C. Time-resolved IR spectroscopy (TRIR) and Time-resolved Fourier Transform Infrared (TRFTIR) Spectroscopy
- Complimentary to the Raman technique is the time-resolved infrared
(TRIR) spectroscopy. In some molecules, fluorescence might be so strong
that Raman studies are very difficult or impossible. In addition, changes
in specific protein residues during photobiological processes can be
followed by TRIR spectroscopy in conjunction with site-directed
mutation.
- There are two ways to obtain a transient IR spectrum. The first one is the two-laser pulse transient optical spectroscopy where an IR pulse from an OPO/OPA is used as the probe. A spectrum is constructed by performing the transient absorption at many different infrared wavelengths. The advantage of this technique is that the time resolution is limited only by the laser pulse width, which is < 200 fs for the Ti:sapphire system. Its disadvantage is that the process of constructing the spectrum point by point is time consuming.
- Another way to obtain the transient IR spectrum is to use a step scan FTIR spectrometer. The current time resolution of the Bruker commercial instrument we have is 5 ns, which will be upgraded to 100 ps in the near future. At this instrument the spectrum is obtained by a step scan procedure which is controlled by a computer. Another advantage of this instrument is the signal-to-noise ratio is better for longer time resolutions due to the averaging procedure available. The disadvantage of this technique is the time resolution being lower than that for the two laser experiment. With both techniques, time-resolved IR spectrum of a transient species with lifetimes of ~200 fs to ms can be obtained.
- There are two ways to obtain a transient IR spectrum. The first one is the two-laser pulse transient optical spectroscopy where an IR pulse from an OPO/OPA is used as the probe. A spectrum is constructed by performing the transient absorption at many different infrared wavelengths. The advantage of this technique is that the time resolution is limited only by the laser pulse width, which is < 200 fs for the Ti:sapphire system. Its disadvantage is that the process of constructing the spectrum point by point is time consuming.
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