Laser Femtosecond-Chemistry

Chemical reactions are based on atomic or molecular collisions. These collisions, which may bring about chemical-bond formation or bond breaking, occur on a time scale of 10 to 10 s. In the past, the events that happened in the transition state between reagents and reaction products could not be time resolved. Only the stages before or after the reaction could be investigated [15.42]. 

The study of chemical dynamics concerned with the ultrashort time interval when a chemical bond is formed or broken may be called real-time femtochemistry [15.43]. It relies on ultrafast laser techniques with femtosecond time resolution [15.44]. 

Assume a dissociating molecule with a fragment velocity of lO m/s. Within 0.1 ps the fragment separation changes by 

The molecule ABC is excited by a pump photon hvi into the dissociating state with a potential curve Vi (7 ). A second probe laser pulse with a tunable wavelength X2 is applied with the time delay At. If X2 is tuned to a value that matches the potential energy difference /zv = V2( ) V (R) at a selected distance R between A and the center of BC, the probe radiation absorption a (A.2, At) shows a time dependence, as schematically depicted in Fig. 15.8b. When A.2 is tuned to the transition BC (BC) — V2 R = 00) — Vi(/ = 00) of the completely separated fragment BC, the curve in Fig. 15.19c is expected. These signals yield the velocity v R) of the dissociation products from which the energy difference V2 R) — Vi(i ) can be derived. 

The experimental arrangement for such femtosecond experiments is exhibited in Fig. 15.9. The output pulses from a femtosecond pulse laser (Sect. 11.1.5) are focused by the same lens into the molecular beam. The probe pulses are sent through a variable optical-delay line and the absorption a At) of the probe pulse as a function of the delay time At is monitored via the laser-induced fluorescence. Cutoff filters suppress scattered laser light. 

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Another example is the detailed investigation of the femtosecond dynamics of iron carbonyl Fe(CO)5 [1417], where the photodissociation after excitation with 267 nm pulses was studied by transient ionization. Five consecutive processes with time constants 21, 15, 30, 47, and 3300 fs were found. The first four short-time processes represent ionization from different excited configurations, which are reached by a pathway from the initially excited Franck-Condon region to other configurations through a chain of Jahn-Teller-induced conical intersections. The experiments also showed that intersystem crossing to the triplet ground states of Fe(CO)4 and Fe(CO)3 takes more than 500 ps. More detailed information on experiments on laser femtosecond chemistry can be found in [1418, 1419]. 

More detailed information on experiments on laser femtosecond chemistry can be found in [15.25]. 

For a time there was concern over just how the uncertainty principle would limit what could be achieved with ultrafast laser pulses. The potential problem turned out to be chimerical, not a problem, once coherence came to be appreciated. This conceptual advance was essential to the successful development of experimental femtosecond chemistry it did not progress through better lasers and nonlinear optical tricks and faster computers alone. 

R. de Vivie-Riedle and J. Manz Prof. Neumark s question about detecting the hole burning in the nuclear wavepacket of the electronic ground state is very stimulating. In this context, we have developed a scheme for detecting the hole in the wavepacket by a femtosecond chemistry laser experiment that involves two laser pulses Our explanation will be for the specific system K2, but more general applications for other systems are obvious ... 

R. D. Levine As emphasized by the speakers on femtosecond pumping schemes, an important point is that the initial excitation is localized within the Franck-Condon regime. The question is whether the sheer localization can be used to advantage to induce laser-selective chemistry (K. L. Kompa and R. D. Levine, Acc. Chem. Res. 27, 91 (1994)]. As we understand better the topography of potential-energy surfaces for polyatomic molecules, it may be possible to launch the system with such initial conditions that it will, of its own accord, proceed to cross a particular transition state and so exit toward a particular set of products. 

Recent progress in laser technology has led to the widespread use of ultrafast lasers with pulse widths shorter than the vibrational periods of most chemical bonds. A localized state, called a nuclear wave packet, is created on a potential surface by exciting a molecule with ultrashort pulses of radiation. The time-evolution of such wave packets can be directly utilized to observe the transition states of chemical reactions. This development is one of the major accomplishments of femtosecond chemistry. 

In the present chapter, the rapidly growing subject of atoms in strong laser fields has been briefly described, the main emphasis being on the novel effects which have been observed. Several aspects of the problem have not been discussed for example, above 1020 Wcm-2, relativistic effects will become important, although these have not yet been observed. Similarly, we have omitted any discussion of coherent control in two-colour excitation and femtosecond chemistry. 

Time-resolved spectroscopy with ultrashort laser pulses gives, for the first time, access to a direct view of the short time interval in which molecules are formed or fall apart during a collision. This femtosecond chemistry is discussed in Sect. 10.1.3. 

The investigation and explanation of the properties of these materials overlap with the interests of physical chemists, too. There are catalytic possibilities, which always perks up the ears of a good ki-neticist (a chemist studying reaction rates). In the late 1900s there are plenty of other things to keep the kineticist interested Lasers now permit kinetic investigations of processes that occur in a quadrillionth of a second—femtosecond chemistry which has been used to probe the elusive transition state and fast intermolecular interactions. Also supported by all the advances in techniques and instrumentation, the field of biochemistry has really taken off in the late 1900s, and it has had... 

D. Goswami, C.W. Hillegas, J.X. Tull, and W.S. Waren, Generation of Shaped Femtosecond Laser Pulses New Appoaches to Laser Selective Chemistry in Femtosecond Reaction Dynamics, D.A. Wiersma (ed.) (North-HoIIand, Amsterdam, 1994), p 291. 

Poitrasson, X.L., Mao, S.S., Freydier, R., Russo, R.E. 2003. Comparison of ultraviolet femtosecond and nanosecond laser ablation inductively coupled plasma mass spectrometry analysis in glass, monazite, and zircon. Analytical Chemistry, 75, 6184-6190. 

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