Femtosecond transition-state spectroscopy

Lee S-Y 1995 Wave-packet model of dynamic dispersed and integrated pump-probe signals in femtosecond transition state spectroscopy Femtosecond Chemistry ed J Manz and L Wdste (Heidelberg VCH)... 

A. H. Zewail If we solve for the molecular Hamiltonian, we will be theorists I do, of course, understand the point by Prof. Quack and the answer comes from the nature of the system and the experimental approach. For example, in elementary systems studied by femtosecond transition-state spectroscopy one can actually clock the motion and deduce the potentials. In complex systems we utilize a variety of template-state detection to examine the dynamics, and, like every other approach, you/we use a variety of input to reach the final answer. Solving the structure of a protein by X-ray diffraction may appear impossible, but by using a number of variant diffractions, such as the heavy atom, one obtains the final answer. 

Bowman, R.M., Dantus, M., and Zewail, A.H. (1990). Femtosecond transition-state spectroscopy of iodine From strongly bound to repulsive surface dynamics, Chem. Phys. Lett. 161, 297-302. 

R. M. Bowman, M. Dantus, and A. H. Zewail, Chem. Phys. Lett., 161, 297 (1989). Femtosecond Transition-State Spectroscopy of Iodine From Strongly Bound to Repulsive Surface Dynamics. 

Figure 16.6 Femtosecond transition-state spectroscopy the figure shows the photodissociation of ICN femtosecond pump-probe technique (see text for details). Data adapted from Rosker et at, J. Chem. Phys., 6113, with permission of the American Institute of Physics...

M. Danfus, G. Roberts Femtosecond transition state spectroscopy and chemical reaction dynamics. Commen. At. Mol. Rhys. 26, 131 (1991)... 

There has already been a great deal of preliminary work on the dynamics and spectroscopy of the transition state [74-76], but Ref 73b now signals the beginning of a new era, that of real time femtosecond transition-state spectroscopy/dynamics. 

Figure 9.8 Femtosecond Transition State (FTS) spectroscopy of I2. By adjusting the center wavelength of the pump pulse (Ai) a wavepacket is launched on the repulsive wall of the weakly bound A3Ifiu state (Ai 700 nm, wavepacket labelled a), the Franck-Condon vertical excitation region of the B3If + state (Ai 620 nm, wavepacket labelled b), or on the repulsive wall of the B3If0+ state (and the repulsive 1Ifu state) near the I(2 P3/2) +1 P1/2)...

The construction of such a TDMEP was in harmony with femtosecond experiments of "transition state spectroscopy" that had been done in the late 1980s [58, 59]. It is also relevant to phenomena in current ultracold collision physics that go under the name "photoassociation" [60]. 

Kovalenko S A, Ernsting N P and Ruthmann J 1996 Femtosecond hole-burning spectroscopy of the dye DCM in solution the transition from the locally excited to a charge-transfer state Chem. Phys. Lett. 258 445-54... 

We shall consider just two examples of the use of femtosecond lasers in spectroscopy. One is an investigation of the transition state in the dissociation of Nal and the other concerns the direct, time-based observation of vibrational energy levels in an excited electronic state of I2. 

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." ... 

Note, however, that the 1999 Nobel Prize for Chemistry was awarded to A. Zewail for his studies of the transition states of chemical reactions using femtosecond spectroscopy (Academy s citation, October 12,... 

History of physical organic chemistry is essentially the history of new ideas, philosophies, and concepts in organic chemistry. New instrumentations have played an essential role in the mechanistic study. Organic reaction theory and concept of structure-reactivity relationship were obtained through kinetic measurements, whose precision depended on the development of instrument. Development of NMR technique resulted in evolution of carbocation chemistry. Picosecond and femtosecond spectroscopy allowed us to elucidate kinetic behavior of unstable intermediates and even of transition states (TSs) of chemical reactions. 

Solvation effects at the transition-state level of electron-transfer reactions is a field that is opening up by means of clusters reactions. Reactions within clusters (see Section 2.7) or at the surface of clusters (see Section 2.8) provide an experimental tool to observe the effect of gradual solvation on electron-transfer reactions. Femtosecond methods appear highly valuable for characterizing the electron transfer in reactions within clusters, since in neutral clusters the electrostatic effects of solvation will show up intensely at the electron-transfer level and for a short period of time, before the products are fully developed. In the gas phase the solvation of electrons is directly sensed by photoelectron spectroscopy, as exemplified in the (nonreactive) case of ultrafast solvation of electrons in water clusters [305]. 

An extremely exciting achievement is the use of real time femtosecond probing of transition states in chemical reactions . The method, which shows promise for the study of bimolecular and unimolecular reactions, has already been applied to the decomposition of ICN. Rulliere has discussed the application of picosecond spectroscopy to study a variety of elementary processes. 

For his studies of the transition states of chemical reactions using femtosecond spectroscopy. ... 

Zaric and HalF predicted a weakly bound structure and a strongly bound intermediate in the reaction path of the activation of methane with the fragment [Tp Rh(CO)] [Tp = HB(3,5-dimethylpyraz()lyl)5 before oxidative addition. Subsequent studies using (microsecond- to femtosecond-scale) time-resolved IR spectroscopy proved that the second intermediate has a strongly bound r -C,H moiety. Similar structures were determined for the transition states in the C-H activation of methane by the [CpM(PH3)CH3)] (M = Rh, Ir) fragment by ab initio orbital theory methods. ... 

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