Femtochemistry

Definition: Femtochemistry is the study of chemical reactions occurring on a femtosecond timescale, that is, from tenths of a femtosecond to hundreds of femtoseconds. A femtosecond (fs) is 10^-15 seconds, or one-quadrillionth of a second. To conceptualize how short a femtosecond is, consider that there are more femtoseconds in a second than there are seconds in thirty million years. Femtochemistry’s importance stems from the fact that many reaction processes occur on the order of hundreds of femtoseconds and move through a number of intermediate stages. The ability to study events on such short timescales is greatly enhancing scientific understanding of such crucial processes as how protons move through water and how energy moves through photosynthetic systems.

Basic Principles

Chemical reactions occur over a wide range of timescales. The study of chemical kinetics, or the rate at which a chemical event occurs, is a relatively new one; the first actual measurement of a reaction rate was performed by Ludwig Wilhelmy in 1850. The Arrhenius equation, used to predict reaction rates, debuted in 1889 and has since become a mainstay of introductory chemistry courses. The first Nobel Prize in chemistry was awarded in 1901 to Jacobus H. van ’t Hoff, who expanded upon Svante Arrhenius’s work. Over the next few decades, different experimental techniques were developed, reaching the millisecond (10^-3 s) and then the microsecond (10^-6 s) timescales.

The next major event in the study of chemical kinetics was Theodore Maiman’s invention of the ruby laser in 1960. The subsequent developments of Q-switching (1961) and mode-locking (1964) enabled the production of nanosecond- and picosecond-scale pulses of light. Each light pulse takes a snapshot of a sample, with the length of the pulse determining the time resolution of the “photograph.” Thus, a picosecond-long laser pulse provides information about what a system looked like at a given picosecond in time.

Femtochemistry developed in the mid-1980s, when Egyptian American scientist Ahmed Zewail and his colleagues studied the dissociation (a kind of splitting) of a simple molecule, iodine cyanide (ICN). Zewail and his team succeeded in viewing the transition stages of the reaction, which occurred on the femtosecond timescale, in 1987; Zewail was later awarded the Nobel Prize in chemistry for his work. Over the next decades, femtochemistry has continued to be refined, achieving much shorter pulses with higher energies, and applied to myriad systems.

While the prefix “femto-” is not commonly used in everyday speech, terms such as “micro-” and “nano-” are. These terms tend to be used interchangeably to mean “small”; however, their precise meanings should not be forgotten. There are one thousand milliseconds in a second, one thousand microseconds in a millisecond, one thousand nanoseconds in a microsecond, one thousand picoseconds in a nanosecond, and one thousand femtoseconds in a picosecond. Each order of magnitude corresponds to different types of chemical processes, and they should not be confused.

Core Concepts

As a field that combines elements from several disciplines, femtochemistry relies heavily on a number of key concepts related to chemistry and physics as well as to subfields such as optics.

Spectroscopy.Spectroscopy is the use of light to investigate the properties of matter. When light passes through or is reflected by matter, its intensity, wavelength, orientation, and direction are changed. By studying the light before and after it interacts with the sample, scientists can learn things about the properties of the sample material. For example, the concentration of a solution can be determined by calculating how much of the light passing through the sample is absorbed, and the thickness of some thin films can be determined based on the change in the orientation of the light after deflection. These properties cannot be measured directly by the naked eye and thus must be determined indirectly using the response of light. In femtochemistry, femtosecond-length light pulses are used to study the structures of transition-state compounds and other “invisible” structures and processes.

Chemical Reactions and Vibrational Spectroscopy. One of the major applications of femtochemistry is in determining precisely how a given chemical reaction involving bond breaking or bond making occurs. Many different timescales are relevant for chemical reactions, but femtochemistry’s unique contribution lies in the study of molecular vibrations, which tend to occur on a femtosecond timescale. To understand why these vibrations are important, consider that molecules are best imaged as balls connected by springs. In a water molecule, two smaller balls (hydrogen atoms) are connected to a larger ball (the oxygen atom). However, these connections are not static; this V-shaped molecule bends, stretches, and scissors in every possible way, with the distances and angles between the atoms varying around some average. This motion is important, because it tells scientists something about the molecule’s environment. If one of the hydrogen atoms is being pulled toward another molecule, the beginnings of a chemical reaction involving an atom transfer, the vibrations slow because the atom is spending more of its time farther away from the parent molecule and thus must travel a longer distance. After it transfers and becomes part of the neighboring molecule, it vibrates at a different frequency determined by its new environment.

