Femtosecond Stimulated Raman Spectroscopy
Transient absorption measurements provide an overview of the number of states or intermediates formed after photoexcitation of a molecule. In addition, such experiments often allow to determine the associated rise and decay times. Bright states can be characterized particularly well by time-resolved fluorescence measurements. Therefore, a combination of both methods allows in many cases e.g. the differentiation of singlet and triplet bands in transient absorption. However, they do not provide direct structural information. In order to obtain such information, vibrational spectroscopy methods (IR or Raman) can be used.
In our group a technique called femtosecond stimulated Raman spectroscopy (FSRS) is used to observe ultrafast photoreactions. In this technique, the sample is first excited with an ultrashort light pulse, as in transient absorption. At a certain delay time, two additional light pulses hit the sample simultaneously, which is called Raman pump and Raman sample. The Raman-probe pulse is identical to the interrogation pulse of the transient absorption, i.e. short in time and spectrally broadband. It is detected by a diode array after passing through the sample. The more intense Raman pump pulse, on the other hand, is spectrally as narrow-band as possible (approx. 10 cm-1 FWHM), since it determines the spectral resolution of the experiment. If both Raman pulses pass the sample simultaneously, a stimulated Raman effect makes intensive Raman bands visible.
With FSRS we elucidate among other things the reaction mechanisms and speeds of organic photoreactions. One example is the oldest known photoreaction of o-nitrobenzaldehyde (see figure), in which a ketene intermediate could be clearly detected.