The multi-pulse ultrafast transient absorption spectrometer

With multi-pulse control spectroscopy, the course of light-induced molecular events is ‘controlled’ by applying additional stimuli at well-defined moments during the reaction. The power of the multi-pulse method lies in the ability to use a second laser pulse (or third, etc.) to selectively remove or transfer population of a transient reaction species with carefully timed and color-adjusted laser pulses, thus disentangling complex elementary events in physics, chemistry and biology.

The Multi-pulse ultrafast transient absorption spectrometer involves an amplified Ti:sapphire laser system (broadband Vitesse-short-pulse Legend, Coherent, Mountain View) of 45 fs pulse duration and 2.5 W output power operating at 1 kHz, equipped with three independently tunable, computer-controlled OPAs (Coherent OperA). Two OPA's have a visible-near IR output for tunable excitation from 475 nm to 2 micron ensuring the wide tunability of its pump and control pulses. Detection takes place with a white light continuum in a multichannel fashion on a shot-to-shot readout basis. A third OPA provides a mid-IR output which will enable the detection of spectral changes in the mid-infrared to selectively probe the dynamic structure of (bio)molecules. This facility will be installed in the near future.

In addition, a novel ultrafast vibrational technique called femtosecond stimulated Raman spectroscopy (FSRS) has been implemented in the multi-pulse setup. In ‘classical’ time-resolved Raman experiments, where two optical pulses are used, the time-resolution is always limited to a few picoseconds since otherwise the spectral resolution would be too low to resolve the individual vibrational frequencies. With FSRS this problem is overcome by using three pulses. A femtosecond excitation pulse is used to excite the sample, stimulated Raman scattering is induced by a narrow-bandwidth pulse of picosecond duration and probed by a short, broad-bandwidth white-light continuum pulse. The time-resolution in this experiment is determined by the duration of the first pulse and the probe pulse. The spectral resolution is determined by the Raman pump pulse. resulting in temporal and spectral resolutions of 100 fs and <10 cm^-1 respectively. To perform FSRS, the Multi-pulse ultrafast transient absorption spectrometer has been equipped with a narrowband Second Harmonic Bandwidth Compressor (SHBC) in combination with a picosecond OPA (TOPAS, both by Light Conversion, Vilnius ) to provide tunable Raman excitation.
Contact Person at LCVU: John Kennis, email: john@few.vu.nl