Scientists can calculate the vibrational frequencies expected for specific systems of molecules and use the measured vibrational frequencies to identify the routes that the different atoms take as they rearrange to form new molecules in chemical reactions. Vibrational spectroscopy, the use of light to measure these vibrations, was developed long before it became possible to take femtosecond measurements; however, slower measurements are unable to take snapshots frequently enough. These slower measurements result in blurry pictures that show only the initial and final molecular arrangements, whereas femtochemistry allows snapshots to be taken frequently enough for scientists to determine exactly how the atoms in the molecules have rearranged themselves.

Transition States and Reaction Control. Most reactions are more complicated than an atom transfer. Although it may not seem as if it would be difficult or important to predict how such a reaction occurs, reactions tend to involve larger molecules, and changes in neighboring molecules, such as the solvent, can be crucial. Furthermore, this discussion has omitted information about electronic energy levels and spin states for the sake of accessibility; these details make the overall description of a chemical reaction even more intricate. Femtochemistry allows researchers to gain information about how reactions occur in more detail than ever before. For example, many transition states, which had previously been theoretical, unmeasurable concepts, have been identified. The transition state is the arrangement of the atoms and electrons at which point the reaction must proceed to the products instead of slipping back to the reactants. Identifying this state and other information about a given reaction may allow for future reaction control. Chemical reactions generally produce numerous products, many of which are undesired, and scientists continue to search for ways to synthesize some compounds. The ability to control reactions would allow researchers to apply a given set of conditions to ensure that a reaction proceeds efficiently to the desired product, minimizing waste and expanding their ability to create any desired molecule.

Coherence. Coherence refers to creating synchronous chemical events. Because any given sample contains many, many molecules, it is useful to have all of the reactions occur at the same time when attempting to record how a reaction occurs using femtosecond “photographs.” One common way to create coherence is to use a “pump-probe” laser experiment. The “pump” laser pulse sets up the system, putting the molecules in an excited state or liberating the target molecules from a surface. The “probe” pulse is then used to measure the properties of interest, acting as the camera in the photography analogy. In addition to creating a defined start time, coherent experiments are useful in signal amplification. Molecules are so small that almost any measurement actually contains information about a very large number of molecules. Depending on the experiment, sometimes the signals from molecules undergoing reactions other than the reaction of interest, reacting at different times, or even reacting in different directions can make it difficult to record the signal for the reaction of interest. Controlling the start time of the reactions can help prevent this.

Applications Past and Present

Femtochemistry has applications in almost every area of chemistry because of its ability to provide “snapshots” of chemical reactions while they are in progress. Through such applications, femtochemistry furthers science and technology in a number of industries.

Water. One might assume that water is one of the best understood substances, due to its ubiquity. However, the opposite is actually the case: Water is as unique and complex as it is common, and much about how it interacts with substances dissolved in it (that is, its properties as a solvent) is only poorly understood. Water is a dynamic network of hydrogen bonding; the hydrogen atoms in each H2O molecule also form loose bonds, known as hydrogen bonds, with nearby oxygen molecules due to the hydrogen’s slightly positive charge and the oxygen’s slightly negative charge. These bonds are not stationary; hydrogen atoms can jump from one oxygen atom to the next, and any change to one water molecule affects all surrounding molecules. The structure of water is different depending on whether it is completely surrounded by other water molecules on all sides (referred to as being in the bulk solution), at an interface (such as the interface between water and air), or near other molecules. This structure is important because it determines which molecules will dissolve in water and how important biological compounds, such as proteins, will function. Because these rearrangements occur on a femtosecond timescale, they can only be accessed using femtochemical techniques.

Atmospheric Chemistry and Combustion. Research related to atmospheric chemistry and combustion tends to focus on very small molecules; however, many bond-breaking and bond-making processes remain unclear, even for triatomic molecules. Femtochemistry is used in atmospheric chemistry to determine what molecules form during the intermediate stages of a reaction. For example, the reaction of OClO to produce Cl (chlorine), which is partially responsible for the depletion of the ozone layer, has been studied in numerous ways using femtosecond spectroscopy. Combustion chemistry entails the study of the reactions and products associated with combustion, such as ketones, of which acetone is the simplest example. Acetone can dissociate in different ways depending on the amount of energy applied. Some of these mechanisms are fairly straightforward, but others have yet to be clearly defined. Femtochemistry again provides a means to take very quick snapshots of the structure of the molecules after inducing dissociation in different ways. Using this knowledge, scientists are better able to predict global-scale changes in the atmosphere’s composition and help regulators enact more effective policies.

