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Pump-probe techniques

Femtochemistry is the area of physical chemistry that studies chemical reactions on extremely short timescales (approximately 10−15 seconds or one femtosecond, 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 journal Science, 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 the Nobel 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 of laser light. [2]


Application of femtochemistry in biological studies has also helped to elucidate the conformational dynamics of stem-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 in attosecond timescales, [6] and will sometimes form intermediate products. These reaction intermediates cannot always be deduced from observing the start and end products.

Pump–probe spectroscopy

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.


Bromine dissociation

Femtochemistry has been used to show the time-resolved electronic stages of bromine 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.

See also

Related Research Articles

An attosecond is a unit of time in the International System of Units (SI) equal to 1×10−18 of a second. For comparison, an attosecond is to a second what a second is to about 31.71 billion years.

A femtosecond is a unit of time in the International System of Units (SI) equal to 10-15 or 11 000 000 000 000 000 of a second; that is, one quadrillionth, or one millionth of one billionth, of a second. For context, a femtosecond is to a second as a second is to about 31.71 million years; a ray of light travels approximately 0.3 μm (micrometers) in 1 femtosecond, a distance comparable to the diameter of a virus.

<span class="mw-page-title-main">Ahmed Zewail</span> Egyptian-American scientist and Nobel Prize in Chemistry recipient

Ahmed Hassan Zewail was an Egyptian-American chemist, known as the "father of femtochemistry". He was awarded the 1999 Nobel Prize in Chemistry for his work on femtochemistry and became the first Egyptian to win a Nobel Prize in a scientific field, and the second African to win a Nobel Prize in Chemistry. He was the Linus Pauling Chair Professor of Chemistry, Professor of Physics, and the director of the Physical Biology Center for Ultrafast Science and Technology at the California Institute of Technology.

In physics and physical chemistry, time-resolved spectroscopy is the study of dynamic processes in materials or chemical compounds by means of spectroscopic techniques. Most often, processes are studied after the illumination of a material occurs, but in principle, the technique can be applied to any process that leads to a change in properties of a material. With the help of pulsed lasers, it is possible to study processes that occur on time scales as short as 10−16 seconds. All time-resolved spectra are suitable to be analyzed using the two-dimensional correlation method for a correlation map between the peaks.

In optics, an ultrashort pulse, also known as an ultrafast event, is an electromagnetic pulse whose time duration is of the order of a picosecond or less. Such pulses have a broadband optical spectrum, and can be created by mode-locked oscillators. Amplification of ultrashort pulses almost always requires the technique of chirped pulse amplification, in order to avoid damage to the gain medium of the amplifier.

<span class="mw-page-title-main">Attosecond physics</span> Study of physics on quintillionth-second timescales

Attosecond physics, also known as attophysics, or more generally attosecond science, is a branch of physics that deals with light-matter interaction phenomena wherein attosecond photon pulses are used to unravel dynamical processes in matter with unprecedented time resolution.

Flash photolysis is a pump-probe laboratory technique, in which a sample is first excited by a strong pulse of light from a pulsed laser of nanosecond, picosecond, or femtosecond pulse width or by another short-pulse light source such as a flash lamp. This first strong pulse is called the pump pulse and starts a chemical reaction or leads to an increased population for energy levels other than the ground state within a sample of atoms or molecules. Typically the absorption of light by the sample is recorded within short time intervals to monitor relaxation or reaction processes initiated by the pump pulse.

Ultrafast laser spectroscopy is a spectroscopic technique that uses ultrashort pulse lasers for the study of dynamics on extremely short time scales. Different methods are used to examine the dynamics of charge carriers, atoms, and molecules. Many different procedures have been developed spanning different time scales and photon energy ranges; some common methods are listed below.

Reaction dynamics is a field within physical chemistry, studying why chemical reactions occur, how to predict their behavior, and how to control them. It is closely related to chemical kinetics, but is concerned with individual chemical events on atomic length scales and over very brief time periods. It considers state-to-state kinetics between reactant and product molecules in specific quantum states, and how energy is distributed between translational, vibrational, rotational, and electronic modes.

Paul Bruce Corkum is a Canadian physicist specializing in attosecond physics and laser science. He holds a joint University of Ottawa–NRC chair in Attosecond Photonics. He is one of the students of strong field atomic physics, i.e. atoms and plasmas in super-intense laser fields.

The International Max Planck Research School for Ultrafast Imaging and Structural Dynamics (IMPRS-UFAST) is a graduate school of the Max Planck Society. It is a joint venture of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD), the University of Hamburg, the Center for Free Electron Laser Science, the Deutsches Elektronen Synchrotron (DESY), and the European XFEL GmbH. It was established in 2011 and is now based at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, Germany.

