Femtochemistry

By means of femtochemistry, investigation of elementary reactions on a timescale of femtoseconds (10-15s) is possible. The method employs a combination of pulsed-laser and molecular-beam technologies. Investigation of a unimolecular reaction by femtosecond spectroscopy involves two ultra-fast laser pulses being passed into a beam of reactant molecules. 

The first experiments involving the femtosecond timescale were carried out by Zewail on the decomposition of iodocyanide. 

The pump pulse (306 nm) had photons of the wavelength needed for absorption by the ground-state ICN, resulting in production of the 

Most chemical reactions can be slowed down by lowering the temperature. With low-temperature studies it is possible to prolong the lifetimes of the reactive intermediates so that they can be characterised by normal techniques. Matrix isolation allows experiments to be carried out at temperatures as low as 4K, in order to study species, such as radicals, that are produced photochemically at very low temperatures. The initial photoproduct is trapped within a rigid matrix that inhibits diffusion of the reactive species. The matrix material must be  

The most useful matrix materials are solid argon, solid neon and solid nitrogen. 

Branch of physical chemistry that studies what happens in a chemical reaction in time intervals in the order of 10 to 10 s. It has enabled the detection of the transition state and reaction intermediates for formation-dissociation of bonds. 

Some reactions occur very slowly, such as when a nail rusts. Other occur very rapidly, such as when methane is combusted in a Bunsen burner. Studying very fast reactions requires very special techniques, usually involving lasers—devices that produce high-energy bursts of light with very precise frequencies. The study of very fast reactions is one of the most important areas of chemical research, as demonstrated by the fact that the 1999 Nobel Prize in chemistry was awarded to Ahmed H. Zewail of the California Institute of Technology in Pasadena, California. Zewail s studies involve reactions that occur on the femtosecond (10—15 s) time scale—the time scale for molecular vibrations. 

In Zewail s laboratory a strong laser flash of a few femtoseconds duration shines on beams of molecules streaming into a vacuum chamber. The laser beam is tuned to excite all of the molecules to the same state where they are vibrating in unison. Subsequent, weaker laser pulses monitor the concentrations of the reactants, intermediates, and products as the reaction occurs. 

One reaction studied by Zewail is the decomposition of cyclobutane to two ethylene molecules  

A laser spectroscopy laboratory at the California Institute of Technology. 

Zewail has shown that the reaction mechanism involves the breaking of one of the carbon-carbon bonds in cyclobutane to produce a tetramethylene intermediate  

The ability to follow chemical reactions at the molecular level has been one of the most relentlessly pursued goals in chemistry. Accomplishing this goal wiU allow chemists to understand when a certain reaction occurs and the dependence of its rate of reaction on temperature and other parameters. On the practical side, this information will help chemists control reaction rates and increase reaction yields. A complete understanding of reaction mechanisms requires a detailed knowledge of the activated complex (also called the transition state). The transition state, however, is a highly energetic species that could not be isolated because of its extremely short lifetime. 

The second mechanism has two steps, with an intermediate where the dot represents an unpaired electron  

The Caltech researchers initiated the reaction with a pump laser pulse, which energized the reactant. The first probe pulse hit the molecules a few femtoseconds later and was followed by many thousands more, every 10 fs or so, for the duration of the reaction. Each probe pulse resulted in an absorption spectrum, and changes in the spectrum revealed the motion of the molecule and the state of the chemical bonds. In this way, the researchers were effectively equipped with a camera having different shutter speeds to capture the progress of the reaction. The results showed that cyclobutane decomposed to ethylene via the second (two-step) mechanism. The lifetime of the intermediate was about 700 fs. 

The femtosecond laser technique has been used 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 d femtochemistry. For the development of the field of femtochemistry. Professor Zewail was awarded the Nobel Prize in Chemistry in 1999. 

The decomposition of cyclobutane to form two ethylene molecules can take place in one of two ways, (a) The reaction proceeds via a single step, which involves the breaking of two C—C bonds simultaneously, (b) The reaction proceeds in two steps, with the formation of a shortlived intermediate in which just one bond is broken. There is only a small energy barrier for the intermediate to proceed to the final products. The correct mechanism is (b). 

In this field, the density operator plays an important and uncontested role. It allows for more than just the above-given series expansion it can be used for consistent approximations by integrating out (read take the partial trace) of a series of states we are not particularly interested in, leading to the so-called reduced density matrix. It can also be used to find representations in spaces, for instance, the Wigner representation [35], that give more insight into the quantum distribution functions, and provide in some cases distribution functions that are more close to the classical. 

