THE THEORY OF ULTRAFAST VIBRATIONAL SPECTROSCOPY

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