One of the best ways to understand the behaviour of atoms in a molecule is by studying how they interact with other particles, atoms or molecules. When two molecules with specific properties are brought into contact in a controlled environment, the ensuing chemical reaction throws light on the molecules' structural, ergonomic and chemical properties. Similarly, when certain particles are shot at atoms in a molecule, the direction in which they “bounce” back and the energy they have lost or gained throws light on the molecule’s topological and “existential” properties.
The use of this technique is called spectroscopy. Technically, spectroscopy is the study of the interaction of radiation with matter. There are multiple components to an investigation that uses this method: a particle source, the target, a detector and a computer. Two important kinds of particles that are used are electrons (in electron diffraction) and photons (X-ray spectroscopy, etc.). Electrons possess a charge and some mass, and therefore, their passage from the source to the target and off the target to the detector can be controlled using a magnetic field.
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The path of an electron in a magnetic field is curved depending on the direction of the magnetic field.[/caption]Photons, on the other hand, have no charge and no mass. However, they do possess a frequency (not that electrons don’t), which is a reflection of the amount of energy they possess. This property is closely tracked when scientists look for an indication of atomic behaviour.
When a particle collides with another, there is a transfer of energy, like when two cars collide head-on. The total energy of the system is conserved: the kinetic energy (KE) before the collision translates as some kinetic energy, some sound energy and so on. There can be no exception, because if there is, the inviolable law of conservation of energy will stand violated. Therefore, such a collision is called an elastic collision. Note that there is a difference between an elastic collision in classical mechanics – as between the two cars – and an elastic collision between two particles. In the former, the total KE is conserved; in the latter, the “individual” KE is conserved.
[youtube http://www.youtube.com/watch?v=NCelD0qr8Do]
However, in many cases, there will be an apparent unaccountability of the particles’ KE. After the collision, the incident particle may have lost or gained some energy, with no signatures visible elsewhere for the energy gained or lost at that location. Such a collision is termed inelastic. And when the incident particle is a photon, the phenomenon is called Raman scattering, named after Sir C V Raman, who first explained the process.
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Sir C V Raman, the eponymous discoverer of the inelastic scattering of photons. February 28, the date he discovered the Raman effect, is celebrated in India as National Science Day.[/caption]Raman and Stokes scattering
During Raman scattering, if the photon loses some energy to the target, its frequency – which is directly proportional to its energy – will drop, producing a shift toward the red-end of the visible spectrum, called a red-shift or Stokes scattering. If energy is transferred from the target to the photon, its frequency will go up, resulting in a blue-shift or anti-Stokes Raman scattering. To make matters simpler, monochromatic (single-colour) light from the visible, near-infrared or near-ultraviolet region of the spectrum is used.
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The x-axis shows the wavenumber (or frequency) of photons scattered from a target. On the y-axis is the transmittance, or the amount of light that passes through the detector. A dip in the frequency corresponding to a particular transmittance implies that, after scattering, the amount of light available at that frequency has dropped. This means a certain fraction of those photons of that frequency lost their energy to the target. In other words: the target absorbed energy at a particular frequency, pointing at a specific form of bond-stretching or molecular vibration. A plot like this is equivalent to the fingerprint of a particular molecule: they're that unique.[/caption]So, is the law of conservation of energy violated? Of course not. I mentioned earlier that the difference between classical inelasticity and quantum inelasticity is the accounting for energy over all entities and over individual entities, respectively. During inelastic scattering, each photon gains or loses some energy, but the total amount over all photons and all the target particles is conserved. That is all.
When a photon loses some energy (Stokes scattering), it is said to have been gained by the target and stored as part of its internal energy. This internal energy manifests itself as the particles’ rotation or vibration about the bonds that hold them in their place. For example, a molecule as a whole may be rotating about an axis in space, but the individual atoms will be vibrating in their positions. Such vibrations usually happen at rates of 1 trillion to 100 trillion times per second. When studying such vibrations and their transition from one energy bracket to another, monochromatic light belonging to the infrared region is suitable.
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If the atoms of a molecule were to be positioned in a certain way on the surface of a drum, one mode of vibration would be like this. See the remaining modes by clicking on the image.[/caption]Fermi resonance
As shown in the chart earlier, there are different energy bands at which energy absorption from the photons occurs. Such bands represent the energies at which a vibrational mode exists in a molecule. Sometimes, when two such vibration bands are close together, they start resonating with which other if both vibrations have a similar symmetry. This phenomenon is called Fermi resonance, named for Enrico Fermi, who first figured it out in 1931.
Fermi resonance is associated with a concept called degeneracy, which is a limiting scenario wherein an object belonging to one class transforms to belong to a simpler class. For example, the point is a degenerate of a circle whose radius is approaching zero. Robert Mulliken, a physicist and Nobel Laureate, simplified things by defining the different classes into which objects could belong, ordering the classification on what he called the Mulliken symbols. There are 10 symbols and so 10 classes, and they range from being singly degenerate to triply degenerate, at the same time accounting for rotational and inverted symmetries.
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The 10 Mulliken symbols[/caption]The connection is that when an atom vibrates in a molecule, it vibrates about the bond it shares with other atoms. When describing the vibration of atoms, scientists are actually describing the changing structure of the bond: if two atoms are wagging on either sides of the bond, the bond can be said to be rocking from side to side depending on how each atom is positioned at a particular moment. If a bond has two vibrational modes, i.e. two energies at which it vibrates, if the modes are close together, and if the natures of the two corresponding vibrations have the same Mulliken symbols, then they seem to exchange and mix energy between themselves.
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The structure of a carbon dioxide molecule[/caption]This results in a distorted scattering spectrum being observed at the detector: instead of one tall peak and one short peak (because one mode is more active at the time than the other), they'd observe two peaks of a comparable height. This doesn't mean that both modes are active but that a resonating mode is - something like a resultant. This phenomenon is frequently observed in carbon dioxide molecules, where the vibrational modes of the two C=O (interpreted as C-double-bond-O) bonds resonate. Consequently, the energy of the more energetic vibrational mode increases while that of the less energetic one decreases. Simultaneously, the more intense mode becomes less intense and vice versa, which translates as the twin peaks.
Stokes scattering and anti-Stokes Raman scattering are basic concepts while Fermi resonance is a relatively advanced one, but they are all equally important when studying molecular, atomic and particle behaviour because the underlying techniques are simple and efficient, making them highly reliable and ubiquitous. Any paper in a journal or in a magazine that investigates the behaviour of matter at such small scales is bound to include spectroscopic data. And knowing what the data translates into can go a long way in understanding the importance of each discovery.
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