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Femtochemistry

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Chemistry of reactions on 10^-15 second timescales

Pump-probe techniques

Femtochemistry is the area ofphysical chemistry that studieschemical reactions on extremely short timescales (approximately 10⁻¹⁵ seconds or onefemtosecond, hence the name) in order to study the very act of atoms within molecules (reactants) rearranging themselves to form new molecules (products). In a 1988 issue of the journalScience,Ahmed Hassan Zewail published an article using this term for the first time, stating "Real-time femtochemistry, that is, chemistry on the femtosecond timescale...".^[1] Later in 1999, Zewail received theNobel Prize in Chemistry for his pioneering work in this field showing that it is possible to see how atoms in a molecule move during a chemical reaction with flashes oflaser light.^[2]

Application of femtochemistry in biological studies has also helped to elucidate the conformational dynamics ofstem-loop RNA structures.^[3]^[4]

Many publications have discussed the possibility of controlling chemical reactions by this method,^{[clarification needed]} but this remains controversial.^[5] The steps in some reactions occur in the femtosecond timescale and sometimes inattosecond timescales,^[6] and will sometimes formintermediate products. These reaction intermediates cannot always be deduced from observing the start and end products.

Pump–probe spectroscopy

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The simplest approach and still one of the most common techniques is known as pump–probespectroscopy. In this method, two or more optical pulses with variable time delay between them are used to investigate the processes happening during a chemical reaction. The first pulse (pump) initiates the reaction, by breaking a bond or exciting one of the reactants. The second pulse (probe) is then used to interrogate the progress of the reaction a certain period of time after initiation. As the reaction progresses, the response of the reacting system to the probe pulse will change. By continually scanning the time delay between pump and probe pulses and observing the response, workers can reconstruct the progress of the reaction as a function of time.

Examples

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Bromine dissociation

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Femtochemistry has been used to show the time-resolved electronic stages ofbromine dissociation.^[7] When dissociated by a 400 nm laser pulse, electrons completely localize onto individual atoms after 140 fs, with Br atoms separated by 6.0 Å after 160 fs.

References

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^Zewail, A. H. (1988-12-23). "Laser Femtochemistry".Science.242 (4886):1645–1653.Bibcode:1988Sci...242.1645Z.doi:10.1126/science.242.4886.1645.ISSN 0036-8075.PMID 17730575.S2CID 220103588.
^The 1999 Nobel Prize in Chemistry, article on nobelprize.org
^Kadakkuzha, B. M.; Zhao, L.; Xia, T. (2009). "Conformational Distribution and Ultrafast Base Dynamics of Leadzyme".Biochemistry.48 (22):3807–3809.doi:10.1021/bi900256q.PMID 19301929.
^Lu, Jia; Kadakkuzha, Beena M.; Zhao, Liang; et al. (2011). "Dynamic Ensemble View of the Conformational Landscape of HIV-1 TAR RNA and Allosteric Recognition".Biochemistry.50 (22):5042–5057.doi:10.1021/bi200495d.PMID 21553929.
^"Femtochemistry: Past, present, and future". A. H. Zewail,Pure Appl. Chem., Vol. 72, No. 12, pp. 2219–2231, 2000.
^Kling, Matthias F.; Vrakking, Marc J. J. (1 May 2008). "Attosecond Electron Dynamics".Annual Review of Physical Chemistry.59 (1):463–492.Bibcode:2008ARPC...59..463K.doi:10.1146/annurev.physchem.59.032607.093532.PMID 18031218.
^Li, Wen; et al. (November 23, 2010)."Visualizing electron rearrangement in space and timeduring the transition from a molecule to atoms".PNAS.107 (47):20219–20222.Bibcode:2010PNAS..10720219L.doi:10.1073/pnas.1014723107.PMC 2996685.PMID 21059945.

External links

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Controlling and probing atoms and molecules with ultrafast laser pulses, PhD Thesis

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