Femtochemistry is the area of physical chemistry that studies chemical reactions on extremely short timescales, approximately 10−15 seconds (one femtosecond, hence the name). The steps in some reactions occur in the femtosecond timescale and sometimes in attosecond timescales, and will sometimes form intermediate products. These intermediate products cannot always be deduced from observing the starting and end products. Femtochemistry allows exploration of which chemical reactions take place, and investigates why some reactions occur but not others. Application of femtochemistry in biological studies has helped to elucidate the conformational dynamics of stem-loop RNA structures.
Many publications have discussed the possibility of controlling chemical reactions by this method,[clarification needed] but this remains controversial. In 1999, Ahmed Hassan 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.
The simplest approach and still one of the most common techniques is known as pump–probe spectroscopy. 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.
Femtochemistry has been used to show the time-resolved electronic stages of bromine dissociation. 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.
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- Femtochemistry. Past, present, and future A.H.Zewail, Pure Appl. Chem., Vol.72, No.12, pp.2219–2231, 2000.
- The 1999 Nobel Prize in Chemistry, article on nobelprize.org
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- Femtochemistry: Ultrafast Dynamics of the Chemical Bond, Ahmed H Zewail, World Scientific, 1994