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Ultrafast Laser Spectroscopy

The Ultrafast Laser Spectroscopy facility in the Frick Chemistry Laboratory offers state-of-the-art equipment for measuring dynamics in photoactive materials on femtosecond-to-millisecond timescales. The facility in Frick Chemistry Laboratory is open to members of any department at Princeton University. Users from outside the University are welcome, as well. The manager of the facility performs the measurements, as well as offers training for long-term users.

Ultrafast laser spectroscopy is a standard method in a number of scientific fields (physics, chemistry, molecular biology, engineering etc.) and is used both as a primary research technique, as well as an accessory characterization tool for supporting the interpretation of the experiments. This technique provides detailed information on the dynamic processes in photoactive materials (in solid and liquid phase). These processes can be excited state relaxation, both inter and intramolecular energy/electron transfer, conformational change (e.g. isomerization), chemical reaction dynamics.

The facility allows measurements of time-resolved change of the absorption spectra (Transient Absorption) with femtosecond (10-14 s) time resolution and up to microsecond timescales with spectral windows in UV-VIS-NIR, allowing probing all possible excited state transitions. It consists of LIBRA Ti:Sapphire amplifier (Coherent Inc.), OPerA Solo optical parametric amplifier (Coherent Inc.), Helios transient absorption spectrometer (Ultrafast Systems), and EOS transient absorption spectrometer (Ultrafast Systems). The Ti:Sapphire amplifier delivers 45fs pulses at 1kHz rate, with total power of 4.0W at 800nm wavelength. Optical parametric amplifier allows tuning of the excitation wavelength in the spectral range from 290 – 2600nm, with average power of 50mW and pulse duration 50fs. Transient absorption spectrometer allows measurements in 350 – 1600nm spectral region (probing range), and on <1ms time range.  A recent addition is the MID-IR system (Ultrafast Systems) for direct probing vibrational modes in molecules (3500cm-1 to 800cm-1).  Allowing for the direct monitoring of molecular bonds on light-initiated processes.  This can even be extended to millisecond timescales for monitoring of diffusion limited processes.

Several options for sample mounting are available in order to prevent photo-bleaching/degradation: stirring, translation, and purging with dry N2.  Helios/EOS software includes dispersion compensation algorithm, anisotropy calculation and other useful tools. The measurements at different excitation intensities, wavelengths or at different temperatures of the sample can be automated. Global analysis of the data (three-dimensional array of time-resolved spectra) allows to map the exact pathways and timescales of the processes occurring in the materials of interest. This software is free and can be downloaded from the internet together with the manual and instructions.

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