Molecular Excitations

• Type of physical science: Chemistry
• Field of study: Chemistry of molecules: nature of chemical bonds

The interaction of electromagnetic radiation with matter is dependent upon the energy levels available for molecular excitation. The energy levels that are available for excitation are specific and depend upon the unique nature of individual molecules. The study of these different molecular excitations provides information concerning the structure and bonding in the molecule.

Overview

Molecular excitation is the change in the energy levels of a molecule when it interacts with electromagnetic radiation. The fundamental relationship for this process is that the energy of the photon of electromagnetic radiation must exactly match the difference in energy between the two levels. The energy levels available for excitation in a molecule are not continuous; that is, they have only certain specific values. These specific, or quantized, energy levels are dependent upon the structure of the molecule.

Although each molecule has a unique set of energy levels, all molecules share the same fundamental equations that relate the energy levels to the structure and motions of that molecule.

The motions of a molecule can be distinguished between three distinct internal perspectives, each depending upon information concerning a different aspect of the structure of the molecule.

The motions of a molecule that depend upon the structure can be divided into three different types of motion. These motions depend upon the perspective taken of the molecule: as a large mass of known shape, as a set of attached bodies, or as distinct bodies in a field of known shape. Each of these perspectives has a unique set of energy levels, which is dependent upon the bonds and their arrangement. Therefore, a molecule can be considered as the entire molecule with size and dimension, as a distinct set of atoms attached by covalent bonds, or as a collection of nuclei held by electrons over the entire molecule.

A molecule is a three-dimensional arrangement of atoms, and as such it has size and shape. These atoms are held together by covalent bonds, the overlap of electron density between nuclei, which can be thought of as stiff springs between atoms. The electrons in the covalent bonds can be thought of as being able to extend over the entire molecule, such that electrons are considered more as a component of the molecule than the atom.

When one considers the motions of molecules within these perspectives, three distinct types emerge: rotational, vibrational, and electronic. First, a molecule considered as a single body has the internal motions associated with rotation around an axis. This rotation can be around any of the three major axes of the molecule (x, y, or z). Second, a molecule considered as a collection of atoms attached by covalent bonds has internal motions associated with the vibration of those bonds. Usually, these vibrations are considered independent of one another; however, there are certain vibrational motions of molecules that entail the movement of a large part or all the atoms in the molecule. Finally, a molecule considered as a collection of nuclei in a molecular cloud of electrons can have these electrons change location, which is called electronic motion. The electronic effects are often seen in the change of electron location between only two atoms; however, they can also move over the entire molecule. These three motions are quantized in molecules, and their values are dependent upon the identity of that molecule.

The rotations of a molecule are the most closely spaced energy levels and depend upon the overall shape, size, and mass of the molecule. These levels are quantized and are inversely proportional to the moment of inertia of a molecule. The moment of inertia of a molecule can be described along its independent axes of rotation (usually x, y, and z). The moment of inertia depends upon the mass and size of a molecule. The greater the mass and size of a molecule, the greater the moment of inertia and the lower the energy separating the energy levels for rotational motion. Therefore, large and massive molecules have rotational energy levels that are close together, while the levels of small molecules are farther apart.

The vibrational energy levels of a molecule are moderately spaced and depend upon the nature of the bonds and the atoms within a molecule. These levels are also quantized. They are approximately evenly spaced and depend directly upon the mass of the atoms and bond strength between atoms in a molecule. The stronger the covalent bonds, the greater the difference in energy levels. The more mass each atom has, however, the less the difference between energy levels. Therefore, having attached atoms of equal mass will result in greater energy separation for those with stronger bonds, while having attached atoms of differing mass and equal bond strength will result in smaller separation of energy levels for those with greater masses.