Biological Processes. Femtochemistry has been used to shed light on many biological processes that were previously inaccessible because of their speed. For example, femtochemistry has been used to determine how the compound retinal, which is crucial for vision, reacts so efficiently with light. Within two hundred femtoseconds of irradiation, retinal twists, acquiring a new structure, and then continues to oscillate. The details of this process allow the retinal molecule to respond to light energy with high efficiency. Chlorophyll has been studied in a similar fashion. In both of these examples, femtochemistry allows the reactions of these molecules with light to be studied with unprecedented time resolution; instead of just seeing the initial and final structures, researchers can identify each step in the process. In this way, scientists can determine exactly how these molecules convert light energy into chemical energy able to be used by organisms.

Catalysts. Catalysts represent a key element of modern chemistry. By definition, catalysts are materials that accelerate chemical reactions and, most important, are regenerated at the end of each reaction for use in the next. Catalysts have improved the scale and speed of chemical manufacturing and decreased waste by reducing the need for large amounts of solvent. However, much about how these catalysts work and how they become inhibited or poisoned (a temporary or permanent reduction in efficiency, respectively) is unknown. Femtochemistry experiments are helping to elucidate these mechanisms, which will in turn help scientists develop better catalysts.

Superconductors. When most materials conduct electricity, some of the electrical energy is lost in the form of heat. Superconductors, in contrast, transmit electricity with no loss, making them extraordinarily useful, with applications ranging from transportation (such as electric rail systems) to improved power distribution. Superconductors can also be used to create very powerful electromagnets, which are required for magnetic resonance imaging (MRI) in medicine and nuclear magnetic resonance (NMR) spectroscopy in chemistry, the latter of which is a crucial technique used to determine the structures of molecules ranging from several atoms to hundreds of atoms. Femtosecond laser pulses can be used to generate terahertz radiation, a useful but difficult-to-access range of frequencies that is uniquely suited to studying superconductors.

Social Context and Future Prospects

Although the field of femtochemistry is relatively new, it has already improved scientific understanding of many key biological processes, which in turn provides an understanding of the causes of and suggests potential treatments for various illnesses and disorders. Femtochemistry has also shed light on how water behaves around biomolecules, allowing scientists to make more accurate predictions about how potential drugs will behave in the body. The application of femtosecond measurements to atmospheric chemistry has and will continue to improve scientific understanding of the gas-phrase reactions that contribute to global warming and respiratory issues. Finally, the knowledge gained by applying femtochemistry to catalysts and superconductors will advance the development of new materials with countless important applications.

In the long term, improvements in instrumentation will increase access to femtosecond pulses with highly specified energies and durations, which will in turn allow femtochemical techniques to be applied to a wider range of chemical systems. Reaction control is also of great interest; the ability to control reactions could be used to reduce waste, generate the desired products in higher yields, and probe complex molecules in new ways. Finally, as the study of reactions occurring on the femtosecond timescale becomes more common and affordable, scientists will likely seek to carry out research on even shorter timescales. Attosecond spectroscopy (1 attosecond equals 10^-18 seconds, or 1/1000 of a femtosecond), is currently in very early development but is already offering interesting insights into chemical and physical phenomena.

Bibliography

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Chergui, Majed, and John Meurig Thomas. “From Structure to Structural Dynamics: Ahmed Zewail's Legacy.” Structural Science, Jul. 2017, vol. 4, no. 4. AIP Publishing, pubs.aip.org/aca/sdy/article/4/4/043802/699430/From-structure-to-structural-dynamics-Ahmed-Zewail. Accessed 28 Aug. 2024.

Telle, Helmut H., Angel González Ureña, and Robert J. Donovan. Laser Chemistry: Spectroscopy, Dynamics and Applications. Hoboken: Wiley, 2007.

Zewail, Ahmed H. “Femtochemistry: Past, Present, and Future.” Pure and Applied Chemistry 72.12 (2000): 2219–32.