Femtochemistry reaction dynamics

I had the honor to review the field, as described by the title of this chapter. I would like to take this opportunity here to focus on some concepts that were essential in the development of femtochemistry reaction dynamics and control on the femtosecond time scale. The following is not an extensive review, as many books and articles have already been published [1-12] on the subject, but instead is a summary of our own involvement with the development of femtochemistry and the concept of coherence. Most of the original articles are given in a recent two-volume book that overviews the work at Caltech [5], up to 1994. 

A. H. Zewail, Femtochemistry Chemical Reaction Dynamics and Their Control, Adv. Chem. Phys. 1997, 101, 3, 892. 

The second advantage of a Q-first approach is that it opens the door to an early introduction of the modem and enthralling. Thermodynamics is perceived (with let s admit, some truth) to be pass6 the modem age is built around quantum theory and its implications for atoms, molecules, and materials. If we want to excite our students, then we are more likely to be able to do so with a quantum than with a thermodynamic function. Through the early introduction of quantum ideas we open the door to the presentation of modem topics, including spectroscopy, molecular reaction dynamics, femtochemistry, computation, and the emerging fields embraced by nanotechnology and nanoscience. In short, we have the opportunity to expose our students to the shock of the new. 

FEMTOCHEMISTRY CHEMICAL REACTION DYNAMICS AND THEIR CONTROL... 

R.A. Marcus. In Femtochemistry <5 Femtobiology Ultrafast Reaction Dynamics at the Atomic-Scale Resolutions. Nobel Symposium 101, V. Sundstrom, ed. (London Imperial College Press, 1997), p. 54. 

Kim HJ, Staib A, Hynes JT. In V Sundstrom, ed. Ultrafast Reaction Dynamics at Atomic-Scale Resolution Femtochemistry and Femtobiology. London Imperial College Press, 1998, pp 510-527. 

More recently, the use of picosecond and femtosecond lasers in reaction dynamics opened up the field of femtochemistry, which was pioneered by Zewail [51-54]. The idea of these reactions is to photoinitiate the reactive process in a van der Waals complex. Sometimes, the process that is initiated is a simple dissociation or the isomerization of a free molecule. In each case, the reaction is initiated by a first ultrashort laser pulse (the pump pulse). It is analyzed after a certain delay by a second pulse (the probe pulse). This gives access to the reaction dynamics on the pertinent time-scale where chemical bonds are broken and others are formed. Depending on the system, this typically lasts between a few tenths of femtoseconds to hundredths of picoseconds. Recently the techniques of stereodynamies have been combined by Zewail and co-workers with femtosecond analysis [55, 56] to label specific reaction channels in electron-transfer reactions. 

V. Sundstrdm, ed., Nobel Symposium Book Femtochemistry and Femtobiology Ultrafast Reaction Dynamics at Atomic Scale Resolution, World Scientific, Imperial College Press, London, 1997. 

Frischkom C, Wolf M (2006) Femtochemistry at metal surfaces nonadiabatic reaction dynamics. Chem Rev 106 4207-4233... 

Manz, J. (1997). Molecular wave packet dynamics theory for experiments 1926-1996. In Femtochemistry and femtobiology ultrafast reaction dynamics at atomic-scale resolution (ed. V. Siindstrom), pp. 80-318. Imperial College Press, London. 

Femtosecond lasers represent the state-of-the-art in laser teclmology. These lasers can have pulse widths of the order of 100 fm s. This is the same time scale as many processes that occur on surfaces, such as desorption or diffusion. Thus, femtosecond lasers can be used to directly measure surface dynamics tlirough teclmiques such as two-photon photoemission [85]. Femtochemistry occurs when the laser imparts energy over an extremely short time period so as to directly induce a surface chemical reaction [86]. 

The experimental and theoretical strategies of femtochemistry have provided telling insights on chemical dynamics over the past 15 years. The breakthrough examples and many of the prototypical organic reactions that have been reported already permit some important generalizations. 

A. H. Zewail, Femtochemistry Recent Progress in Studies of Dynamics and Control of Reactions and Their Transition States, J. Phys. Chem. 1996, 100, 12701. 

In a classical Bohr orbit, the electron makes a complete journey in 0.15 fs. In reactions, the chemical transformation involves the separation of nuclei at velocities much slower than that of the electron. For a velocity 105 cm/s and a distance change of 10 8 cm (1 A), the time scale is 100 fs. This is a key concept in the ability of femtochemistry to expose the elementary motions as they actually occur. The classical picture has been verified by quantum calculations. Furthermore, as the deBroglie wavelength is on the atomic scale, we can speak of the coherent motion of a single-molecule trajectory and not of an ensemble-averaged phenomenon. Unlike kinetics, studies of dynamics require such coherence, a concept we have been involved with for some time. 

Femtochemistry at High Pressures The Dynamics of an Elementary Reaction in the Gas-Liquid Transition Region, C. Lienau, J. C. Williamson, and A. H. Zewail, Chem. Phys. Lett. 213, 289 (1993). 

The variety of manifestations in time of coherent development of molecular dynamics also includes such phenomena as mono- and bimolecular chemical reactions. Thus, Seideman et al [342] suggest the idea of governing the yield of a reaction by suddenly creating coherent superposition of two states of the transient complex and applying a second pulse with fixed delay for the dissociation of the complex. The appearance of coherent beats in femtochemistry , in particular, at photodissociation, has been analyzed by Zewail (review [404]). 

Zewail AH. Femtochemistry recent progress in studies of dynamics and control of reactions and their transition states. J Phys Chem 1996 100 12701-12724. 

The invention of schemes to induce and observe dynamical processes in molecules employing ultrashort laser pulses has founded research areas with the names Femtochemistry and Femtobiology [1-8], culminating with the Nobel Prize awarded to Ahmed Zewail for his achievements in femtosecond spectroscopy [9,10]. More recently, the technology of pulse shaping [11,12] has opened up the field of laser control of chemical reactions, until then populated only by theorists [13-17]. This has been an active area of research in recent years [18— 23], and textbooks [24-26] as well as many reviews [27-40] have been published on the topic. 

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