Femtosecond flash photolysis

The femtosecond (10-15 s fs) is arguably the final limit in the time-scale of chemical processes. The duration of a molecular vibration is of the order of 100 fs, and even an electronic transition takes some 1 fs (this is of course the 

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Wulff M, Schotte F, Naylor G, Bourgeois D, Moffat K, Mourou G (1997) Time-resolved stmctures of macromolecules at the ESRF single-pulse Laue diffraction, stroboscopic data collection and femtosecond flash photolysis. Nucl Instr Meth Phys Res A 398 69-84... 

Fig. 8.10 a Differential absorption spectra (visible and near-infrared) obtained upon femtosecond flash photolysis (387 nm) of 4b in nitrogen-saturated THF solutions with several time delays between 0 and 20 ps at room temperature and b between 0 and 1600 ps at room temperature... 

Fig. 9.6 a Differential absorption spectrum (visible and near-infrared) obtained upon femtosecond flash photolysis (477 nm) of solutions of monomer 9b in nitrogen-saturated THF with time delays between 0 and 3000 ps at room temperature (black = 0 ps, red = 1 ps, and orange = 2900 ps). b Differential absorption spectrum (visible and near-infrared) obtained upon femtosecond flash photolysis (477 nm) of solutions of dimer 9c in nitrogen-saturated THF with several time delays between 0 and 3000 ps at room temperature (black = 0 ps, red = 1 ps, and orange = 2900 ps)... 

Femtosecond flash photolysis studies on Q-state CdS [107] indicate that reaction (4a) proceeds via two recombination processes a 50 ps decay at low excitation intensities, postulated to correspond to geminate e h+ recombination, and a faster 2 ps decay at higher flash fluences, corresponding to non-geminate or possibly three body Auger charge carrier recombination. Other studies by Nosaka and Fox [118] indicate that the second order rate coefficient for electron-hole recombination within CdS particles is of the order 9 x 10 t7 m3 s l. 

Methylnitrene is the simplest of all alkylnitrenes. Photolysis of methyl azide (CH3N3, 13) does not produce 1NCH3 ( 14) as a trappable species, and even attempts to detect it by femtosecond flash photolysis have failed.Triplet methylnitrene was not formed on direct irradiation of 13, and only methyleneimine (15), the product of formal isomerization of 14, was detected in cryogenic matrices. 

FIGURE 10 Schematic diagram illustrating layout of femtosecond flash photolysis technique. The pump and probe pulses are separated in time by adjusting the light path of the latter. Beams of molecules in the sample tube are excited or dissociated by the pump pulse, and the fragments monitored by the probe pulse on its path to the detector. Reproduced with permission from Scientific American (see Bibliography). 

FIGURE 11 Femtosecond flash photolysis of gaseous sodium iodide. As the excited ion-pair vibrates, it gradually decays (lower curve) and the resulting free atoms are detected (upper curve). 

Techniques have now been developed for studying the course of chemical change in great detail. In particular, femtosecond flash photolysis has been used to observe transition species, notably by Phihp R. Brooks and by Ahmed H. Zewail. 

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