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Femtochemistry is the science that studies chemical reactions on extremely short timescales, approximately 10-15 seconds (one femtosecond, hence the name). In 1999, Ahmed H. Zewail received the Nobel Prize in Chemistry for his pioneering work in this field. Zewail's technique uses flashes of laser light that last for a few femtoseconds. Femtochemistry is the area of physical chemistry that addresses the short time period in which chemical reactions take place and investigates why some reactions occur but not others. Zewail's picture-taking technique made possible these investigations. One of the first major discoveries of femtochemistry was to reveal details about the intermediate products that form during chemical reactions, which cannot be deduced from observing the starting and end products. Many publications have discussed the possibility of controlling chemical reactions by this method, but this remains controversial.
The ability to follow chemical reactions at the molecular level has been one of the most relentlessly pursued goals in chemistry. Accomplishing this goal means chemists will be able to understand when a certain reaction occurs and the dependence of its rate on temperature and other parameters. A complete understanding of reaction mechanism requires a detailed knowledge of the activated complex (or transition state). However, the transition state is a highly energetic species that could not be isolated because of its extremely short lifetime. This lifetime is further figured out by experiments via lasers to calculate its lifetime in femtoseconds. That is why this new chemistry is been found which is known as FEMTOCHEMISTRY.
EXPERIMENTS FOR FEMTOCHEMISTRY:-
ULTRAFAST LASER TECHNIQUIE:-
The dynamics of a chemical reaction in the region of its transition state is the principal information that must be collected for a detailed description of the reaction mechanism. Essentially two experimental routes lead to this information:
in the time domain as done in the organic femtochemistry activity of the reaction dynamics group at Laboratoire Francis Perrin,
spectroscopically as done in other activities of the group.
In both directions, the chemical reaction under study is turned on by laser excitation. The latter occurs with no geometrical change of the reactive system but the gradients along the excited potential energy surface induce the necessary momentum to the system to push it to reaction. The spectroscopical information carries essentially on the Franck Condon region of the photoexcitation and on the final state of the reaction products. In contrast, the temporal information allows to explore the system in its way through geometrical conformations and electronic configurations that are very different than those accessible in the Franck Condon region. Of course these two information are complementary, hence justifying that our group conducts experiments of both types.
In this experiment, we get to know about compound which detects its motion in femtoseconds by calculating through lasers.
1) Infrared Spectroscopy during Chemical Reactions: Femtosecond IR:-
The Infrared Spectrum of a molecule provides a unique fingerprint enabling us to recognize and characterize the molecule. With our Femtosecond Infrared Spectrometer, we can perform IR spectroscopy on molecules during a chemical reaction and thus directly monitor how bond are broken and how energy is transferred to the solvent.
We focus on small molecules, with few vibrational modes, in water and use strong UV- laser pulses to initiate the photochemical changes. Our goal is gradually to move to larger molecules of biological interest -amino acids or proteins- and we have initially studied molecules containing the relevant functional groups.
Biomolecules are effectively protected against UV light and we are interested in understanding the detailed molecular mechanism that prevents for example formation of free radicals. The mechanism is thaught to combine an effective deactivation of the electronic excited state followed by an equally effective dissipation of the energy to the solvent. Our combined UV and IR femtosecond lasers are well suited for this. In addition, we collaborate with Svend Knak Jensen who is performing theoretical calculations and simulation to model the detailed interaction between the molecules and the solvent (water)
2) Femtosecond Photochemistry:-
We have for some years studied the photochemical reactions of on of the (apparently) Â simplest photochemical systems in water: the photolysis of nitrate (and nitrite) in water. Using the femtosecond UV laser system, we can excite nitrate (and nitrite) and follow the reaction products with broad band probe laser pulses.Â Part of the nitrate molecules isomerize to peroxynitrate (ONOO-). This molecule is one of the key molecules in inter-cellular signalling and we are very interested in learning more of its fundamental properties in water. Is is formed in cis- og trans-configuration, how and how fast does it protonate, what is its lifetime, â€¦â€¦â€¦â€¦ Apart from the biological relevance of this molecule, it is also a very good system when testing and challenging the methodes of theoretical and computational chemists.Â We use both our IR and UV setup in the studies of the nitrate/nitrite photochemistry.
