Forbidden transition

Definition

A forbidden transition refers to an electronic transition between two energy levels that is highly unlikely to occur due to selection rules, which govern the allowed changes in quantum states during transitions. These transitions often involve changes in quantum numbers that are not permitted by the dipole approximation, making them less probable and usually resulting in much weaker spectral lines compared to allowed transitions. Understanding forbidden transitions is crucial for interpreting certain spectral features and the behavior of molecules in spectroscopy.

5 Must Know Facts For Your Next Test

1. Forbidden transitions are often associated with specific types of electronic states, such as those involving changes in spin multiplicity.
2. While they are unlikely to occur, forbidden transitions can still be observed in spectroscopic measurements, typically with lower intensity than allowed transitions.
3. The time required for a forbidden transition to occur is generally longer compared to allowed transitions, leading to weaker spectral features.
4. Forbidden transitions can become more probable under certain conditions, such as when there is a perturbation from an external field or interaction with other states.
5. In some cases, forbidden transitions can provide valuable information about molecular environments and dynamics, revealing insights into excited state lifetimes and molecular symmetry.

Review Questions

• What are the key selection rules that determine whether a transition is forbidden or allowed?
  • Key selection rules dictate the changes in quantum numbers that can occur during a transition. For electric dipole transitions, which are the most common, changes in the orbital angular momentum quantum number (\
• How does the intensity of spectral lines relate to forbidden transitions, and what factors influence this relationship?
  • The intensity of spectral lines is closely related to the probability of their associated transitions occurring. Forbidden transitions have significantly lower probabilities compared to allowed ones, leading to weaker and less intense spectral lines. Factors influencing this relationship include the nature of the electronic states involved, any external perturbations affecting the system, and the overall molecular symmetry. Additionally, longer lifetimes for forbidden transitions may allow for some indirect observations through techniques like resonance or stimulated emission.
• Evaluate the implications of forbidden transitions on molecular spectroscopy and how they can enhance our understanding of molecular behavior.
  • Forbidden transitions play a significant role in molecular spectroscopy by providing insights into molecular environments and dynamics that might not be captured through allowed transitions alone. Their detection can reveal information about electronic configurations, spin states, and interactions within a molecule. Moreover, studying these transitions can enhance our understanding of phenomena such as phosphorescence or delayed fluorescence in photophysical processes. By analyzing forbidden transitions, researchers can also gain deeper knowledge about energy transfer processes and molecular symmetry adaptations in response to external influences.

Related terms

Selection Rules: Rules that dictate the allowed transitions between energy levels based on changes in quantum numbers and symmetries of molecular states.

Spectral Line Intensity: The strength or brightness of a spectral line, which is influenced by the probability of the transition associated with that line.

Spin Multiplicity: The number of different spin states available for a given electronic state, which can affect the likelihood of transitions occurring.