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From Gas to Solid: How Absorption Bands Reveal the Hidden Secrets of Matter?
In spectroscopy, an absorption band is a range of specific wavelengths, frequencies, or energies in the electromagnetic spectrum that characterizes the transition of a substance from its initial state to its final state. According to quantum mechanics, atoms and molecules can only possess certain defined amounts of energy, or exist in specific states. When these quanta of electromagnetic radiation are emitted or absorbed by atoms or molecules, the energy of the radiation changes the state of the atom or molecule, causing it to change from an initial state to a final state.
The nature of the absorption process
When electromagnetic radiation is absorbed by atoms or molecules, the energy of the radiation causes the substance to change its internal state. The process is not random; orbits and energy levels are discrete in gases or rarefied systems, but have a continuous distribution of states in condensed systems like liquids or solids. The energy change of a certain substance must be achieved through a series of "steps", which requires the absorption of photons to provide energy.
The influence of electromagnetic transition
When a photon is absorbed, its electromagnetic field disappears, causing the state of the system that absorbed the photon to change. This process follows a series of physical laws, such as the conservation of energy, momentum, and angular momentum. At the same time, not all transitions can occur, and this restriction generates a series of so-called selection rules that ensure that only certain energies or frequency ranges are observable.
The strength of the electromagnetic absorption process is mainly determined by two factors: first, transitions that only change the magnetic dipole moment of the system are usually weak, while transitions that change the electric dipole moment are much stronger.
Shape and analysis of absorption bands
The existence of various absorption bands and line shapes makes analyzing band or line shapes an effective way to obtain system information. In many cases, narrow spectral lines are assumed to be Lorentzian or Gaussian, depending on the decay mechanism or fine structure-related temperature effects. By analyzing the intensity, width, and shape of the spectrum, researchers are able to obtain important data about the observing system.
In systems with a large number of states, such as macromolecules and large conjugated systems, it is not always possible to clearly distinguish individual energy levels in the absorption spectrum.
Types of absorption bands
Electronic transition
Electronic transitions occur mainly in the ultraviolet and visible spectral ranges, causing the interaction of photons with atoms, molecules or condensed matter.
Vibration transition
Vibrational transitions and optical phonon transitions occur in the infrared spectrum.
Rotation transition
Rotation transition mainly occurs in the far-infrared and microwave regions.
Other transitions
In the radio frequency range, absorption bands also appear in nuclear magnetic resonance (NMR) spectra.
Application Scope
Materials with broad absorption bands are widely used in pigments, dyes and optical filters. For example, titanium dioxide, zinc oxide, and dyes serve as UV absorbers and reflectors in sunscreens.
Attention to atmospheric physicists
In the process of studying the atmosphere, the absorption bands of oxygen and ozone have attracted great attention from scientists. These absorption bands not only attract the attention of physicists, but also help us understand the earth's environment and its changes.
The absorption bands corresponding to oxygen and ozone have different ranges and strengths, providing us with a window into the structure of the atmosphere.
With the continuous development of science and technology, how will more research on absorption bands and their potential applications change our fundamental understanding of matter and energy conversion?
