Atomic and molecular physics

Fields of Study:Physics; chemistry; physical chemistry/chemical physics; spectroscopy; optics; photonics; quantum mechanics; statistical mechanics; electromagnetism; condensed-matter physics; nuclear and particle physics; fluid physics; solid-state physics; computational chemistry/physics; atomic and optical physics; biophysics; plasma physics; theoretical particle physics; classical mechanics; mathematics; calculus; differential equations; linear algebra; statistics; numerical analysis; computer science.

Definition:Atomic and molecular physics is as broad a subject as the name implies. In addition to the study of the physics of atoms and molecules—atoms and ions in isolation and in combination, respectively—atomic and molecular physics has close ties to spectroscopy, which is used to study these systems, and to such fields as nanotechnology and biophysics, which extend insights from atomic and molecular physics to particular systems. Applications range from understanding the behavior of atmospheric species contributing to climate change to engineering new pharmaceutical delivery systems and developing sensors to detect trace levels of explosives or contaminants.

Basic Principles

The idea that all matter is composed of fundamental building blocks traces back to ancient Greek and Indian philosophers. However, it was not until the turn of the nineteenth century that John Dalton established atomic theory, which states that all matter is composed of atoms and that atoms are the smallest unit of matter. One hundred years later, at the turn of the twentieth century, atomic theory was confirmed by Albert Einstein, who used it to explain the random motion of a particle suspended in a liquid or gas. Einstein was also one of the fathers of quantum mechanics, the model of physics used to explain how atoms and molecules behave.

In its original incarnation, atomic theory states that atoms are the smallest units of matter. However, scientists later discovered that atoms are composed of protons, electrons, and neutrons, which are in turn composed of quarks; the six types of quarks are called up, down, strange, charm, bottom, and top. These subatomic particles are relevant to atomic and molecular physics, but their study as entities in and of themselves is the province of particle physics. Molecular physics sometimes bleeds into the realm of chemical physics or physical chemistry. Although there is no cut-and-dried distinction between the two, physical chemistry generally concerns molecules only and approaches the topic with a focus on its application to the broader realm of chemistry. Chemical physics lies somewhere between and within atomic and molecular physics and physical chemistry, concerning molecules more than atoms but focusing on the physical phenomena rather than chemical applications.

Core Concepts

Quantum Mechanics. Very small objects, such as atoms and molecules, behave very differently from human experience of the laws of physics. All matter exhibits the same wave-particle duality commonly used to describe photons or electrons; that is, matter is not the perfectly corporeal entity observed on this scale. Rather, all matter has a characteristic wavelength called a de Broglie wavelength. One may observe a human hand as being solid and well defined, but it actually has a wavelength, period, and amplitude, just like light. The quantum world seems strange in other ways, too. If a person were playing with a spring, for example, he or she might think that it could be made to bounce at any frequency chosen; however, the energy used to make the spring bounce is actually only available in discrete packets. Thus, if that person can bounce the spring with energy A, bouncing it at energy A+B is impossible unless B is an integer number of energy units, called quanta. As the term quantum mechanics suggests, it was the discovery of the quantization of energy by Einstein and his colleagues that launched scientists’ understanding of small-scale physics. Because quantum mechanics dictates how atoms and molecules behave, it is a crucial element of atomic and molecular physics.

Structures of Atoms.High-school science teaches that atoms comprise a small core containing protons and neutrons orbited by electrons, much like the sun is orbited by planets. However, this is a vast simplification, and the omitted details are crucial for understanding atomic and molecular physics. For example, both the electrons and the nuclei have a property called spin. Despite its seemingly straightforward name, this type of spin is a purely quantum-mechanical property and has nothing to do with any kind of actual spinning motion. Electrons also do not orbit around the nucleus per se, often being found in different shapes around the nucleus, such as dumbbell or clover shapes.

Structures of Molecules. Chemical bonds can be covalent, in which electrons are shared; ionic, in which opposing charges attract; or metallic, in which the electrons travel throughout the material, surrounding the nuclei but not being bound to any one nucleus. Although in chemistry the word molecule tends to refer to covalently bonded atoms, it is typically used more broadly in physics. Molecules undergo many different types of motion. Translational motion is the movement of an entire molecule in space, vibrational motion is the stretching and bending of chemical bonds, and rotational motion involves rotation around chemical bonds. The concepts of nuclear and electron spin, electronic energy levels, and energy quantization are also relevant for molecules. All of these properties can be studied using spectroscopy, as explained below.

