Allowed transition

Definition

An allowed transition refers to a spectral transition that complies with the selection rules governing electronic, vibrational, or rotational changes in molecules. These transitions are characterized by the ability to occur with a significant probability when a molecule interacts with electromagnetic radiation, resulting in observable spectral lines. The concept of allowed transitions is crucial for understanding how and why certain spectral lines appear in spectroscopic studies.

5 Must Know Facts For Your Next Test

1. Allowed transitions typically involve changes in the quantum numbers associated with electronic, vibrational, or rotational states that meet the criteria set by selection rules.
2. In electronic spectroscopy, allowed transitions usually involve changes in spin multiplicity that follow the spin selection rule (ΔS = 0).
3. Allowed transitions generally produce stronger spectral lines compared to forbidden transitions due to their higher probability of occurring.
4. In vibrational spectroscopy, allowed transitions can often be observed through techniques like infrared spectroscopy, where specific vibrational modes correspond to allowed transitions.
5. The intensity of allowed transitions can be influenced by factors such as the concentration of the absorbing species and the path length of the light through the sample.

Review Questions

• What are the key criteria that define an allowed transition, and how do these relate to selection rules?
  • An allowed transition is defined by its compliance with selection rules, which dictate the changes in quantum numbers that can occur during a transition. For instance, in electronic transitions, an allowed transition must maintain the same spin multiplicity (ΔS = 0). This relationship ensures that certain energy state changes can occur with a significant probability, leading to observable spectral lines while forbidden transitions do not follow these criteria and hence have much lower probabilities.
• How do allowed transitions differ from forbidden transitions in terms of spectral line intensity and probability of occurrence?
  • Allowed transitions are characterized by a higher probability of occurring compared to forbidden transitions, resulting in much stronger spectral line intensities. Forbidden transitions do not meet certain selection rule criteria, which leads to their low probability and often weak or imperceptible spectral lines. This distinction is essential for interpreting spectra, as it helps scientists identify which transitions are likely contributing to the observed data.
• Evaluate the implications of allowed transitions on the analysis of molecular spectra and their role in practical applications such as spectroscopy.
  • Allowed transitions play a pivotal role in molecular spectroscopy by providing clear and strong spectral lines that allow for precise identification and quantification of substances. Their significance lies in their ability to enhance the sensitivity and accuracy of spectroscopic methods such as UV-Vis and IR spectroscopy. Understanding these transitions aids researchers in interpreting complex spectra, designing experiments, and developing applications across various fields like chemistry, environmental science, and pharmaceuticals where accurate molecular characterization is crucial.

Related terms

Selection rules: Criteria that determine whether a transition between energy states is permitted or forbidden based on quantum mechanical principles.

Forbidden transition: A transition that does not comply with selection rules, resulting in a significantly lower probability of occurrence and often leading to weak or no observable spectral lines.

Spectral line: A distinct line in a spectrum that corresponds to a specific wavelength or frequency of light emitted or absorbed during a transition between energy levels.