Infrared Radiation: Beyond Visible Light on the Electromagnetic Spectrum - Essay Sample

Introduction

Infrared Radiation (IR) is electromagnetic radiation that the human eye cannot see but can be felt as heat. IR is named so because they occur just below visible red light in the electromagnetic spectrum. Ranked in ascending frequency, the electromagnetic spectrum includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma-rays. IR gets released when atoms in a molecule absorb and release energy, and all matter in nature above minus 268 degrees Celsius emit some degree of infrared (Ferraro, 2014). Because of its ubiquity, infrared has been exploited in various technologies, including infrared cameras and night vision glasses. This paper describes the theory of FTIR spectroscopy and spectrometer spectra.

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Infrared Radiation (IR) was discovered in the early 19th century by British astronomer William Herschel. Its wavelength ranges between 12800 to10 cm-1 and frequency between 3 GHz (gigahertz) to nearly 400 (THz) terahertz. Infrared has three zones; the region from 12800 to around 4000 wavenumbers is near-infrared section, 4000 to around 200 wavenumbers is mid-infrared zone, and 50 to around 1000 wavenumbers is the far-infrared range (Lin & Dence, 2012). From inception, infrared has had many uses and applications; for example, near-infrared light that TV remotes use to communicate to the TV, and far-infrared heat that people feel and use from sunlight for various purposed and from fire to cook.

Infrared absorption spectroscopy is where Scientists use molecules' infrared light absorption characteristics to know their structures and composition. Upon exposure to infrared, compounds absorb the rays selectively and at particular wavelengths, which changes their dipole moments (Lin & Dence, 2012). The change moves the sample particles from the ground state to excited state energy levels. The vibrational energy gap is thus used to find the absorption peak frequency. The vibrational capacity of the molecule is hence shown by its number of absorption peaks. A scientist can, therefore, analyze the molecule's IR spectrum to deduce its structure. Infrared spectroscopy can analyze all solid, liquid, and gas samples because they are all infrared active.

Most molecules in nature absorb infrared best at the range of 4000 ~ 400 cm-1 wavelength. The most modern IR spectrometers are the FTIR (Fourier Transform Infrared) spectrometers. They use an interferometer (called the Michelson interferometer) in place of the monochrometer used in previous IR spectrometers (Ferraro, 2014). The advancement comes with more power, a higher signal-to-noise ratio, higher wavenumber accuracy, shorter scan time, wider scan range, and lesser disturbance from stray light. An FTIR spectrometer has a source, detector, sample compartment, interferometer, A/D converter, amplifier, and a computer. The source sends radiation through the sample and into the detector. The A/D converter changes this signal into a digital signal and then passes it to the amplifier for amplification (Lin & Dence, 2012). The data then reaches the computer for Fourier transform.

The interferometer's work is to split a beam of rays into two, so they follow different paths and recombines them later and then directs them into the detector. It comprises of a beamsplitter and two perpendicular mirrors. The detector quantifies the difference in the intensity between the beams as a function of the path difference. One if the interferometer's mirrors is stationary while the other is moveable (Ferraro, 2014). The beamsplitter reflects half of the incoming light and transmits the other half. The transmitted and reflected light then hit the stationary and moveable mirrors in that order, and are reflected back and recombine at the beamsplitter.

Fourier transform occurs in the computer. Here, the signals received are analyzed mathematically to generate a plot called an interferogram, which is also further processed to create the final image of the sample (Lin & Dence, 2012). The calculations link the beam intensities to the distances they traveled while considering that the second mirror is moveable. The process makes the possible accurate representation of what the sample's molecular structure is.

References

Ferraro, J. R. (2014). Practical Fourier Transform Infrared Spectroscopy: Industrial and laboratory chemical analysis. Saint Louis: Elsevier Science.

Lin, S. Y., & Dence, C. W. (2012). Methods in Lignin Chemistry. Berlin, Heidelberg: Springer Berlin Heidelberg.