Photon Interactions With Molecules

Type of physical science: Chemistry

Field of study: Chemistry of molecules: structure

Photon interactions with molecules are the basic processes that occur in natural chemical reactions and in human technology. Natural phenomena include plant respiration and vision. Human technological processes include photography, spectroscopy, and photochemistry.

Overview

Light is a form of energy that can be described in terms of a vibrating electric field that oscillates perpendicularly to a vibrating magnetic field. Thus, light is electromagnetic radiation and has properties that are characteristic of waves. Some properties of electromagnetic radiation, however, are more characteristic of particle behavior than waves. Therefore, a photon is a "particle of light" that has no rest mass and no charge.

The energy of a molecule (or atom or ion) may be regarded as the sum of electronic, vibrational, rotational, and other types of energies. Each of these categories is characterized by a range of energies, and each range of energy is quantized. That is, only specific levels of energy within each range are possible, and, because of the stability associated with lower energy, molecules tend to exist in the lowest energy level in each category. A molecule (or atom or ion) in its lowest energy state is said to be in its ground state.

It is possible to raise the energy of a molecule in a number of different ways, all of which must involve the addition of energy to the molecule. When a molecule is in a higher energy level than its ground, it is said to be in an excited state. Excitation energy can be added by an electric current, sound, or electromagnetic radiation (photons). An excited particle returns to its ground state by emitting energy that is equal to the difference in energy between the two states. Often, this energy is emitted as a photon.

By analyzing photons that are absorbed by a sample, an absorption spectrum is obtained. If emitted photons are analyzed, an emission spectrum is obtained. In either case, the analysis reveals the energy levels involved in the transitions, which are characteristic of structural features present in molecules of the sample.

The energy of electromagnetic radiation, or photons, is dependent upon the wavelength of the radiation. It follows that the energy transmitted to a molecule as a result of photon interaction will depend on the energy of the photon. Thus, the category of molecular energy affected, whether it is electronic, vibrational, rotational, or some other type, is determined by the photon energy.

The electromagnetic spectrum refers to the range of energies, or wavelengths, of photons. The highest-energy photons are γ rays, which have very short wavelengths of about 1 picometer. In the analysis of molecular structure, however, lower-energy photons are preferred.

X rays are the type of photons that occur in the 10-picometer to 10-nanometer wavelength range of the electromagnetic spectrum. X rays are produced when electrons (for example, cathode rays) collide with matter. Like γ photons, X rays have sufficient energy to penetrate deeply into matter and also cause the removal of electrons from molecules with which they collide. Absorption of an X-ray photon causes the ejection of one of the innermost electrons from an atom, producing an ion in an excited state.

Photons in the 10- to 375-nanometer wavelength region of the spectrum are called ultraviolet rays. These are of high energy, but not so high as γ photons or X rays.

Ultraviolet photons interact with matter to cause electronic transitions in molecules. Although the energy transferred from the photon to the electron is not great enough to remove it from the molecule completely, the electron transition to a higher level means that the molecule as a whole is no longer in its ground state.

When a sample is irradiated with a beam of ultraviolet radiation, molecules in the sample absorb energy and become excited. If the transmitted photons are analyzed to determine which photon energies were absorbed by the sample, an ultraviolet absorption spectrum is obtained.

An excited molecule may undergo several processes. It may return to the ground state; because it is an electron that actually has been excited, it is the excited electron that returns to the ground state and emits energy that is equivalent to the energy difference between the two states.

Alternatively, the excited molecule may collide with another molecule and transfer the excitation energy to the other molecule. This process, called sensitization, is often used in photochemically induced reactions.

Another possibility arises because the excited molecule may be in a higher vibrational state within the higher electronic level. Therefore, collision with another molecule may result in a partial transfer of excitation energy corresponding to the vibrational excitation only. This is called vibrational relaxation. The result is that the molecule is still in an excited electronic state, but is in the lowest vibrational level of the higher electronic level. Two possible pathways are open: the molecule may return to the ground state and emit energy lower than the excitation energy--this sequence of events gives a fluorescence spectrum; or, the molecule may undergo intersystem crossing (in which the electron spin changes) to an intermediate, lower-energy excited state and then return to the ground state by emission of energy that is much lower than the original excitation energy--this process gives a phosphorescence spectrum.

Finally, the excited molecule may collide with a different molecule and undergo a chemical reaction with it. Although this is the desired event in photochemical reactions, it is an undesirable one in the analysis of molecular structure.

