What is the basic principle of electronic spectroscopy?

Basic Principle of Electronic Spectroscopy

Electronic spectroscopy, also known as electronic absorption spectroscopy or UV/Vis spectroscopy, is a technique used to study molecules and atoms through their interactions with electromagnetic radiation in the ultraviolet (UV), visible, and near-infrared regions. The basic principle revolves around the absorption of electromagnetic radiation by molecules, leading to transitions between different electronic energy levels.

Key Concepts

Absorption of Photons: When a molecule absorbs a photon of light, it can be excited from a lower energy state to a higher energy state. This process is governed by the quantum mechanical principle that the energy difference between the initial and final states must equal the energy of the absorbed photon.

Electronic Transitions: Electronic transitions involve the movement of electrons from one orbital to another within an atom or molecule. These transitions can be between bonding and anti-bonding orbitals, non-bonding orbitals, or between different molecular orbitals.

Beer-Lambert Law: The intensity of absorption can be quantified using the Beer-Lambert Law, which states that the absorbance is directly proportional to the concentration of the absorbing species and the path length of the light through the sample.

Types of Electronic Transitions

  • Sigma to Sigma*: Involves sigma electrons. Common in saturated molecules.
  • Pi to Pi*: Involves pi electrons and is typical in unsaturated and aromatic molecules.
  • n to Sigma* and n to Pi*: Involve non-bonding electrons. These transitions require relatively lower energy and often occur in molecules with lone pairs.

These electronic transitions are responsible for the absorption of specific wavelengths of light, leading to the characteristic absorption spectra of different molecules. The spectra can provide valuable information about the molecular structure, electronic environment, and interaction of molecules.


Electronic spectroscopy has a wide range of applications including but not limited to:

  • Identifying functional groups in organic compounds.
  • Studying the dynamics of photochemical and photophysical processes.
  • Monitoring the concentration of substances in solutions.
  • Investigating the energy levels and electronic structures of molecules.

Thus, electronic spectroscopy serves as a fundamental tool in both chemical and physical studies, offering insights into molecular behaviors under various conditions.

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