Fs-pump-and-probe spectroscopy on squaraine dyes

In this paper we present our investigations on the first step of the optically induced multistep electron transfer in squarilic systems, i.e. we show the evolution of the transient intramolecular charge transfer state on a fs-timescale. For the chosen pump-and-probe setup a mode-locked Ti-sapphire laser in combination with a regenerative amplifier provides the used 100 fs-laserpulses. As probe light we use white light which is generated in a thin sapphire plate and which allows us to measure the excited state absorption in a wide spectral range (240 nm) simultaneously. When exciting the squaraine molecule with the frequency-doubled laser light at 400 nm, the local charge densities inside the squaraine vary. It can be noticed that an intramolecular electron transfer from the nitrogen towards the squaric acid has taken place.