Infrared (IR) radiation is part of the electromagnetic spectrum, falling between visible light and microwaves. Its wavelengths are longer than those of red light, making it invisible to the human eye, but it is experienced primarily as heat. All objects with a temperature above absolute zero emit this radiation, with most energy radiated by objects near room temperature falling into the infrared band. The absorption of infrared radiation is governed by the molecular structure of the material it encounters, dictating which substances absorb IR and which allow it to pass through.
The Molecular Mechanism of Absorption
The absorption of infrared radiation begins on the molecular level, involving the movement of atoms within a molecule. Molecules are not rigid structures; their atoms constantly undergo vibrational and rotational movements. When the frequency of the incoming IR radiation exactly matches one of the molecule’s natural vibrational or rotational frequencies, the molecule absorbs the energy. This absorbed energy increases the amplitude of the molecular vibration, storing the radiation’s energy as kinetic energy.
For IR absorption to occur, a molecule must experience a change in its dipole moment during vibration or rotation. The dipole moment measures the separation of positive and negative electrical charges, creating an electrical field. When the molecule vibrates, the fluctuation of this charge separation creates an oscillating electric field that couples with the infrared light wave. This requirement explains why highly symmetrical diatomic molecules, such as nitrogen ($\text{N}_2$) and oxygen ($\text{O}_2$), do not absorb IR radiation.
Since $\text{N}_2$ and $\text{O}_2$ are composed of identical atoms, their motion does not change their zero dipole moment. In contrast, molecules with three or more atoms typically have complex vibrational modes that result in a changing dipole moment. Even symmetrical carbon dioxide ($\text{CO}_2$), which has no permanent dipole moment, absorbs IR when it undergoes asymmetrical stretching or bending. These movements temporarily shift the charge distribution, creating the necessary fluctuating dipole moment to absorb infrared energy.
The Atmosphere’s Primary Absorbers
The molecular mechanism of absorption has implications for the Earth’s climate system, as certain atmospheric gases efficiently absorb outgoing infrared radiation. The primary IR absorbers are water vapor ($\text{H}_2\text{O}$), carbon dioxide ($\text{CO}_2$), and methane ($\text{CH}_4$). Water vapor is the most effective absorber, covering a broad range of the infrared spectrum. These gases absorb the longwave IR energy radiated upward from the Earth’s surface, preventing it from escaping directly into space.
Once absorbed, these gas molecules re-emit the energy in all directions, redirecting a substantial portion back toward the Earth’s surface. This re-emission is known as the greenhouse effect, which raises the planet’s average temperature. $\text{CO}_2$ is particularly effective at absorbing energy near 15 micrometers, a region overlapping with the peak of the Earth’s thermal emission.
There is a specific band of wavelengths between 8 and 12 micrometers where absorption by water vapor is weaker. This gap is known as the “atmospheric window,” allowing IR radiation within this range to largely escape directly to outer space. However, trace gases like ozone absorb within this window, and increasing concentrations of greenhouse gases continue to narrow this escape route.
Engineered Materials and Everyday Absorbers
Many common solids and liquids readily absorb infrared radiation. Liquid water is a strong IR absorber, contributing to the heating of bodies of water exposed to sunlight. Materials such as glass, wood, brick, and asphalt absorb IR radiation, which is why they feel warm to the touch after exposure to heat. The ideal absorber, a theoretical black body, absorbs all incident electromagnetic radiation, including IR.
Engineered materials are designed either to maximize IR absorption for energy collection or to minimize it for insulation. Solar thermal collectors use dark coatings to maximize the absorption of solar infrared energy, converting it efficiently into heat. Conversely, thermal insulation incorporates substances that scatter or absorb the radiation to minimize heat transfer.
Specialized coatings use inorganic metallic oxides, such as indium tin oxide (ITO) or antimony tin oxide (ATO), to selectively block near-infrared (NIR) light. These nanoparticle films are engineered to absorb the NIR region while remaining highly transparent to visible light, making them suitable for energy-efficient windows. Highly reflective metals, like polished aluminum or silver, block IR by reflecting the radiation away, which is a different mechanism than absorption.
How Scientists Measure IR Absorption
Scientists use infrared spectroscopy to study precisely how materials absorb different infrared wavelengths. This method involves shining a beam of IR light through a sample and recording which frequencies are absorbed and how intensely. The resulting graph, known as an IR spectrum, shows the absorbance against the wavenumber or frequency of the radiation. This spectrum acts as a unique molecular fingerprint for the substance being analyzed.
The specific wavelengths absorbed correspond to the distinct vibrational and rotational modes of the chemical bonds within the molecule. Because every compound has a unique set of bonds, its IR spectrum provides a reliable method for identification. The mid-infrared region, typically between 4000 and 400 reciprocal centimeters, is the most commonly used range for identifying unknown substances.