably excites molecules aligned along the direction of polarization, the orientational distribution of the unexcited OH
⁻
ions becomes anisotropic after the excitation. The decay of the anisotropy, by picosecond reorientation of the OH
⁻
ions, can be measured by probing the CTTS transition with a time delayed pulse, polarized either along or orthogonal to the pump pulse. A key property of this technique, different from femtosecond IR techniques, is that we are selectively probing the motion of the ground state OH
⁻
ions located in molecular environments unaffected by the strong pump pulse. In other words, we are probing the motion of OH
⁻
in thermal equilibrium. Furthermore, as the CTTS transition is localized on the OH
⁻
ion, we measure a local molecular reorientation and not a distributed effect. We perform the experiments over a range of temperatures and measure reorientation times and extract activation energies, thus providing a set of experimental data that can be used as benchmark for future theoretical investigations.