Forbidden transitions

Definition

Forbidden transitions refer to electronic transitions between quantum states of an atom or molecule that are not allowed under certain selection rules, typically because they do not have the required change in angular momentum or parity. These transitions are less probable compared to allowed transitions and often occur at a significantly lower rate, resulting in weaker or absent spectral lines in emission or absorption spectra.

5 Must Know Facts For Your Next Test

1. Forbidden transitions often involve changes in angular momentum that violate selection rules, making them less likely to occur during interactions with electromagnetic radiation.
2. These transitions can still happen, but they generally occur with much lower probabilities, sometimes leading to observable effects in long-lived excited states.
3. The presence of forbidden transitions can result in weak lines in spectra that are crucial for identifying certain molecules or atoms in astrophysical contexts.
4. In many systems, forbidden transitions can gain some intensity through mechanisms like spin-orbit coupling or higher-order multipole moments.
5. Understanding forbidden transitions is essential in fields like laser physics and quantum optics, where they can affect the design and functioning of various optical devices.

Review Questions

• What are the conditions that classify a transition as forbidden, and how do selection rules play a role in this classification?
  • A transition is classified as forbidden when it does not meet the criteria established by selection rules, which typically involve restrictions on changes in quantum numbers such as angular momentum and parity. For instance, electric dipole transitions require specific changes in angular momentum that forbidden transitions fail to satisfy. Consequently, these transitions occur much less frequently than allowed ones, affecting their presence in spectral lines.
• Discuss how the concept of forbidden transitions impacts the interpretation of spectral lines in atomic or molecular spectroscopy.
  • In spectroscopy, forbidden transitions lead to spectral lines that are often weaker and less prominent than those from allowed transitions. Since these transitions have much lower probabilities of occurring, they may result in faint lines that can be critical for identifying species within a sample. The study of these lines can provide insight into the environment of the emitting species and can indicate longer radiative lifetimes associated with metastable states.
• Evaluate the significance of forbidden transitions in advanced applications such as laser technology and astrophysics, providing examples.
  • Forbidden transitions play a significant role in advanced applications such as laser technology and astrophysics by influencing the behavior and characteristics of laser output. For instance, certain laser systems leverage the properties of metastable states associated with forbidden transitions to achieve population inversion. In astrophysics, the detection of weak lines due to forbidden transitions can yield valuable information about cosmic environments, like electron density and temperature in nebulae, offering insights into stellar evolution and galactic chemistry.

Related terms

Selection Rules: Criteria that determine whether a transition between quantum states is allowed or forbidden based on changes in quantum numbers and angular momentum.

Dipole Moment: A measure of the separation of positive and negative charges in a system, significant for determining the strength of allowed transitions.

Radiative Lifetime: The average time an excited state exists before it returns to a lower energy state, impacting the intensity of spectral lines.