Absorption spectrum

Definition

An absorption spectrum is a spectrum that displays the wavelengths of light absorbed by a substance, typically represented as dark lines or bands on a continuous spectrum. This spectrum is essential for understanding how molecules interact with electromagnetic radiation, particularly in electronic spectroscopy, where transitions between electronic states occur. The pattern of absorption reveals valuable information about the electronic structure and energy levels of the molecules involved, making it a critical tool in physical chemistry.

5 Must Know Facts For Your Next Test

1. Absorption spectra are crucial for identifying substances, as each molecule has a unique pattern of absorbed wavelengths.
2. The intensity of the absorption peaks can provide insights into the concentration of the absorbing species according to Beer-Lambert Law.
3. In electronic spectroscopy, the absorption spectrum corresponds to transitions between different electronic states and can reveal information about molecular structure.
4. The Franck-Condon factors play a key role in determining the shapes and intensities of peaks in an absorption spectrum due to vibrational transitions.
5. Temperature and pressure can affect the absorption spectrum by altering energy levels and molecular interactions, leading to shifts in peak positions.

Review Questions

• How does an absorption spectrum relate to electronic transitions in molecules?
  • An absorption spectrum is directly linked to electronic transitions, as it represents the specific wavelengths of light absorbed when electrons move from lower energy levels to higher ones. Each transition corresponds to a distinct energy difference, which manifests as specific lines or bands in the spectrum. Understanding these transitions helps chemists deduce the electronic structure and properties of the molecules involved.
• Discuss how the Franck-Condon principle affects the appearance of an absorption spectrum.
  • The Franck-Condon principle plays a significant role in shaping the absorption spectrum by influencing the intensity and position of spectral lines. It suggests that transitions occur most readily between vibrational levels that are closely spaced in energy, leading to stronger peaks for certain transitions. This principle helps explain why some absorptions are more pronounced than others and how vibrational states contribute to the overall spectral pattern.
• Evaluate how varying experimental conditions might influence the interpretation of an absorption spectrum.
  • Variations in experimental conditions such as temperature and pressure can significantly impact an absorption spectrum's interpretation. Higher temperatures may lead to broadening of spectral lines due to increased molecular motion and collision frequencies, while changes in pressure can shift energy levels and alter peak positions. These factors must be carefully considered when analyzing spectra, as they can affect conclusions drawn about molecular behavior and interactions.

Related terms

Electronic Transition: The process by which an electron moves from one energy level to another within an atom or molecule, usually induced by the absorption of a photon.

Franck-Condon Principle:

A principle that describes the intensity of spectral lines in electronic transitions, based on the overlap of vibrational wave functions in different electronic states.

Emission Spectrum: A spectrum that shows the wavelengths of light emitted by a substance when electrons transition from higher energy levels to lower ones, often complementary to absorption spectra.