Absorption band

An absorption band is a specific range of wavelengths, frequencies, or energies within the electromagnetic spectrum. These bands are characteristic of a particular transition in a substance from an initial state to a final state. This phenomenon occurs because of the rules of quantum mechanics. According to these rules, atoms and molecules can only hold certain defined quantities of energy. They can only exist in specific, discrete states. When a substance absorbs a photon, the energy of that radiation changes the state of the atom or molecule.

This process is essential for understanding how matter interacts with light and other forms of radiation.

The mechanism of absorption follows a very specific sequence of events. First, a photon of electromagnetic radiation must interact with an atom or a molecule. To change its energy, the substance must absorb the photon in a series of steps. This absorption can move a particle, such as an electron, from an occupied state to an unoccupied state. It can also move an entire system, like a molecule, from one vibrational or rotational state to another. In solid materials, this process can even create a quasiparticle known as a phonon or a plasmon.

When the photon is absorbed, its electromagnetic field disappears. The energy, momentum, angular momentum, and magnetic or electric dipole moments are all transported from the photon to the system.

Not all transitions are possible due to various physical constraints. These constraints are known as selection rules. A transition must satisfy these rules to occur within a specific energy or frequency range. The strength of the absorption process depends on several factors. Transitions that change the electric dipole moment are much stronger than those that only change the magnetic dipole moment. Furthermore, transitions to higher order moments, such as quadrupole transitions, are weaker than dipole transitions. The intensity of these absorptions is also influenced by temperature and statistical mechanics. For example, in the microwave or radio frequency ranges, the occupation numbers of states determine the observed intensity.

Absorption bands can appear as sharp lines or broad bands depending on the system. In gaseous or diluted systems, the energy levels are discrete, leading to specific states. In condensed systems like liquids or solids, there is a continuous density of states. This often results in continuous energy bands rather than sharp lines. The shape of these bands provides vital information about the system. Scientists often analyze the spectral density and the width of these lines. In some cases, a narrow line can be assumed to be Lorentzian or Gaussian. For example, Mössbauer spectra of 57Fe show very sharp lines that can be used for detailed analysis.

There are several distinct types of transitions categorized by their energy levels. Electronic transitions typically occur at energies found in the ultraviolet (UV) and visible parts of the spectrum. In the X-ray energy range, core electrons in atoms can be observed through X-ray absorption spectroscopy. Vibrational transitions and optical phonon transitions occur in the infrared part of the spectrum. These typically involve wavelengths between 1 and 30 micrometres. Rotational transitions take place in the far infrared and microwave regions. Finally, absorption bands in the radio frequency range are found in NMR spectroscopy. Each type of transition reveals different characteristics of the atoms or molecules involved.

Many important absorption bands in Earth's atmosphere have been named after the scientists who discovered them. In oxygen, the Hopfield bands are very strong and exist between 67 and 100 nanometres in the ultraviolet. The Schumann–Runge bands and continuum are also significant in the ultraviolet range. These are named after Victor Schumann and Carl Runge. In ozone, the Hartley bands are found between 200 and 300 nanometres, with a maximum absorption at 255 nanometres. They are named after Walter Noel Hartley. Other notable bands include the Huggins bands, named after Sir William Huggins, and the Chappuis bands, named after J. Chappuis. These specific names allow scientists to communicate precisely about atmospheric behavior.

Understanding these bands has significant practical applications in technology and safety. Materials with broad absorption bands are used to create pigments, dyes, and optical filters. For instance, titanium dioxide and zinc oxide are used as UV absorbers and reflectors in sunscreens. This protects skin by absorbing harmful ultraviolet radiation. Radio technicians also rely on this science to manage communications. Radio waves traveling through the atmosphere are affected by absorption bands in oxygen and water vapor. For example, water vapor has a resonance peak at 22.24 GHz. Oxygen has a peak around 5 millimetres, which impacts radio communication in the V band.