Ultrafast X-rays or ultrashort X-ray pulses are femtosecond x-ray pulses with wavelengths occurring at interatomic distances. This beam uses the X-ray's inherent abilities to interact at the level of atomic nuclei and core electrons. This ability combined with the shorter pulses at 30 femtosecond could capture the change in position of atoms, or molecules during phase transitions, chemical reactions, and other transient processes in physics, chemistry, and biology.

Ultrafast electron diffraction (UED), also known as femtosecond electron diffraction (FED), is a pump-probe experimental method based on the combination of optical pump-probe spectroscopy and electron diffraction. UED provides information on the dynamical changes of the structure of materials. It is very similar to time resolved crystallography, but instead of using X-rays as the probe, it uses electrons. In the UED technique, a femtosecond (fs) laser optical pulse excites (pumps) a sample into an excited, usually non-equilibrium, state. The pump pulse may induce chemical, electronic or structural transitions. After a finite time interval, a fs electron pulse is incident upon the sample. The electron pulse undergoes diffraction as a result of interacting with the sample. The diffraction signal is, subsequently, detected by an electron counting instrument such as a CCD camera. Specifically, after the electron pulse diffracts from the sample, the scattered electrons will form a diffraction pattern (image) on a CCD camera. This pattern contains structural information about the sample. By adjusting the time difference between the arrival of the pump and probe beams, one can obtain a series of diffraction patterns as a function of the various time differences. The diffraction data series can be concatenated in order to produce a motion picture of the changes that occurred in the data. UED can provide a wealth of dynamics on charge carriers, atoms, and molecules.

Michael David Fayer is an American chemical physicist. He is the David Mulvane Ehrsam and Edward Curtis Franklin Professor of Chemistry at Stanford University.

<span class="mw-page-title-main">Two-photon photoelectron spectroscopy</span>

Time-resolved two-photon photoelectron (2PPE) spectroscopy is a time-resolved spectroscopy technique which is used to study electronic structure and electronic excitations at surfaces. The technique utilizes femtosecond to picosecond laser pulses in order to first photoexcite an electron. After a time delay, the excited electron is photoemitted into a free electron state by a second pulse. The kinetic energy and the emission angle of the photoelectron are measured in an electron energy analyzer. To facilitate investigations on the population and relaxation pathways of the excitation, this measurement is performed at different time delays.

Pump–probe microscopy is a non-linear optical imaging modality used in femtochemistry to study chemical reactions. It generates high-contrast images from endogenous non-fluorescent targets. It has numerous applications, including materials science, medicine, and art restoration.

Helen H. Fielding is a Professor of physical chemistry at University College London (UCL). She focuses on ultrafast transient spectroscopy of protein chromophores and molecules. She was the first woman to win the Royal Society of Chemistry (RSC) Harrison-Meldola Memorial Prize (1996) and Marlow Award (2001).

Ultrafast scanning electron microscopy (UFSEM) is an innovative consolidated facility that combines two microscopic modalities, Pump-probe microscopy and Scanning electron microscope, to gather temporal and spatial resolution phenomena. In fact, this technique is very wonderful at which ultrashort laser will be used for pump excitation of the material and the sample response will be detected by an Everhart-Thornley detector. Acquiring data depends mainly on formation of images by raster scan mode after pumping with short laser pulse at different delay times. The characterization of the output image will be done through the temporal resolution aspect. Thus, the idea is to exploit the shorter DeBroglie wavelength in respect to the photons which has great impact to increase the resolution about 1 nm. That technique is an up-to-date approach to study the dynamic of charge on material surfaces.

<span class="mw-page-title-main">Fabrizio Carbone</span> Italian and Swiss physicist

Fabrizio Carbone is an Italian and Swiss physicist and currently an Associate Professor at École Polytechnique Fédérale de Lausanne (EPFL). His research focuses on the study of matter in out of equilibrium conditions using ultrafast spectroscopy, diffraction and imaging techniques. In 2015, he attracted international attention by publishing a photography of light displaying both its quantum and classical nature.

Olga Smirnova is a Russian physicist who is Head of the Strong Field Theory Group at the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy and Professor at the Technical University of Berlin. Her research considers the interaction of strong fields with atoms and molecules.


  1. 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.
  2. The 1999 Nobel Prize in Chemistry, article on nobelprize.org
  3. 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.
  4. 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.
  5. "Femtochemistry: Past, present, and future". A. H. Zewail, Pure Appl. Chem., Vol. 72, No. 12, pp. 2219–2231, 2000.
  6. 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.
  7. 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.

Further reading

Andrew M. Weiner (2009). Ultrafast Optics. Wiley. ISBN   978-0-471-41539-8.