The interaction between carbon dioxide (\(\text{CO}_2\)) and infrared (IR) radiation is central to understanding atmospheric science. \(\text{CO}_2\) is a linear molecule composed of one carbon atom double-bonded to two oxygen atoms. The Earth’s surface constantly emits thermal energy in the form of infrared radiation, and the interaction between this energy and atmospheric gases determines the planet’s temperature. The ability of \(\text{CO}_2\) to absorb energy in a resonant way makes it a crucial component in the atmosphere’s temperature regulation.
Clarifying Resonance: Chemical vs. Vibrational
The term “resonance” is used in two different contexts in chemistry and physics. In chemistry, chemical resonance refers to delocalization, describing bonding where a single Lewis structure is insufficient, such as in benzene. Carbon dioxide’s stable, linear structure does not exhibit this type of chemical resonance.
The relevant concept in atmospheric physics is vibrational resonance. This physical phenomenon occurs when the frequency of incoming electromagnetic energy, like infrared light, precisely matches one of the molecule’s natural vibrational frequencies. When this resonance happens, the molecule efficiently absorbs the energy, causing its atoms to move more vigorously. This absorbed energy is directly linked to the retention of heat in the atmosphere.
The Mechanics of \(\text{CO}_2\) Molecular Vibration
A carbon dioxide molecule is linear, with the carbon atom positioned symmetrically between the two oxygen atoms. As a three-atom linear molecule, it possesses four fundamental ways to vibrate, known as normal modes. For a molecule to absorb infrared radiation, its vibration must create a temporary, oscillating change in its electric dipole moment. An electric dipole moment is a separation of positive and negative charges.
The molecule’s vibrations fall into three types. The symmetric stretch involves the two oxygen atoms moving away from the central carbon atom and then back toward it simultaneously. This symmetrical motion does not alter the molecule’s overall charge distribution, meaning it cannot absorb infrared light directly.
The asymmetric stretch is highly effective at absorbing IR radiation. Here, one oxygen atom moves toward the carbon atom while the other moves away, creating a temporary shift in charge distribution and an oscillating dipole moment. The bending mode involves the atoms moving perpendicular to the molecule’s axis, causing the molecule to bend and straighten. This motion also creates a temporary dipole moment change and is degenerate, occurring in two planes with the same frequency. The asymmetric stretch and the two bending modes are considered “IR active.”
Interaction with Infrared Radiation
Resonance occurs because the vibrational frequencies of \(\text{CO}_2\)‘s IR-active modes align with the wavelengths of thermal energy emitted by the Earth’s surface. The Earth radiates energy in the infrared region between 5 and 25 micrometers (\(\mu\text{m}\)), and \(\text{CO}_2\) molecules have major absorption bands within this range.
The most intense absorption band is the asymmetric stretch, absorbing energy at approximately \(4.25\) micrometers. A second, broader absorption band comes from the bending mode, centered around \(15\) micrometers. When an infrared photon with a matching frequency encounters \(\text{CO}_2\), the molecule absorbs the photon, causing it to jump to a higher vibrational energy state.
The excited molecule releases this absorbed energy in two primary ways: by colliding with another molecule, transferring vibrational energy into kinetic energy (heat), or by re-emitting a new infrared photon in a random direction. This re-emission of energy in all directions is the fundamental process by which \(\text{CO}_2\) effectively traps heat, slowing its escape into space.
Comparison to Major Atmospheric Gases
Carbon dioxide’s vibrational resonance contrasts sharply with the two most abundant atmospheric gases, nitrogen (\(\text{N}_2\)) and oxygen (\(\text{O}_2\)). These two gases make up about \(99\%\) of the atmosphere, yet they do not contribute significantly to atmospheric heat retention due to their molecular structure and symmetry.
Both \(\text{N}_2\) and \(\text{O}_2\) are diatomic molecules composed of two identical atoms bonded together. Because the atoms are the same, the molecules are perfectly symmetrical and have no permanent electric dipole moment. When these molecules vibrate, the stretching motion does not create a temporary change in their charge distribution. Lacking the required oscillating dipole moment, \(\text{N}_2\) and \(\text{O}_2\) cannot efficiently absorb infrared radiation and are essentially transparent to the Earth’s thermal energy. This structural difference highlights the significant role polyatomic molecules like \(\text{CO}_2\) and water vapor play in determining the Earth’s temperature.