The electronic energy levels of a molecule are spaced the farthest apart and result from the various arrangements that electrons may take within a molecule, and yet hold the molecule together. Unlike the vibrational and rotational energy levels, electronic energy levels depend primarily on the electronic structure of the molecule, which is influenced by molecular structure, bonding, and electron delocalization. In most molecules, the electrons in the bonds that compose the molecule are localized between the atoms in those bonds. The energy levels available to the electrons in localized bonds are far apart; in fact, the addition of energy to them will often destroy the bond. Nevertheless, in other molecules, some of the electrons in the molecule are not localized between one pair of atoms in one bond but are delocalized over other atoms of the molecule. The more atoms that participate in this delocalization, the greater the number of energy levels that occur without the destruction of the bonds of the molecule. These electronic energy levels are also quantized and depend upon the extent of delocalization for the separation. The greater the delocalization (greater number of bonds that share electrons), the smaller the energy difference.

In each of these cases of molecular excitation—rotational, vibrational, and electronic—the energy levels are determined by a structural feature of the molecules. Each of these excitations usually occurs in a different region of the electromagnetic spectrum, as the difference in energy levels is vastly different. The rotational excitations occur in the radio frequency and microwave regions. Vibrational excitations occur in the infrared (thermal) region, and electronic excitations usually occur in the visible and ultraviolet regions of the electromagnetic spectrum.

Molecules are excited to these different energy levels by the absorption of energy. This is accomplished by the absorption of a photon of exact energy; in some cases, excitation can occur via multiphoton absorption under high-intensity radiation conditions. Usually, the excitation of molecules is accomplished by the absorption of a single photon that results in a molecule at a higher energy level. This energy level may have a combination of electronic, vibrational, and rotational energy, which is dependent upon the energy of the photon. In other words, energy in the microwave region excites rotational levels; energy in the infrared region excites vibrational and rotational energy levels; and energy in the visible-ultraviolet region excites electronic, vibrational, and rotational energy levels.

Once a molecule is in an excited state, it has three possible fates. One of these fates, and the most common, is nonradiative relaxation, where energy is dissipated into vibrational motion and transferred as heat to its neighboring molecules. This thermal degradation of the energy transforms molecular excitation into thermal excitation of the environment of the molecule. If this is not the path that is most easily accomplished, the molecule loses energy by emitting it as a photon of electromagnetic radiation.

The radiative process can occur as fluorescence or phosphorescence. In fluorescence, the emitted radiation occurs as soon as excitation ceases. In phosphorescence, the emitted radiation may persist for long periods of time. Fluorescence is the rapid emission of radiation from an excited singlet state after absorption and partial energy relaxation. In the process, a photon is absorbed into one of the excited states of the molecule, where some nonradiative relaxation occurs until an energy gap that is too large to be absorbed by surrounding molecules is reached. This energy is emitted as a single photon. Since it is the reverse of absorption, it occurs rapidly and ends once excitation stops.

Phosphorescence is the time-dependent conversion of absorbed radiation into reemitted energy. As in fluorescence, a photon is absorbed, and nonradiative relaxation occurs. As the molecule loses energy, however, it overlaps a state of equal energy for the molecule that possesses different properties from those of the usual excited or ground states. In the usual states, the energy of electrons changes, but not their spin properties. In phosphorescence, the spin properties of the electrons are different in this intermediate energy state of the molecule. If this is the only route available to the molecule, it will lose its energy by this means.

Once a molecule is in this intermediate energy state, however, it is unfavorable for the energy to be emitted, returning the molecule to its ground state. Instead, the energy slowly leaks from this excited state over a long period of time, resulting in phosphorescence.

Applications

The nature of molecular excitation levels depends upon different structural parameters concerning a molecule. The energy of those excitation levels can be determined so that information concerning the molecule can be obtained. This information can be of three distinct types: qualitative, quantitative, and structural. All three are used quite extensively by scientists in a variety of fields.

Qualitative spectroscopy is based on the uniqueness of molecular energy levels to a molecule. A molecule can absorb photons of specific energy into its rotational, vibrational, and electronic levels and, therefore, identify the molecule. Of the three available energy types, the large number of covalent bonds in most molecules affords the greatest possibility for absorption of energy. Therefore, vibrational spectroscopy is the most widely used for the identification of molecules.

