Femtosecond Titanium Sapphire Laser

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Ultrafast spectroscopy enables the investigation of the dynamics of excited states as well as the detailed analysis of reaction mechanisms of light-induced reactions. As intramolecular electron transfer processes take place on time scales down to the fs range (10^(-15) s), a femtosecond laser system is indispensable for performing such ultrafast studies. For this purpose, a basic system consisting of a diode-pumped maintenance-free Ti:sapphire seed laser, a diode-pumped Nd:YLF pump laser, a temperature-stabilized regenerative amplifier and a temperature-stabilized stretcher and compressor unit as well as an optical parametric amplifier (OPA) system is required. The laser pulses of 800 nm with pulse widths of <60 fs obtained with this setup possess a sufficient temporal resolution to characterize the processes described above in e.g. pump-probe experiments. In the aforementioned method, the sample is first optically excited with a spectrally defined excitation pulse and then examined with a broadband white light pulse after a selected delay time Δt. For the analysis of our photoactive samples it is essential that different excitation wavelengths are available. For these reason, a non-collinear optical parametric amplifier (OPA) must be used to extend the original wavelength of the femtosecond laser from 800 nm plus/minus 5 nm to 290-2600 nm so that excitation can subsequently be performed in a broad range as wide as possible (depending on the substance and scientific question). To be able to use the OPA in this broad range and to have the necessary energies of at least 1microJ per pulse at the end, an output power of > 5 mJ at 800 nm is crucial. In this way, transient absorption spectra can be obtained with the required high temporal resolution and good signal-to-noise ratio. The instrument parameters described here ultimately enable the investigation of large variety of different samples and complexes, which play an important role in the research of the participating groups and cooperation partners. As a result, elementary questions such as: "What are the intermediate steps of light-induced reactions and what are their time constants? Is it a singlet or triplet state?" or "Are intramolecular charge transfer processes involved? On which time scales do charge separation and recombination processes occur?" can be answered.