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Molecular vibrations
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Atoms in molecules behave as springs, giving infrared spectra.
Atoms in a molecule oscillate like masses on springs, with frequencies in the infrared (10¹³ Hz). Absorption spectra reveal bond strengths.
CO₂ absorbs infrared strongly, causing the greenhouse effect.
At high energy the potential is anharmonic.
In more detail
Molecules vibrate about their equilibrium bond lengths, with frequencies in the infrared range. Each vibrational mode absorbs light at specific wavelengths, forming a fingerprint of the molecule.
Real-world uses
- Infrared spectroscopy identifies chemicals by their vibrational spectra.
- Greenhouse gases absorb infrared radiation through molecular vibrations.
- Chemists use infrared spectroscopy to identify molecules, because molecular bonds vibrate at characteristic frequencies.
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Frequently asked questions
Why do molecules absorb infrared light?
Because its photon energy matches the energy spacing of vibrational levels.
Is a molecular bond a spring?
As a first approximation, yes, with the bond stiffness acting as the spring constant.
Why do bonds absorb infrared light?
The light frequency matches the bond's vibrational frequency, so the energy is absorbed and shows up as a spectral line.
Related topics in Oscillations & SHM
Oscillation of a floating body · Liquid in a U-tube · Tunnel through the Earth · Quantum harmonic oscillator