Chemical reactions femtochemistry

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]. 

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

FEMTOCHEMISTRY CHEMICAL REACTION DYNAMICS AND THEIR CONTROL... 

The fastest that anything happens in a chemical reaction is on a time scale of approximately 1 femtosecond (1 fs = 10 15 s). That is the time it takes for a bond to stretch or bend, and perhaps break. If we could follow atoms on that time scale, we could make a movie of the changes in molecules as they take part in a chemical reaction. The new field of femtochemistry, the study of very fast chemical processes, is bringing us closer to realizing that dream. Lasers can emit very short but intense pulses of electromagnetic radiation, so they can be used to study processes on very short time scales. 

Important classes of chemical reactions in the ground electronic state have equal parity for the in- and out-going channels, e.g., proton transfer and hydride transfer [47, 48], To achieve finite rates, such processes require accessible electronic states with correct parity that play the role of transition structures. These latter acquire here the quality of true molecular species which, due to quantum mechanical couplings with asymptotic channel systems, will be endowed with finite life times. The elementary interconversion step in a chemical reaction is not a nuclear rearrangement associated with a smooth change in electronic structure, it is aFranck-Condon electronic process with timescales in the (sub)femto-second range characteristic of femtochemistry [49],... 

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]). 

Femtochemistry following Zewail s work has become a Buzz word in experimental physical chemistry research today. It has changed our view of chemical reactions and we can ask detailed questions which could not be asked before. 

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. 

The emphasis in Part 1 has been on an introduction to chemical kinetics and the analysis of reaction mechanism. A main theme has been to describe how experimental measurements of reaction rates, both as a function of concentration and temperature, can provide information on the mechanisms of chemical reactions. The coverage has been wide-ranging from fundamental well-established concepts to leading-edge research in femtochemistry. 

The femtosecond laser technique has been applied to unravel the mechanisms of many chemical reactions and biological processes such as photosynthesis and vision. It has created a new area in chemical kinetics that has become known as femtochemistry. 

The 1999 Nobel Prize in (Zhemistry was awarded to Zewail for pioneering the field of femtochemistry. In its citation for the award the Nobel committee noted that in our quest for ever faster probes, "we have reached the end of the road no chemical reactions take place faster than this."... 

One of the most exciting possibilities of ultrafast laser techniques is to follow the course of fundamental chemical reactions on the relevant timescale at which they occur. Previously, it was only possible to know the individual states of molecules A and B before reacting and the final state of the compound molecule AB. In contrast, the details of the chemical reaction can now be followed on a femtosecond scale with information on how chemical bonds are formed and broken. In particular, the existence of transition states has been demonstrated. This new field of science is frequently referred to as femtochemistry [9.191-9.204], for which A. Zewail was awarded a Nobel prize in chemistry (1999). 

Experimental methods of femtochemistry are based on the achievements of femtosecond spectroscopy (see Section 3.2.11). Three main directions of this new area can be distinguished dynamics of intramolecular process and transition state during chemical transformation kinetics of superfast chemical reactions and control of the intramolecular dynamics and elementary chemical act. These three directions are briefly described in the next sections. The examples are taken from the review by A. Zewail. 

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. 

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. 

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. 

The first section of this book covers liquids and. solutions at equilibrium. I he subjects discussed Include the thcrmodvnamics of solutions, the structure of liquids, electrolyte solutions, polar solvents, and the spectroscopy of solvation. The next section deals with non-equilibrium properties of solutions and the kinetics of reactions in solutions. In the final section emphasis is placed on fast reactions in solution and femtochemistry. The final three chapters involve important aspects of solutions at interfaces. Fhese include liquids and solutions at interfaces, electrochemical equilibria, and the electrical double layer. Author W. Ronald Fawcett offers sample problems at the end of every chapter. The book contains introductions to thermodynamics, statistical thermodynamics, and chemical kinetics, and the material is arranged in such a way that It may be presented at different levels. Liquids, Solutions, and Interfaces is suitable for senior undergr.iduates and graduate students and will be of interest to analytical chemists, physical chemists, biochemists, and chemical environmental engineers. 

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