Spectroscopy ultrafast

Many of the most interesting current developments in electronic spectroscopy are addressed in special chapters of their own in this encyclopedia. The reader is referred especially to sections B2.1 on ultrafast spectroscopy. Cl.5 on single molecule spectroscopy, C3.2 on electron transfer, and C3.3 on energy transfer. Additional topics on electronic spectroscopy will also be found in many other chapters. 

The development of ultrafast spectroscopy has paralleled progress in the teclmical aspects of pulse fomiation [Uj. Because mode-locked laser sources are tunable only with diflSculty, until recently the most heavily studied physical and chemical systems were those that had strong electronic absorption spectra in the neighbourhood of conveniently produced wavelengths. 

These limitations have recently been eliminated using solid-state sources of femtosecond pulses. Most of the femtosecond dye laser teclmology that was in wide use in the late 1980s [11] has been rendered obsolete by tliree teclmical developments the self-mode-locked Ti-sapphire oscillator [23, 24, 25, 26 and 27], the chirped-pulse, solid-state amplifier (CPA) [28, 29, 30 and 31], and the non-collinearly pumped optical parametric amplifier (OPA) [32, 33 and 34]- Moreover, although a number of investigators still construct home-built systems with narrowly chosen capabilities, it is now possible to obtain versatile, nearly state-of-the-art apparatus of the type described below Ifom commercial sources. Just as home-built NMR spectrometers capable of multidimensional or solid-state spectroscopies were still being home built in the late 1970s and now are almost exclusively based on commercially prepared apparatus, it is reasonable to expect that ultrafast spectroscopy in the next decade will be conducted almost exclusively with apparatus ifom conmiercial sources based around entirely solid-state systems. 

Fleming G R 1986 Chemical Applications of Ultrafast Spectroscopy (New York Oxford University Press)... 

Jimenez R and Fleming G R 1996 Ultrafast spectroscopy of photosynthetic systems Biophysical Techniques In Photosynthesis ed J Amesz and A J Hoff (Dordrecht Kluwer) pp 63-73... 

Applications of ultrafast spectroscopy to chemical dynamics, especially in the condensed phase and in proteins. 

For near-field imaging based on nonlinear or ultrafast spectroscopy, light pulses from a femtosecond Ti sapphire laser (pulse width ca. 100 fs, repetition rate ca. 

Recently, Zewail and co-workers have combined the approaches of photodetachment and ultrafast spectroscopy to investigate the reaction dynamics of planar COT.iii They used a femtosecond photon pulse to carry out ionization of the COT ring-inversion transition state, generated by photodetachment as shown in Figure 5.4. From the photoionization efficiency, they were able to investigate the time-resolved dynamics of the transition state reaction, and observe the ring-inversion reaction of the planar COT to the tub-like D2d geometry on the femtosecond time scale. Thus, with the advent of new mass spectrometric techniques, it is now possible to examine detailed reaction dynamics in addition to traditional state properties." ... 

A final study that must be mentioned is a study by Hartmann et al. [249] on the ultrafast spectroscopy of the Na3F2 cluster. They derived an expression for the calculation of a pump-probe signal using a Wigner-type density matrix approach, which requires a time-dependent ensemble to be calculated after the initial excitation. This ensemble was obtained using fewest switches surface hopping, with trajectories initially sampled from the thermalized vibronic Wigner function vertically excited onto the upper surface. 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Ultrafast spectroscopy is so important because it provides dynamical information that is very hard or impossible to access from IR and Raman spectra. For systems with a single chromophore, this dynamical information is often characterized by the frequency TCF,... 

It is more difficult to perform ultrafast spectroscopy on neat H20 (than it is on H0D/D20 or HOD/H20) since the neat fluid is so absorptive in the OH stretch region. One innovative and very informative technique, developed by Dlott, involves IR pumping and Raman probing. This technique has a number of advantages over traditional IR pump-probe experiments The scattered light is Stokes-shifted, which is less attenuated by the sample, and one can simultaneously monitor the populations of all Raman-active vibrations of the system at the same time. These experimental have been brought to bear on the spectral diffusion problem in neat water [18, 19, 75 77],... 

One problem yet to be solved theoretically involves ultrafast echo and pump-probe experiments on H20. Jansen has extended the time-averaging approximation to nonlinear ultrafast spectroscopy [164], meaning that one is now in the position of calculating 2DIR spectra for liquid water, which would allow for direct comparison with results from the exciting new experiments [73, 74]. 

M. M. Martin, P. Plaza, N. Dai Hung, Y. H. Meyer, J. Bourson, and B. Valeur, Photoejection of cations from complexes with a crown-ether-linked merocyanine evidenced by ultrafast spectroscopy, Chem. Phys. Lett. 202, 425 (1993). 

G.R. Fleming, Chemical Applications of Ultrafast Spectroscopy Press, N.Y., 1986). 

Centre for Atom Optics and Ultrafast Spectroscopy, School of Biophysical Sciences and Electrical Engineering, Swinburne University of Technology, Melbourne, Australia 3122... 

Tunable operation with bandwidth-limited sub-10-fs pulses in the visible (550-700 nm) and near infrared (900-1300 nm) was also performed by changing the seed delay with respect to the pump after increasing the seed chirp [10]. The NOPA is one of most useful light sources for ultrafast spectroscopy in the present stage on an extremely short time scale. 

Laboratory of Condensed Phase Ultrafast Spectroscopy, ICMB, BSP, Ecole Polytechnique Federale de Lausanne, CH-1015 Lausanne, Switzerland... 

To summarize, Jean shows that coherence can be created in a product as a result of nonadiabatic curve crossing even when none exists in the reactant [24, 25]. In addition, vibrational coherence can be preserved in the product state to a significant extent during energy relaxation within that state. In barrierless processes (e.g., an isomerization reaction) irreversible population transfer from one well to another occurs, and coherent motion can be observed in the product regardless of whether the initially excited state was prepared vibrationally coherent or not [24]. It seems likely that these ideas are crucial in interpreting the ultrafast spectroscopy of rhodopsins [17], where coherent motion in the product is directly observed. Of course there may be many systems in which relaxation and dephasing are much faster in the product than the reactant. In these cases lack of observation of product coherence does not rule out formation of the product in an essentially ballistic manner. 

To sum up, this chapter has endeavored to show that chemical processes in solution often proceed in a deterministic fashion over chemically significant distances and time scales. Ultrafast spectroscopy allows real-time observation of relative motions even when spectra are devoid of structure and has stimulated moleculear level descriptions of the early time dynamics in liquids. The implication of these findings for theories of solution phase chemical reactions are under active investigation. 

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