Visible light has wavelengths in the range of 375 to 760 nanometers. Photons with energies in this range also interact with matter to cause electronic transitions. Such photons do not have high energies, and will induce only electron transitions in molecules whose structure is such that the electron energy levels have appropriate differences in energy. When such a transition occurs, the molecule undergoing the transition absorbs a specific wavelength of the visible region of the spectrum. The result is that the light that is transmitted (or reflected) by the substance lacks this specific wavelength of light. Because the wavelength of visible light is related to the color of the light, the substance containing the absorbing molecules will be colored.

Photons with wavelengths longer than visible light have low energy and do not induce electronic transitions in molecules. Infrared (760 nanometers to 1 millimeter wavelength) and microwave radiation (1 millimeter to about 50 millimeters) fall into this category. Instead, vibrational energy levels of the molecule exist within each energy level, and the molecule as a whole is excited to a higher vibrational level by interacting with such photons. Also, within each vibrational energy level are the rotational energy levels, and these are also involved. Infrared spectroscopy, in which the transmitted beam is analyzed to determine which photon energies were absorbed by the sample, is a useful analytical tool in molecular structure analysis. In general, excitation of molecules to higher vibrational and rotational energies results in an increase in molecular motion, which is perceived as heat.

Photons that constitute radiowaves have very low energies, with wavelengths longer than about 50 millimeters. Such photons interact with conductors of electricity, such as metals, but pass through nonconducting materials. The interaction with conductors is based on the transfer of energy from the electric component of the electromagnetic radiation to the "free" electrons of the conductor, which induces an electric current.

Applications

Gamma photons are not used in routine molecular analysis. They are extremely penetrating and can pass through several meters of concrete. They are often referred to as "ionizing radiation" because they cause the ejection of electrons from molecules with which they interact, resulting in bond breaking. In living tissue, most of the damage is caused by ionization of water. The unstable ions produced as a result of this initial interaction fragment to give hydroxyl and hydrogen ions and radicals. These react with important biomolecules that control growth, metabolism, and reproduction, changing their structures and disrupting all these functions.

X-ray photons can be used in several ways for molecular analysis. If the transmitted X-ray photons (those that pass through the sample without interacting) are analyzed, the energies of the absorbed photons can be determined, giving an X-ray absorption spectrum. If the process by which the ion returns to its ground state is monitored, the analysis is called X-ray fluorescence. If the ejected electron is monitored, the method is called photoelectron spectroscopy.

Another method of analysis that uses X-ray photons is called X-ray diffraction. In this method, photons are momentarily retained (but not absorbed) by molecules in a solid, which then reemit the photons in all directions, resulting in scattering. The scattered beams provide information about the crystal structure of the solid.

Ultraviolet-visible absorption spectroscopy is a technique used in research to reveal various structural characteristics of the substance being investigated. A dilute solution of the substance is scanned by irradiation through the range of wavelengths of about 200 to 750 nanometers. The transmitted light is analyzed to determine which energies were absorbed and the degree of absorption. The energies absorbed correspond to the energy of the electron transition, and these are indicative of various structural features. As examples, absorption occurring near 300 nanometers is characteristic of an atom with a lone pair of electrons adjacent to a double bond (for example, a carbonyl group); absorptions in the 200- to 250-nanometer range are indicative of a molecule containing two or three alternating single and double bonds. Once a substance has been characterized, it is often possible to monitor its appearance or disappearance in a chemical process by analyzing samples at specific wavelengths. Thus, adenosine triphosphate (ATP) may be monitored by its known absorption at 260 nanometers.

Ultraviolet light, which is of lower energy than γ photons or X rays, does not penetrate matter to the same degree. The electronic transitions induced by ultraviolet photons, however, produce excited-state molecules that can react with nearby molecules. This can be harmful to living organisms because the chemical structures of molecules on their surfaces (skin) will be disrupted, giving rise to dysfunctioning cells.

Photochemistry is an area of chemistry that deals with the interaction of photons with molecules. In practice, the area is generally confined to chemical reactions stimulated by ultraviolet photons and, to a much lesser extent, visible light. Nevertheless, the basic process is the excitation of electrons in a specific part of a molecule (selected by energy requirements for the particular structure in the region) to a higher energy level, followed by reaction with another molecule.

Visible light also induces electronic transitions in absorbing molecules. A substance will appear to have a certain color if it contains molecules that have absorbed the energy corresponding to the complementary color. The complementary colors are red-blue, orange-indigo, and yellow-violet, with some overlap occurring. Thus, a substance will be blue if it absorbs energy that corresponds to red light.

