Pump pulse time-resolved femtosecond dynamics

So far we have exclusively discussed time-resolved absorption spectroscopy with visible femtosecond pulses. It has become recently feasible to perfomi time-resolved spectroscopy with femtosecond IR pulses. Flochstrasser and co-workers [M, 150. 151. 152. 153. 154. 155. 156 and 157] have worked out methods to employ IR pulses to monitor chemical reactions following electronic excitation by visible pump pulses these methods were applied in work on the light-initiated charge-transfer reactions that occur in the photosynthetic reaction centre [156. 157] and on the excited-state isomerization of tlie retinal pigment in bacteriorhodopsin [155]. Walker and co-workers [158] have recently used femtosecond IR spectroscopy to study vibrational dynamics associated with intramolecular charge transfer these studies are complementary to those perfomied by Barbara and co-workers [159. 160], in which ground-state RISRS wavepackets were monitored using a dynamic-absorption technique with visible pulses. 

Initial work with respect to Fe(CO)5 photodissociation includes the work mentioned previously by Waller [37] and Seder [31] who studied the state-resolved photochemical breakdown. A number of smdies have been reported which looked at the ultrafast (i.e., sub-picosecond) photodissociation dynamics of Fe(CO)5. Examples of such smdies include that of Banares et al. [57], which looked at the photodissocation dynamics of Fe(CO)5 in a molecular beam using femtosecond laser pulses via two-photon pumping at 400 nm followed by non-resonant ionisation at 800 nm. Detection of the photoproducts was by means of a time-of-flight mass-spectrometer. The timescale for the dissociation of the CO ligands was measured, and it was found that Fe(CO)4 was formed after 20 5 fs, Fe(CO) formed after 100 fs, and complete dissociation of the metal and all ligands sometime after 230 fs. 

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