Femtochemistry: Capturing Chemical Reactions in a Quadrillionth of a second
- Feb 10
- 5 min read
Written by: Salma Awad
Edited by: Violet So
Introduction
For centuries, chemistry was characterised by change: energy is transferred between atoms, bonds break and form, and reactants change into products. However, chemistry was limited to indirect observation of reactions for the majority of its history. Before and after reactions, scientists examined stable molecules and measured reaction rates. But, the actual process of atoms rearranging themselves was still unknown. This resulted from chemical reactions taking place on timescales that were much shorter than those that could be reached using traditional experimental methods. Atoms move in femtoseconds, or one quadrillionth of a second, during bond formation and bond breaking.[1] This measure of time was essentially undetectable until the late 20th century. This was completely altered by the advancement of femtochemistry, which made it possible for researchers to observe chemical reactions in greater detail.[2]
What is femtochemistry?
The study of chemical reactions on femtosecond (10⁻¹⁵ s) timescales is the focus of femtochemistry, a subfield of physical chemistry.[4] Femtochemistry examines the constant motion of atoms and electrons during chemical transformations in order to track changes in the reaction, as opposed to merely concentrating on reactants and products. Molecules are dynamic at this scale. Before settling into new configurations, chemical bonds vibrate, stretch, bend, and reorganise.[2] Using mathematical models like the transition-state theory, traditional chemistry deduced these processes.[5] This was made possible by femtochemistry, which signalled a fundamental shift in chemists' understanding of reactions from static models to dynamic processes.
Atomic bond vibrations occur between 10 and 100 femtoseconds; to put this into perspective, a second is equivalent to a quadrillion femtoseconds. Therefore, the right equipment is required to observe reactions that occur on such a short timescale.[5]
The pump-probe technique:
An experimental technique called the pump-probe technique is used to examine how quickly molecules and materials change after absorbing energy.[6] The sample is first excited by a brief pump pulse, which starts a chemical or physical reaction. A weaker probe pulse interacts with the excited system after a carefully regulated delay to gauge changes in its characteristics, such as energy levels or light absorption. Scientists gather a series of snapshots taken at various times by repeating the experiment with different delays between the two pulses. The molecular and energetic changes that take place on femtosecond timescales are then revealed by combining these snapshots.[7]
The birth of femtochemistry and a Nobel Prize:
Ahmed H. Zewail, a chemist at the California Institute of Technology, established the field of femtochemistry.[8] Zewail investigated basic chemical systems in the mid-1980s by combining molecular beam methods with femtosecond laser pulses. He directly witnessed a chemical bond stretching and breaking (lasting only a few hundred femtoseconds) through experiments on molecules containing iodine.[2] This was the first documented instance of a transition state in chemistry. Zewail received the 1999 Nobel Prize in Chemistry "for his studies of the transition states of chemical reactions using femtosecond spectroscopy" as a result.[8]
Transition State:
The transition state, where new bonds start to form and existing bonds are partially broken, is the most unstable configuration along a chemical reaction pathway. Transition states were thought to be inferred from reaction rates and theoretical enthalpy calculations since they could not be directly observed.[4] This was altered by femtochemistry, which tracks changes in electronic states and molecular structure on femtosecond timescales.[1] This enables us to detect extremely short reaction intermediates, measure the rate at which bonds stretch and compress, and differentiate between alternative reaction pathways.
Applications of Femtochemistry:
Femtochemistry initially concentrated on extremely small molecules in gases.[2] Scientists were able to test whether chemical reactions could actually be seen as they occurred thanks to these straightforward systems. Femtochemistry grew to study much more complicated systems as technology advanced, such as liquid reactions, industrial chemical processes, and biological systems.[4] These days, it is crucial to physics, biology, and chemistry.
