Infrared

Infrared radiation

In physics, infrared radiation (IR, first discovered in 1800 by astronomer William Herschel) is the electromagnetic radiation with a frequency band of the electromagnetic spectrum lower than that of visible light but greater than that of radio waves, i.e. wavelength between 700 nm and 1 mm (infrared band). The range of Infrared region is 12800 ~ 10 cm-1 and can be divided into near-infrared region (12800 ~ 4000 cm-1), mid-infrared region (4000 ~ 200 cm-1) and far-infrared region (50 ~ 1000 cm-1). This range of wavelengths corresponds to a frequency range of approximately 430 THz down to 300 GHz and includes most of the thermal radiation emitted by objects near room temperature. The term means “under the red” (from the Latin infra, “under”), because red is the visible color with the lowest frequency.

It is often associated with the concepts of “heat” and “thermal radiation”, since every object with a temperature above absolute zero spontaneously emits radiation in this band (according to Wien’s law increasing the temperature the peak of emission moves more and more towards the visible until the object becomes incandescent).

Infrared light is emitted or absorbed by molecules when they change their rotational-vibrational movements. Infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for the study of these energy states for molecules of the proper symmetry. Slightly more than half of the energy from the Sun arrives on Earth in the form of infrared radiation. The balance between absorbed and emitted infrared radiation has a critical effect on the Earth’s climate. Infrared light is used in industrial, scientific, and medical applications. Night-vision devices using active near-infrared illumination allow people or animals to be observed without the observer being detected. Incandescent bulbs convert only about 10 percent of their electrical energy input into visible light energy, while the other 90 percent is converted to infrared radiation, according to the Environmental Protection Agency.

Infrared spectroscopy

Infrared spectroscopy is the study of the interaction of infrared light with matter; it is the analysis of infrared light interacting with a molecule. This can be analyzed in three ways by measuring absorption, emission, and reflection. The fundamental measurement obtained in infrared spectroscopy is an infrared spectrum, which is a plot of measured infrared intensity versus wavelength (or frequency) of light. The main use of this technique is in organic and inorganic chemistry. It is used by chemists to determine functional groups in molecules. Infrared spectroscopy measures the vibrations of atoms, and based on this it is possible to determine the functional groups. Generally, stronger bonds and light atoms will vibrate at a high stretching frequency (wavenumber). Infrared spectroscopy is an analytical technique that takes advantage of the vibrational transitions of a molecule, has been of great significance to scientific researchers in many fields such as protein characterization, nanoscale semiconductor analysis, and space exploration.

Fourier transform infrared (FTIR) spectroscopy is a measurement technique that allows one to record infrared spectra. FTIR spectrometers (Fourier Transform Infrared Spectrometer) are widely used in organic synthesis, polymer science, petrochemical engineering, pharmaceutical industry, and food analysis. In addition, since FTIR spectrometers can be hyphenated to chromatography, the mechanism of chemical reactions and the detection of unstable substances can be investigated with such instruments. Up till FTIR spectrometers, there have been three generations of infrared radiation spectrometers:

  1. the first generation infrared radiation spectrometer was invented in the late 1950s. It utilizes a prism optical splitting system. The prisms are made of NaCl. The requirement of the sample’s water content and particle size is extremely strict. Furthermore, the scan range is narrow. Additionally, the repeatability is fairly poor. As a result, the first generation infrared radiation spectrometer is no longer in use;
  2. the second generation IR spectrometer was introduced to the world in the 1960s. It utilizes gratings as the monochrometer. The performance of the second generation infrared radiation spectrometer is much better compared with infrared radiation spectrometers with prism monochrometer, But there are still several prominent weaknesses such as low sensitivity, low scan speed and poor wavelength accuracy which rendered it out of date after the invention of the third generation infrared radiation spectrometer;
  3. The invention of the third generation infrared radiation spectrometer, Fourier transform infrared spectrometer, marked the abdication of monochrometer and the prosperity of interferometer. With this replacement, infrared radiation spectrometers became exceptionally powerful. Consequently, various applications of infrared radiation spectrometers have been realized.

Infrared astronomy

Astronomers have found that infrared radiation is especially useful when trying to probe areas of our universe that are surrounded by clouds of gas and dust. Because of infrared’s longer wavelength, it can pass right through these clouds and reveal details invisible by observing other types of radiation. Especially interesting are areas were stars and planets are forming and the cores of galaxies where it is believed huge black holes might reside.

Infrared astronomy uses sensor-equipped telescopes to penetrate dusty regions of space, such as molecular clouds; detect objects such as planets, and view highly red-shifted objects from the early days of the universe. Infrared thermal-imaging cameras are used to detect heat loss in insulated systems, to observe changing blood flow in the skin, and to detect overheating of electrical apparatus.

Infrared waves have longer wavelengths than visible light and can pass through dense regions of gas and dust in space with less scattering and absorption. Thus, infrared energy can also reveal objects in the universe that cannot be seen in visible light using optical telescopes. The James Webb Space Telescope (JWST) has three infrared instruments to help study the origins of the universe and the formation of galaxies, stars, and planets.

One of the advantages of infrared radiation observation is that it can detect objects that are too cool to emit visible light. This has led to the discovery of previously unknown objects, including comets, asteroids and wispy interstellar dust clouds that seem to be prevalent throughout the galaxy.

Infrared astronomy is particularly useful for observing cold molecules of gas and for determining the chemical makeup of dust particles in the interstellar medium, said Robert Patterson, professor of astronomy at Missouri State University. These observations are conducted using specialized CCD detectors that are sensitive to IR photons. Another advantage of infrared radiation is that its longer wavelength means it doesn’t scatter as much as visible light, according to NASA. Whereas visible light can be absorbed or reflected by gas and dust particles, the longer IR waves simply go around these small obstructions. Because of this property, IR can be used to observe objects whose light is obscured by gas and dust. Such objects include newly forming stars imbedded in nebulas or the center of Earth’s galaxy.

References

  1. What are Infrared Waves? NASA science. https://science.nasa.gov/ems/07_infraredwaves
  2. Fourier Transform Infrared Spectrometer. LibreTexts. https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Vibrational_Spectroscopy/Infrared_Spectroscopy/How_an_FTIR_Spectrometer_Operates
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