3) Femtosecond IR spectroscopy of Water (HOH, HOD, DOD):-
The experiments described above study how energy deposited in a molecules rapidly leaves the molecule, as a results broken bonds, isomerizations, deexcitations, and interactions with the solvents. Consequently, the solvent -the acceptor of the excess energy- is excited and perhaps it is possible to monitor the uptake of energy by the solvent by studying the femtosecond infrared spectrum of the solvent. This we recently demonstrated in an experiment on nitrite in water, where we observed a very rapid and efficient excitation of the inter molecular hydrogen bonds of the water solvent. We did not observed specific excitations of the different water modes (stretching and bending) but rather observed a general excitation of all the H-bonds linking the water molecules together. This is an extremly interesting observation at the heart of chemistry: where does the energy go and why is water so useful as a solvent and as the matrix for life! We are continuing these experiments and are changing both temperature and the isotopic content of water to gain more insight into water and its unique role in chemistry.
4) Femtochemistry in Liquids:-
Often when we think of a chemical reaction in water, we see before us the reacting molecules roaming around the water as rigid bodies submerged in a homogenious substance. However, it is important to realize that the water consists of an enourmous number of water molecules placed in a vast flexible network held together by hydrogen bonding. When a molecule is solvated, the water molecules will arrange themselves in a structure around the solvated molecule and interact strongly with it. Considering that all biological processes take place in water, it is important to understand the interaction between a molecule and the solvent, and how the solvent might change the properties of a molecule.Â
To study how molecules behave in liquids we rely on femtosecond lasers. These lasers give out very short bursts of light lasting roughly 10-13 s (100 fs). At first we iniate the reaction by sending a laser pulse into the sample containing the molecules and, of course, the water. This pulse is called the pump pulse. When the molecule absorps the photons from the laser pulse it is transformed into an excited state. This means that we decide exactly at what time the reaction will take its beginning. Because the molecule is now excited it may begin to dissociate and split up into different fragments, what is known as photodissociation.
At this point we have to remember that the water molecules essentially form a cage surrounding the molecule, and it may very well be that the fragments cannot escape this cage, and will have to recombine to the original molecule. This is known as a geminate recombination. It is also possible that the fragments escape the solvent cage, and start to swim around in the water network. They may then react with other fragments in what is known as secondary reactions. The dissociation and recombination occurs on a very fast time scale on the order of femtoseconds, and this is exactly why we need so short laserpulses.
At a later time we send in yet another femtosecond laser pulse. This is known as the probe pulse, and by measuring the absorption of the probe pulse we can see what is going on in the sample. Since we can decide at which time the probe pulse will be sent into the sample, we also control the time that passes between the pump and the probe.Â It is then possible for us to record a sort of movie of the processes and dynamics that take place in the liquid, although they proceed at timescales so fast that it is hard to imagine.
5) Shaping laser pulses: How do we design and the create the optimal laser pulse for a given chemical reaction:-
The femtosecond laser used in our experiments are inherently spectrally very broad. From Heisenbergs uncertainty relation one knows that a very short pulse must be undetermined in energy (energy Âµ 1/wavelength).Â A femtosecond pulse is typically 10 nm wide (spectral bandwidth) and this opens for a new and exciting way of controlling the molecules we excite. Almost arbitrary pulse shapes can be obtained by "shaping" the femtosecond laser pulses.
We use an optical grating in conjunction with an 2D-shaper, where we can change the amplitude and phase of the individual spectral components inside the bandwith of the pulse. This allows use to design optial pulses that are optimized for specific excitations and molecules! The project involves understanding the shaper, the light-molecule interactions, and designing optimization schemes for the laser pulses. We plan to use shaped pulses for both our Cars, bpw and femtochemistry experiments.
FUTURE ASPECTS OF FEMTOCHEMISTRY:-
Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum-mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user-defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so-called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas-phase as well as liquid-phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas-phase as well as liquid-phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer-guided optimization methods. The general approach to active control of light-matter interaction has also applications in many other areas of modern physics and related disciplines.