THE THEORY OF ULTRAFAST VIBRATIONAL SPECTROSCOPY

This paper discusses the use of ultrafast time-resolved IR spectroscop y to probe vibrational modes that are coupled to photochemically induc ed reactions. A detailed theory is derived that describes the IR probe signals in three different schemes: pulsed-pump, pulsed-probe spectro scopy in which the total power of the transmitted probe pulse is measu red, pulsed-pump, pulsed-probe spectroscopy in which the probe pulse i s spectrally dispersed after the sample and pulsed-pump, cw-probe with gating after the sample. It is found that the three schemes lead to s ubtly different signals which may greatly affect the interpretation of experiments. Also the coherent excitation of vibrational modes that a re both IR and Franck-Condon active is considered.