The first 15 wave functions were used in a dynamical simulation of the Liouville equation for the density operator. The Franck-Condon factors were generated by assuming that the ground-state vibrational wave function is a Gaussian centered at —1 A, with a width of 0.1 A. The central frequency of the light pulse was chosen 

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Femtosecond pump-probe experiments have burgeoned in the last ten years, and this field is now connnonly referred to as laser femtochemistry [26, 27, 28 and 22],... 

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]. 

Femtochemistry 97 1998 (The American Chemical Society) J. Phys. Chem. 102 (23))... 

Zewail A H 1995 Femtochemistry concepts and applications Femtosecond Chemistry ed J Manz and L waste (New York VCH) pp 15-128... 

Zewail A and Bernstein R 1992 Real-time laser femtochemistry viewing the transition from reagents to products The Chemical Bond Structure and Dynamics ed A Zewail (San Diego, CA Academic) pp 223-79... 

Zewail A H 2000 Femtochemistry atomic-scale dynamics of the chemical bond using ultrafast lasers (Nobel lecture) Angew. Chem. 39 2586-631... 

Zewail A H 1994 Femtochemistry. Uitrafast Dynamics of the chemicai Bond (Worid Scientific Series in 20th Century Chemistry, voi 3) (Singapore World Scientific)... 

Problems arise if a light pulse of finite duration is used. Here, different frequencies of the wave packet are excited at different times as the laser pulse passes, and thus begin to move on the upper surface at different times, with resulting interference. In such situations, for example, simulations of femtochemistry experiments, a realistic simulation must include the light field explicitely [1]. 

Zewail, A. H. (1994) Femtochemistry, World Scientific Publishing, New Jersey. 

Vol. 3 Femtochemistry Ultrafast Dynamics of the Chemical Bond by Ahmed H. Zewail... 

Despite their transient existences, it is possible to study transition states of certain reactions in the gas phase with a technique called laser femtochemistry Zewall, A.H. Bernstein, R.B. Chem. Eng. News, 1988, 66, No. 45 (Nov. 7), 24. For another method, see Ceilings, B.A. Polanyi, J.C. Smith, M.A. Stolow, A. Tarr, A.W. Phys. Rev. Lett., 1987, 59, 2551. See Smith, M.B. Organic Synthesis, McGraw-Hill NY, 1994, p. 601. 

Zewail, A. H. 1996 Femtochemistry, Vols 1, 2. Singapore World Scientific. 

Douhal, A. and Santamaria, J. (eds) (2002) Femtochemistry and Femtobiology, World Scientific, Singapore. 

Zewail, A. H. Femtochemistry Ultrqfast Dynamics of the Chemical Bond World Scientific Singapore, 1994, Volumes I and II. 

Zewail, H. A. FEMTOCHEMISTRY. Ultrafast dynamics of the chemical bond, World Scientific, Singapore, 1994... 

Femtosecond studies being performed intensely the world over, using not only molecular beams but studying also processes on surfaces and clusters and in polymers. In addition, femtochemistry has been applied to the study of many important biological systems. 

Lozovoy, V. V., and Dantus, M. 2005. Coherent control in femtochemistry. Chem. Phys. Chem. 6 1970-2000. 

There are technical challenges still to be overcome before every thought experiment in this brave new realm of molecular science can be realized in practice, but there is good reason for optimism. While simple gas-phase reactions of comparatively small molecules will continue to attract serious attention, the forefronts of femtochemistry today encompass far wider perspectives. 

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 Chemical Reaction Dynamics and Their Control, Adv. Chem. Phys. 1997, 101, 3, 892. 

A. H. Zewail, Femtochemistry. Atomic-scale Resolution of Physical, Chemical and Biological Dynamics, Proc. Robert A. Welch Foundation Conf. Chem. Res. 1997, 41, 323. A. H. Zewail, Femtochemistry Atomic-Scale Dynamics of the Chemical Bond Using Ultrafast Lasers (Nobel lecture), Angew. Chem., Int. Ed. Engl. 2000, 39, 2586. 

A. H. Zewail, Femtochemistry. Past, Present, and Future, Pure Appl. Chem. 2000, 72, 2219. 

The references cited here depend heavily on investigations reported by the Professor Ahmed H. Zewail, the 1999 Nobel Laureate in Chemistry, and his collaborators at the California Institute of Technology. In them, one will find extensive citations of work leading up to recent advances in femtochemistry as well as to contemporary studies from other laboratories. 

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