Molecular vibration

Molecular vibration is periodic motion of atoms relative to one another within a molecule. Its quantized energy levels produce characteristic absorption and emission of infrared light.

Normal mode: A coordinated pattern of motion in which every part of a system oscillates at one frequency. Each molecular vibration can be described as a normal mode near equilibrium.
Infrared spectroscopy: A technique that measures how matter absorbs infrared radiation as a function of frequency. Infrared absorption commonly excites vibrations that change a molecule’s dipole moment.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule. Equilibrium geometry defines the reference structure around which atoms vibrate.
Molecular rotation: The rotational motion of a molecule as a whole about its center of mass. Rotation changes molecular orientation, unlike vibration, which changes relative atomic positions.
Harmonic oscillator: A system whose restoring force is proportional to displacement from equilibrium. The harmonic approximation models small-amplitude molecular vibrations.
Raman spectroscopy: A spectroscopy method that measures frequency shifts in light scattered by molecular vibrations. Raman scattering detects vibrations that change molecular polarizability.
Chemical bond: An interaction that holds atoms together in a molecule or solid. Bond stiffness supplies the restoring forces underlying many molecular vibrations.
Molecular translation: Motion of a molecule’s center of mass through space. Translation moves all atoms together and is excluded from internal vibrational modes.
Anharmonicity: Deviation of an oscillator’s restoring force or energy levels from the harmonic model. Real molecular vibrations depart from harmonic behavior, especially at high energies.
Vibrational spectroscopy: Spectroscopic methods that use molecular vibrational transitions to study matter. Infrared and Raman measurements are complementary forms of vibrational spectroscopy.

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