Many kinds of chemical reactions are now studied using femtosecond techniques. These include catalytic reactions, in which catalysts accelerate chemical reactions, and photochemical reactions, in which light supplies the energy required for bonds to change.[4] the study of reactions in liquids, where the surrounding solvent molecules can influence the speed and efficiency of reactions, is another application of femtochemistry.[1] This method of observing these reactions aids scientists in comprehending not only the final products but also the precise mechanism of reactions. Also, because many biological processes occur very quickly, femtochemistry is particularly useful in biology. For instance, vision starts when light causes the retina, a molecule in the eye, to undergo a rapid change in a matter of femtoseconds. In plants, ultrafast energy transfer between pigment molecules also initiates photosynthesis. Femtochemistry helps scientists comprehend how living systems function so precisely and efficiently by examining these processes at their natural speed.[5]
Ultrafast Electron Diffraction
Scientists created a method known as ultrafast electron diffraction (UED) to obtain even more precise information. Ahmed H. Zewail created this technique to supplement femtosecond laser spectroscopy. Ultrafast electron diffraction uses extremely brief electron pulses in place of light. These electrons create patterns that show the locations of atoms when they interact with a molecule or solid. Ultrafast electron diffraction enables scientists to observe how atoms move during a chemical reaction by combining femtosecond timing (when change occurs) with atomic-scale (spatial detail/where atoms are). This method gave femtochemistry a new
perspective and resulted in the creation of four-dimensional electron microscopy, which allows for the tracking of changes in atomic structure over time and space.[2]
Femtoseconds and Attoseconds:
Studying even faster processes was made possible by femtochemistry. The attosecond timescale, which is a thousand times shorter than a femtosecond, has been reached by scientists thanks to developments in laser technology. At this stage, scientists can examine the motion of electrons inside atoms and molecules before the atoms themselves start to move. Attosecond science is a field of study that directly builds upon the concepts and methods established in femtochemistry.[5]
Conclusion:
Femtochemistry is far more significant than just seeing quick reactions. It enhances knowledge of how energy moves through molecules and materials, supports the development of more effective catalysts and light-based technologies, and helps validate predictions made by quantum mechanics. Most significantly, femtochemistry alters scientists' perceptions of chemistry.[2] Our knowledge of reactions and their rates as seen over the course of the reaction has been completely transformed by femtochemistry.[1] Additionally, Attoscience research and studies are further supported. To sum up, femtochemistry is a major shift in molecular chemistry, specifically in the way molecules move during a reaction.[5] Scientists were able to witness chemical reactions as they actually occur by creating instruments and scientific methods like the pump-probe or the UED, which are quick enough to match the natural timescale of atomic motion. From femtosecond lasers to ultrafast electron diffraction and attosecond experiments, femtochemistry has made it possible for scientists to see and manipulate the basic motions of atoms and electrons, which can lead to the development of high-speed electronics, sustainable energy systems, and more potent medications.[6]
Bibliography:
1. Yoshitaka Tanimura, Koichi Yamashita, and Philip A. Anfinrud, “Femtochemistry,” August 3,1999,
2. Zewail, A. H. “Femtochemistry: Atomic-Scale Dynamics of the Chemical Bond, using Ultrafast Lasers,” Nobel lecture Notes (2000).
http://polymer.chem.cmu.edu/~kmatweb/2000/July_00/Angebrante/AngwChemIntEd%2 C%202000%2C%2039%2C%202587-2631.pdf
3. EBSCO. “Femtochemistry.” EBSCO Research Starters: Chemistry, n.d. https://www.ebsco.com/research-starters/chemistry/femtochemistry
4. ScienceDirect. “Transition State Theory.” ScienceDirect Topics: Chemistry, https://www.sciencedirect.com/topics/chemistry/transition-state-theory
5. Massachusetts Institute of Technology. “Explained: Femtoseconds and Attoseconds.” MIT News, April 25, 2012. https://news.mit.edu/2012/explained-femtoseconds-and-attoseconds
6. ScienceDirect. “Pump–Probe Spectroscopy.” ScienceDirect Topics: Medicine and Dentistry
7. Zurich Instruments. “Pump–Probe Spectroscopy.” Applications: Optics & Photonics https://www.zhinst.com/ch/en/applications/optics-photonics/pump-probe-spectroscopy
8. Nobel Prize Committee. “The Nobel Prize in Chemistry 1999: Press Release.” NobelPrize.org, October 12, 1999.
Comments