Development of a total internal reflection ultrafast transient lens method for studying molecular dynamics on an interface.

We have developed the total internal reflection ultrafast transient lens (TIR-UTL) method to detect nonradiative chemical processes at interfaces and surfaces with subpicosecond time resolution. In the TIR-UTL measurements, the evanescent field of a pump beam irradiated under the TIR condition generates a refractive index change. The refractive index change is attributed to changes of the molecular electronic state, of density by molecular orientation/structure change, and of temperature by vibrational relaxation processes. The refractive index change is detected as a change of the power intensity of the probe beam adjusted coaxially with the pump beam. At first, we discuss a theoretical principle of a coaxial configuration in the TIR-UTL measurement. This configuration has an advantage of versatility over the established TIR configuration. Then, we evaluate time resolution of TIR-UTL and obtain a value of less than 400 fs. We measure the ultrafast molecular dynamics of the cationic chromophore Auramine O (AuO) at a silica/water interface. Two slow time constants originating from AuO adsorbed on the silica surface are detected by TIR-UTL. These are attributed to AuO, whose twisting motion is strongly hindered by adsorption on a silica surface.