Pump-Probe Spectroscopy

Transient absorption spectroscopy (TAS) is a non-linear spectroscopic technique wich allows to follow the time evolution of the excited state of a sample created by the interaction with a short laser pulse. An intense laser pulse, the pump, interacts with the sample creating an electronic or vibrational excitation. After a variable time delay, a less intense probe pulse  reads out the absorption spectrum of the sample after the arrival of the pump.

The setup used for the TAS measurements in the visible range is based on a Ti:sapphire regenerative amplifier (Amplitude Pulsar) pumped by a Ti:sapphire home-made oscillator. The system produces 100 fs pulses at 815 nm, 1 kHz repetition rate and average power of 450-500 mW. A home-made Non Linear Optical Parametric Amplifier (NOPA) is available for the generation of the excitation pulses in the visible spectral range (480-750 nm). Second or third harmonic generation of the fundamental laser output is also possible.

The broadband probe pulse is routinely generated by focusing a small portion of the fundamental laser output radiation on a 2 mm thick calcium fluoride or sapphire window. Pump-probe delays are introduced by sending the portion of fundamental beam used for the probe generation through a motorized stage. After crossing the pump beam at the sample position, the white light continuum is sent to a flat field monochromator coupled to a home-made CCD detector.

Samples are usually placed in commercial quartz cells  mounted on a movable stage to avoid sample photo degradation and multiple photon excitation. Customized sample holders with several optical paths are available, as well as holders for solid state samples.

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