Quantitative spectroscopy stems from the consideration that molecules absorb photons in proportion to their concentration. This means that the more molecules are present, the more photons will be absorbed. This quantitative relationship enables the determination of the number of molecules present in a sample. In practice, quantitative spectroscopy is usually performed by visible and ultraviolet spectroscopy upon the electronic energy levels.

The use of molecular excitation to determine structural information is a technique widely invoked to understand the nature of molecules. In its simplest uses, each type of spectroscopy reveals information about a molecule. Rotational spectroscopy, in the microwave region, yields information about bond lengths and molecular geometry in small molecules. Vibrational spectra, taken in the infrared region, give information about the strength of chemical bonds in molecules. Electronic spectra in the visible-ultraviolet region give information about the extent of electron delocalization in a molecule. In addition to information about a molecule in its natural state, spectra can reveal the effects of different environments or other molecules upon the structure of a molecule. In practice, this is most often done in the vibrational and electronic excitation regions. Changes in individual bonds, as revealed by changes in vibrational levels, and changes in molecular electrons, as revealed by changes in electronic levels, can be monitored in this region.

On a more practical note, one use of molecular excitation occurs quite often in cooking.

In a microwave oven, water molecules absorb the microwave radiation through dipole rotation, increasing their thermal energy. These “relax” (attain an equilibrium state by transferring energy) to the other food molecules, which become heated. Eventually, the excitation of the many rotational levels in the water molecules is like taking small steps to climb between floors of a building. The floors of the building can be thought of as the vibrational levels, which can be excited independently by infrared energy. The food is heated through dielectric heating caused by molecular motion and collisions.

Structural information concerning the molecule can be determined from emission techniques as well, where the techniques of fluorescence and phosphorescence can be employed to study molecules. Like absorption techniques, emission techniques are useful qualitative and quantitative tools for understanding molecular structure.

Context

The modern, detailed understanding of molecular excitation began with Max Planck at the beginning of the twentieth century. Planck’s discovery that electromagnetic radiation was quantized led to the discoveries and models of quantized atomic energy levels by Niels Bohr. As attention shifted to molecular systems, the theoretical foundations of quantum mechanics by Erwin Schrödinger and Paul Adrien Maurice Dirac were realized by scientists to begin the understanding of molecular structure.

Although the theoretical framework was derived and described in the early part of the twentieth century, the arrival of more sensitive and accurate instrumentation in the late twentieth century has enabled the confirmation and utilization of molecular excitation for study and application. Ultrafast spectroscopy techniques, such as two-dimensional fluorescence–excitation methods, allow clearer observation of excited-state dynamics by resolving overlapping signals and capturing energy transfer processes on femtosecond timescales. Advancements in artificial intelligence and quantum–classical computing methods have further improved the ability to calculate and predict multiple molecular excited states with high accuracy.

These developments may lead to a greater ability to determine the structure and functions of different molecules. Researchers have demonstrated electronic structures, such as half-Möbius electronic topology, revealing unusual excited-state behavior and expanding the known possibilities of molecular structure. In addition, as the energy requirements for specific molecules and their bonds are determined, molecule-specific and bond-specific spectroscopy may enable advances in synthesis, analysis, degradation, and understanding of molecules.

Principal terms

ABSORPTION: energy from a photon of electromagnetic radiation is added to a molecule in a low-energy level that results in a change to a higher-energy level of the molecule

ELECTROMAGNETIC RADIATION: the continuous energy spectrum,, which includes radio waves, microwaves, infrared, visible light, ultraviolet, and X-rays

EMISSION: energy is released by a molecule in a high energy level that results in a change to a lower energy level of the molecule, producing a photon of electromagnetic radiation

ENERGY LEVEL: a quantized state of energy for a molecule that depends upon the motions and positions of atoms and their electrons in that molecule

MOLECULE: a three-dimensional arrangement of atomic nuclei held together typically by covalent (electron sharing) bonds between the nuclei

PHOTON: the carrier of electromagnetic radiation; in quantized units, energy is proportional to wave number and frequency of the radiation

SPECTROSCOPY: the study of the interaction between matter and electromagnetic radiation, where the energy changes in a molecule match exactly the energy of interacting radiation

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