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Quantum particles are the smallest possible discrete, interactable units of matter. Advances in our understanding of the universe’s smallest scale have been crucial for the development of infrared (IR) spectroscopy, the study of interactions between matter and IR radiation. The role of quantum particles in this field is intrinsic to it.
IR Spectroscopy: An Overview
Many techniques are covered by the term IR spectroscopy. These are mainly forms of absorption spectroscopy, which measures the absorption of radiation in the subject material.
In IR spectroscopy, an infrared spectrum is produced using an infrared spectrometer. This spectrum is a graph visualization showing infrared absorbance against the frequency or wavelength of infrared light emitted.
To achieve this, the IR spectrometer emits a beam of infrared light (light which vibrates at a higher frequency to visible light) through the sample material. The amount of energy absorbed by the subject at each set frequency of light is revealed by how light is transmitted through the subject. These absorption levels can be measured with a monochromator that scans the wavelength range or a Fourier transform instrument which measures the entire range.
Infrared spectra can provide information about the molecular structures of the material by revealing an absorption fingerprint of its covalent bonds (the chemical bonds between atoms created by electrons passing between them).
Because molecular structure determines which frequencies are absorbed in the subject material, IR spectroscopy (like other forms of absorption spectroscopy) identifies the different molecules that make up a subject material with relative ease and complete reliability.
Modern IR spectroscopy has numerous diverse applications in a wide range of industries and research fields. IR gas analyzers, for example, are in situ IR spectrometers used to measure concentrations of CO2 in greenhouses and growth chambers. Applications as diverse as forensic science investigations and art history research have been found for IR spectroscopy techniques.
Quantum Particles in IR Spectroscopy
Techniques of IR spectroscopy rely on the behavior of particular quantum particles, and the method has developed along with our understanding of quantum chemistry and physics.
The Born–Oppenheimer (BO) approximation relates to the quantum particles that make up atoms: nuclei and electrons. In this theoretical and experimental approach, it is assumed that the motion of nuclei and electrons in one atom can be considered separately. Named after Max Born and J. Robert Oppenheimer who proposed it in 1927, the BO approximation enables IR spectroscopy to consider molecular energy as a sum of independent terms.
The harmonic approximation details the ways the molecular Hamiltonian (the entire energetic structure of nuclei and electrons in an atom) resonates in harmony with the normal modes that correspond with the molecule’s electronic ground state potential energy surface.
IR spectroscopy causes excitation in the subject matter. Excitation is a feature of quantum mechanics, where quantum particles increase energy levels in matter where radiation is transmitted.
Summary
IR spectroscopy exploits energy and matter that are both formed of quantum particles at the most minute scale possible to observe. The interaction of electrons and nuclei in bonded atoms that make up molecules of matter with the energetic particles in transmitted infrared light results in precise absorption of certain frequencies or wavelengths of transmitted light.
The precision that frequencies are absorbed where molecular structures depend on the quantum phenomenon in which molecular Hamiltonians vibrate at a consistent frequency in the normal mode. This consistent frequency observable among quantum particles provides each molecule in a subject with a uniquely identifying vibrational fingerprint.
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