Resonance Raman spectroscopy excited-state spectroscopic probes

The most widely used vibrational spectroscopic technique is time-resolved resonance Raman spectroscopy (TR ) [65]. This has been used successfully to obtain structural information about organic excited states in SCCO2. McGar-vey and co-workers probed the excited triplet state of anthracene in SCCO2 [66]. However, TR experiments involve data collection over many laser pulses, with all of the problems associated with secondary photolysis. These problems have prevented TR being used effectively to follow chemical reactions apart from highly photoreversible processes. To our knowledge, TR has not yet been used to follow chemical reactions in SCFs. Recently, however. 

Besides various detection mechanisms (e.g. stimulated emission or ionization), there exist moreover numerous possible detection schemes. For example, we may either directly detect the emitted polarization (oc PP, so-called homodyne detection), thus measuring the decay of the electronic coherence via the photon-echo effect, or we may employ a heterodyne detection scheme (oc EP ), thus monitoring the time evolution of the electronic populations In the ground and excited electronic states via resonance Raman and stimulated emission processes. Furthermore, one may use polarization-sensitive detection techniques (transient birefringence and dichroism spectroscopy ), employ frequency-integrated (see, e.g. Ref. 53) or dispersed (see, e.g. Ref. 54) detection of the emission, and use laser fields with definite phase relation. On top of that, there are modern coherent multi-pulse techniques, which combine several of the above mentioned options. For example, phase-locked heterodyne-detected four-pulse photon-echo experiments make it possible to monitor all three time evolutions inherent to the third-order polarization, namely, the electronic coherence decay induced by the pump field, the djmamics of the system occurring after the preparation by the pump, and the electronic coherence decay induced by the probe field. For a theoretical survey of the various spectroscopic detection schemes, see Ref. 10. 

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