Abstract

We use the charge transfer to solvent (CTTS) transition of OH in water to probe the reorientation of OH ions in water(l-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 mn, 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 enviromnents 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 benclunark for future theoretical investigations.

© 2009 IEEE