Spectroscopy.Spectroscopy is the use of the interaction between light and matter to gain information about the properties of that matter—or, sometimes, the properties of the light. Spectroscopy is used extensively in atomic and molecular physics because it is often the only way to obtain detailed information about atoms and molecules. The information that can be accessed using spectroscopic techniques includes the connectivity of atoms within molecules, the lattice orientation of a solid, the electron configuration of an atom or molecule, and electron and nuclear spins. Spectroscopy is categorized by the wavelength of radiation used and by the experimental design. In the case of the former, different parts of the electromagnetic spectrum are able to probe different aspects of an atomic or molecular system. For example, infrared light is often used to study vibrations in molecules, radio waves can be used to elucidate information about spin, and ultraviolet and visible light provide information about the energy levels occupied by electrons in molecules or atoms. Regarding the experimental design, numerous details can differentiate experiments: whether the experiment measures the absorption, reflection, or emission of light by a sample; the use of a single wavelength or a range of wavelengths; the method used to analyze the data; and other details.

Arrangements of Matter. The following terms are used heavily in atomic and molecular physics to describe the phase or arrangement of the matter being discussed. Three of the states of matter—solid, liquid, and gas—should already be familiar to students. A fourth state of matter is plasma, which is gas that has been ionized, producing a gaseous substance that responds strongly to electricity. Plasma globes can be found in many science museums as novelty items; neon-colored tendrils of electricity seem to extend from a center orb (which is actually an electrode) to the glass and respond to hands placed on the outside of the glass. Another common term in atomic and molecular physics is solid state, such as in the phrase solid-state physics. As the name suggests, it refers to the solid state of matter. Condensed matter incorporates both solids and liquids, as well as other, less common states of matter, such as Bose-Einstein condensates—matter cooled to a point where low-temperature quantum phenomena dictate its behavior. Matter changes states by undergoing a phase transition, involving a change in temperature, pressure, or both. However, matter can be “tricked” into remaining in one phase despite being in a pressure and temperature regime that would favor another phase. For example, water can remain a liquid below zero degrees Celsius (thirty-two degrees Fahrenheit) at standard pressure if it is pure enough that the ice lattice has no imperfection to trigger crystallization; this process is known as supercooling.

Applications Past and Present

Air-Pollution Reduction. Scientists use atomic and molecular physics to study the atmospheric reactions relevant to air pollution, the accumulation of greenhouse gases, and the destruction of the ozone layer. By studying the reactions between key small molecules and radicals, scientists have identified the causes of air pollution. Using this information, regulations can be created to reduce emissions accordingly. For example, research revealed that chlorofluorocarbon, or CFC (e.g., Freon), responds to the intense ultraviolet light found at the top of the atmosphere by decomposing and reacting with ozone in a chain reaction, depleting it in the process. Chlorofluorocarbon was then subjected to intense regulation to reduce and eventually eliminate its use.

Atomic and molecular physics is also applied in finding ways to remove CO2 from the atmosphere to offset the disproportionate amount of CO2 being released by humans. For example, a common CO2 removal and storage strategy known as a carbon sink is the use of amine solutions to trap CO2 molecules from the air in the solution. Strategies such as this are based on an understanding of the physical properties of small molecules.

Lasers. Lasers are an example of the utilization of a quantum-mechanical phenomenon that would not be possible without the field of atomic and molecular physics. Lasers produce light composed of photons with the same energy, that is, a beam of a single color of light. Since the development of lasers, they have been heavily applied in the medical field as a surgical device; in biological, chemical, and physical research as a way to study a system’s response to a very specific energy increment; and in everyday life in DVD players, laser pointers, and barcode scanners. Since their invention in 1960, many different types of lasers have been developed, all utilizing different features of the constituent material to emit light at previously inaccessible wavelengths or with properties useful for a specific application, such as high-intensity radiation. The design of new lasers remains an active research area.

Energy. Atomic and molecular physics, the basis for materials science, plays a crucial role in the development of more energy-efficient devices by applying insights gained from a ground-level understanding of matter to develop more efficient processes. In addition, atomic and molecular physics is used in a similar vein to develop better photovoltaics for capturing solar energy and cleaner coal-burning technology, among others. One of the motivations behind superconductor research is that, by employing a resistance-free, lossless conduction medium, superconductors would vastly improve the efficiency of the grid used to distribute electricity throughout the United States. This technology has not yet arrived; although the number and variety of superconductors continues to increase, they still require quite cold temperatures, making them unfeasible for this type of practical application.

Timekeeping. As science has progressed, recording time in a very precise way has become increasingly important, and precise time measurement requires a standard. Just as there exists a material defined as weighing exactly one kilogram, so too are there clocks used to define exactly how long a second is. However, this is no easy task, as clocks are subject to inaccuracies due to friction and general degradation. To overcome this problem, clocks relying on the intrinsic atomic properties of certain materials have been created. The current US standard is a cesium fountain clock located at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, which takes over 100 million years to lose a second’s worth of accuracy. Although this value may seem impressive, research groups continue to work on creating even more accurate clocks. In 2005, Jun Ye’s research group at JILA (NIST and the University of Colorado’s Joint Institute for Laboratory Astrophysics), also located in Boulder, made a clock using strontium that, improved upon since then, became three times more accurate than the cesium fountain clock.

