Using a streak camera to resolve the motion of molecular excited states with picosecond time resolution and 150 nm spatial resolution

A one-dimensional streak camera imaging (1D-SCI) experiment to visualize the spatial motion of luminescent excited-state species in condensed phase systems is presented. A femtosecond pulse is focused through an objective lens to create a localized excited-state population, using either one-photon or two-photon excitation conditions. The time-dependent emission from this spot is then imaged onto the horizontal entrance slit of a streak camera. The resulting one-dimensional projection of the emission spot is detected with picosecond time resolution. Using this setup, the ability to resolve broadening of the excitation spot due to diffusive motion on the order of 150 nm is demonstrated. The 1D-SCI technique is successfully employed to image the translational diffusion of excited-state platinum octaethylporphyrin molecules in a toluene solution at various concentrations. In addition, the absence of long-range (> 1 mu m) electronic energy transfer in the polymer polyvinylcarbazole is confirmed. In both of these samples, the absorption and emission spectra are well-separated, and the luminescence can escape the sample without reabsorption playing a role. Preliminary experiments on disodium fluorescein, a molecule with a small fluorescence Stokes shift, exhibit anomalously rapid broadening. This is most likely due to fluorescence reabsorption and re-emission, although the simplest model of nonradiative transport fails to capture the observed behavior. In principle, changes in the optical arrangement could improve the spatial resolution of this experiment to approximately 60 nm, given the current experimental signal-to-noise ratio. Other potential improvements and limitations of the technique are also discussed.