Molecules interacting with photons of visible light must have appropriate electronic energy levels. Because electronic transitions resulting from interaction with visible light are lower in energy than those corresponding to ultraviolet light, the excited- and ground-state levels must be closer in energy. Thus, the visible part of the scan in the ultraviolet-visible spectroscopic analysis, which indicates structural features, is used to identify or monitor substances that have been characterized.

Many transition metal compounds are colored because they absorb specific wavelengths of visible light. The electronic transitions induced in transition metal ions by interactions with photons occur between d-orbitals whose energies differ as a result of bonding with ligands. This results in the familiar blue color of aqueous copper sulfate solutions and the use of transition metal compounds as inorganic pigments in paints.

In black-and-white photography, the photon energy of visible light is sufficient to remove an electron from silver ions in the silver compound (usually silver bromide) that is a component of the emulsion of the photographic film. Areas of the film that have been exposed to more photons produce a greater number of silver atoms, and subsequent treatment of the film gives the images that correspond to the light that struck the film.

The oxidation of carotenoids in the human intestine gives vitamin A (retinol) and retinal, which, in combination with proteins, constitute the light-sensitive rhodopsin molecules in the retina of the eye. The extensive system of alternating single and double bonds in the retinal moiety produces the difference in electronic energy levels that responds to interactions with photons in the visible energy region of the spectrum.

Such systems of double bonds are present in many naturally occurring molecules, including chlorophyll (the green colorant in plants that utilizes the energy of absorbed photons for biological processes), heme (the red colorant in blood), betanin (a red colorant of beetroot), betaxanthins (the yellow pigments of cactus flowers), and melanin (dark pigment of skin and hair).

The preservation of artwork is often an attempt to inhibit the excitation of electrons in paint-pigment double bonds, which is followed by reaction of the excited electrons with oxygen in the air. The resulting disruption of the double-bond system alters the color of the pigment.

Infrared spectroscopy is also a useful analytical tool. Absorptions that occur at specific photon energies are characteristic of various structural features in the absorbing molecule. Thus, absorptions that occur at about 5,000 nanometers are indicative of a carbonyl functionality in the molecule. Many other functional groups can be identified in this manner.

Substances that undergo strong vibrational transitions when they are exposed to infrared light have allowed the development of infrared photographic film and infrared goggles, which respond to heat and therefore can "see" warm objects in total darkness.

Although molecular transitions between vibrational energy levels are generally characterized by excitation or emission of photons of infrared light, a noteworthy exception can be observed in the excitation of water molecules (in water or ice) to higher vibrational levels. In this case, the excitation energy is in the visible region because the bonds are strengthened by hydrogen bonding between molecules and therefore absorb higher energies than are usually required for vibrational transitions. The excitation energy corresponds to red light, which gives large bodies of water or ice a familiar blue tint.

Context

The present knowledge of the way in which photons interact with molecules received significant contributions in the early 1900's. During this period, scientists investigated the intimate relationship between the structure of the atom and the nature of light.

In 1752, the Scottish physicist Thomas Melvill analyzed the light emitted by excited gases by diffracting the emitted beam into its component wavelengths. He found that the emitted light consisted of separate, distinct energies. The origin of line spectra (as they came to be called) could not be explained until the structure of the atom had been clarified to a great degree.

Niels Bohr defined the concept of energy levels for electrons, abandoning classical theories in favor of a quantum-theory interpretation of atomic structure. He was awarded the Nobel Prize in Physics in 1922. By using the experimental results and postulates of Max Planck, Albert Einstein, Ernest Rutherford, and others regarding the wave-particle duality of light and the structure of the atom, Bohr deduced a model for atomic structure that has come to be known as the Bohr model. Although it has undergone many additions since its conception, it still forms the basis of an understanding of the interaction of photons with molecules (or atoms or ions). The fundamental idea that atoms and molecules can be excited into higher-energy states and then return to their ground state by emission of energy is an integral part of modern science.

The understanding of photon interactions with molecules has led to many technological advances. Many modern methods of instrumental analysis have been developed, permitting faster research with greater sensitivity and greater accuracy. Such research has shown that the interaction of ultraviolet photons with biomolecules of surface cells of organisms causes structural changes in those molecules that control growth, metabolism, and reproduction. It is known that ultraviolet light from the sun is absorbed in the stratosphere, about 15 kilometers from the surface of the earth, by the ozone layer. Mario J. Molina and F. Sherwood Rowland revealed in 1974 that chlorofluorocarbons catalyze the decomposition of ozone, and the widespread and indiscriminate use of chlorofluorocarbons is a threat to the protective ozone layer. Continued monitoring and research throughout the 1980's validated this concern. Holes in the ozone layer above the Antarctic and the Arctic occur every spring, and the holes are larger each year. Furthermore, the depletion of the ozone layer in general has been established. The threat to the ozone layer is a threat to life itself.