Medicine. Atomic and molecular physics is crucial in modern medicine, providing scientists with a better understanding of the human body and how different treatments create change. Consider the routine procedure of getting an x-ray, perhaps to image teeth or a broken bone. Atomic and molecular physics provides the information needed to interpret the x-rays, to understand which materials create light spots and which create dark spots. Scientists’ understanding of the way that biological tissue reacts to radiation also informs the sparing use of x-rays and the use of appropriate lead protection to isolate the x-rays to the location of interest. This field also helps scientists understand the mechanisms of lead poisoning, radiation sickness, and “the bends,” an issue encountered by divers who travel back to the surface more quickly than their body can adapt, causing gas bubbles to form in the blood.

Reaction Control. As scientists become better able to apply light with specific characteristics to matter and predict how the matter will respond, the question arises of whether scientists will ever be able to control reactions using lasers rather than just monitoring them as passive observers. This field is still in development, but the results thus far are promising, particularly given the development of increasingly short, attosecond-scale (10^-18 s) laser pulses. Reaction control would enable scientists to steer chemical reactions in certain directions using laser pulses, allowing them to study less common reaction pathways and to produce purer products by eliminating undesired side reactions.

Quantum Computing.Quantum computing is another field in development with exciting future applications. Quantum mechanics includes a phenomenon called entanglement, wherein some part of the identity of one particle becomes inherently related to that of another. For example, imagine that atom A is in state 1 and atom B is in state 2. If these two atoms are entangled—specifically such that one of the two is always in each state—then regardless of how far apart the atoms are, changing the state of one instantaneously changes the state of the other. This feature can be used to do basic computing at extremely fast speeds. Some encouraging strides have been made in this area, including the creation of actual, if small and limited, quantum computers that can perform basic logical functions.

Social Context and Future Prospects

As a field of basic science, the impact of atomic and molecular physics in one’s daily life is so enormous that it is impossible to find an area of life that has not been affected by advances in this field. Scientists’ increasing understanding of the properties of atoms and molecules enables biomedical engineers to design drugs based on their expected efficacy and side effects. Everything from breakfast cereal to transmission fluid to long underwear has been improved by scientists’ ability to identify how molecules behave. Without atomic and molecular physics, scientists would be unable to assess air pollution and find remedies for it, and electronics would never have been invented. Due to its importance and expansiveness, there is no danger of this field becoming obsolete. The job outlook for atomic and molecular physicists will fluctuate due to cyclical trends in the importance attached to basic versus applied science and in the relative popularities of biology, chemistry, and physics. The broad range of subdisciplines under the umbrella of atomic and molecular physics ensures opportunities for interested students to specialize in areas of high demand.

Future research directions will most likely involve taking advantage of atomic- and molecular-scale quantum phenomena to create novel devices. For example, the ongoing development of quantum computing relies on quantum entanglement to perform calculations much faster than classical computers, which would constitute a giant leap forward in computer technology. Another promising area of research is the creation and investigation of Bose-Einstein condensates, which can be used to create an atom laser: a material that emits a beam of atoms with approximately the same energy. One potential use of atom lasers is studying surfaces with resolutions of single nanometers or less; surfaces are particularly interesting in applications because they are often the site of chemical reactions and can also be used to form monolayers. In the long term, atomic and molecular physicists seek to completely understand atoms and molecules, and there is no danger of this ambitious goal being achieved anytime soon.

Bibliography

"Atomic and Molecular Physica." Center for Astrophysics, Harvard and Smithsonian, 2023, www.cfa.harvard.edu/people/atomic-and-molecular-physics. Accessed 29 Sept. 2023.

“Atomic and Molecular Physics.” JILA: CU Boulder and NIST, jila.colorado.edu/research/atomic-molecular-physics. Accessed 29 Sept. 2023.

Cohen-Tannoudji, Claude, Bernard Diu, and Franck Laloë. Quantum Mechanics. 2nd ed., vol. 2, Wiley, 1977.

Demtröder, Wolfgang. Atoms, Molecules, and Photons: An Introduction to Atomic-, Molecular-, and Quantum-Physics. 2nd ed., Springer, 2010.

Ford, Kenneth W. The Quantum World: Quantum Physics for Everyone. Harvard UP, 2005.

Neffe, Jürgen. Einstein: A Biography. Johns Hopkins UP, 2009.

Svanberg, Sune. Atomic and Molecular Spectroscopy: Basic Aspects and Practical Applications. 4th ed., Springer, 2004.

About the Author

Cassandra Newell graduated from Colby College with a bachelor’s degree in chemistry and attended MIT for graduate school, completing 2.5 years of work toward a PhD in physical chemistry before leaving to pursue other interests. She worked as a research assistant at both institutions, studying such topics as molecular recognition, guest-host chemistry, computational chemistry, terahertz spectroscopy, acoustic spectroscopy, and proton/electron transfer in chemical systems. She also has experience as a teaching assistant and tutor in the fields of spectroscopy and quantum mechanics. Since leaving academia, she has pursued projects pertaining to science communication, such as editing scientific manuscripts and writing about science for nonspecialists.