Transient absorption

Future devices and applications that will use photochemical processes will ultimatively depend on the understanding of transient reactions of species embedded in solid systems. Currently we are establishing experiments with the aim of extending our ultrafast studies on thin, molecular films that allow the characterization of photoinduced reactions on more integrated samples.

Our method of choice is called ‘transient absorption’: A femtosecond laser pump pulse triggers some photochemical reaction in a thin film that is deposited on a substrate of transparent glass. The change in electronic structure of the sample is probed by a time delayed ultrashort probe pulse that has a broad frequency width all over the visible spectrum (“white light pulse”). Transient changes in the valence shell, structure and composition of the molecule is detected via a change in absorption at specific bands with time.

Figure 1: Schematic of the transient absorption setup. Femtosecond pulses are split into two paths (pump and probe). The pump pulse is frequency doubled and induces the photochemical process in the sample. The time delayed probe pulse is focused into a CaF2 window to generate a white light continuum, which is then guided into the sample. Using a Chopper wheel, Delay Stage and spectrum analyzer, the time and wavelength dependent change in absorption is measured.