Understanding the processes involved in the interaction of photons with molecules has not only advanced humanity's ability to carry out research and monitor the environment but also resulted in military and commercial applications.

In the military, infrared devices have been used to carry out military operations at night. Lasers, which have been used to guide weapons systems, are undergoing further testing. A laser is a device that produces an intense beam of monochromatic (consisting of one wavelength) light. Molecules (or atoms) are excited to a higher energy state and induced to return to the ground state in such a way that the energy emitted is coherent (in phase) and therefore intensified. Because the light from a laser is monochromatic, the device is also used in research spectroscopy to excite molecules by means of a specific energy of light.

Commercially, lasers are used for compact disc players and computers. The use of microwave radiation as a heat source forms the basis of the technology in microwave ovens.

Molecules that have interacted with these photons are excited to a higher energy state, and friction heats the food.

In medicine, γ photons are used in the treatment of cancer; irradiating a cancerous region with γ photons kills the cancerous cells. In the detection of disease, a controlled amount of radioactive technetium is injected to detect abnormal growths in humans, because the γ photons produced inside the body can be photographed, giving a picture of the entire region where the technetium is located. The penetrating power of X rays also enables them to provide an internal photograph of the body.

In summary, the understanding of photon interactions with molecules continues to provide technological progress that enhances the ability of human beings to understand and monitor the environment, fight disease, and improve the quality of life.

Principal terms

CARBONYL GROUP: a functional group in chemistry composed of a carbon doubly bonded to an oxygen; this group is present in many organic and biological molecules

EXCITATION: the process by which a molecule gains energy and undergoes a transition to an energy level that is higher than its ground state

GROUND STATE: the lowest energy state of a molecule, atom, or ion

PHOTOCHEMISTRY: the area of chemistry dealing with chemical processes and reactions induced by photons

PHOTON: a "bundle" or particle of light; the term is interchangeable with light wave and radiation

PICOMETER: 10 to the power of -12 meter

SPECTRUM: the photon energies absorbed or emitted by a substance that are characteristic of the substance or phenomenon

TRANSITION: the process of an electron, molecule, or some other particle passing from one energy level to another

WAVE-PARTICLE DUALITY: the concept that light has properties characteristic of waves and particles

Bibliography

Ebbing, Darrell D. GENERAL CHEMISTRY. 3d ed. Boston: Houghton Mifflin, 1990. An excellent text that is quite easy to read. Chapter 7 is especially relevant, since it discusses atomic structure, line spectra, lasers, and electromagnetic radiation and provides short biographical notes.

Gillespie, Ronald G., et al. FOUNDATIONS OF MODERN PHYSICAL SCIENCE. Reading, Mass.: Addison-Wesley, 1965. This book provides a historical viewpoint of the development of science. Biographical information is included throughout. Chapters 29 through 35 provide relevant information on atomic structure, spectroscopy, and the like.

Nassau, Kurt. THE PHYSICS AND CHEMISTRY OF COLOR. New York: John Wiley & Sons, 1983. An excellent book that is recommended for nontechnical and technical readers. All the chapters are well written and provide a wealth of knowledge regarding many aspects of photon interactions with molecules. References are provided for each topic in Appendix G.

Ohanian, Hans C. PHYSICS. New York: W. W. Norton, 1985. This physics text has a well-written, informative, and nontechnical feature on radiation (Interlude B).

Roan, Sharon L. OZONE CRISIS: THE FIFTEEN-YEAR EVOLUTION OF A SUDDEN GLOBAL EMERGENCY. New York: John Wiley & Sons, 1989. This is a well-written, chronological account of the discoveries and developments surrounding the depletion of the ozone layer. Conveys the excitement and dangers of the Antarctic research expeditions, and provides some insight into the personalities of the scientists.

Robinson, William R. CHEMISTRY. 2d ed. Boston: Allyn & Bacon, 1989. This introductory, college-level textbook contains an excellent chapter 7 on electromagnetic phenomena and atomic and molecular structure, and provides biographical features (for example, Albert Einstein). Includes a feature on the ozone layer.

Skoog, Douglas A. PRINCIPLES OF INSTRUMENTAL ANALYSIS. Philadelphia: Saunders College Publishing, 1985. A recommended, comprehensive text that covers all routine methods of instrumental analysis. Easy-to-read text.

Photochemistry, Plasma Chemistry, and Radiation Chemistry

The Chemistry of Photography

X-Ray Determination of Molecular Structure