Allowed transitions

Definition

Allowed transitions refer to the specific electronic transitions that can occur between energy levels in an atom or molecule, governed by quantum mechanical rules. These transitions are determined by selection rules, which dictate the conditions under which a transition can take place, considering factors like symmetry and conservation of angular momentum. Understanding allowed transitions is crucial for grasping how light interacts with matter, particularly in terms of absorption and emission processes.

5 Must Know Facts For Your Next Test

1. Allowed transitions generally occur when there is a change in the quantum number associated with the energy levels, such as the principal or angular momentum quantum numbers.
2. In allowed transitions, the probability of the transition happening is high, which means they dominate processes like absorption and emission of light.
3. Selection rules vary depending on whether the system has spin, parity, or rotational symmetry, affecting which transitions can occur.
4. Not all transitions are allowed; forbidden transitions have very low probabilities and typically involve a change in parity or certain symmetry considerations.
5. Allowed transitions are fundamental to understanding phenomena like fluorescence and phosphorescence, where materials absorb light and re-emit it at different wavelengths.

Review Questions

• What are the key factors that determine whether an electronic transition is allowed or forbidden?
  • The key factors that determine whether an electronic transition is allowed or forbidden are the selection rules that arise from quantum mechanics. These rules take into account changes in quantum numbers associated with energy levels, as well as considerations of symmetry and conservation laws. For example, transitions involving changes in angular momentum must satisfy specific conditions related to the parity of the states involved, which ultimately dictates if a transition can proceed.
• Discuss how allowed transitions impact the behavior of light when interacting with matter.
  • Allowed transitions have a significant impact on how light interacts with matter because they define the energy levels at which atoms or molecules can absorb or emit photons. When light encounters a substance, it can excite electrons from lower to higher energy levels through these allowed transitions. This absorption leads to observable phenomena like color and fluorescence. The efficiency of these interactions is also tied to how likely these allowed transitions are to occur compared to forbidden ones.
• Evaluate the importance of allowed transitions in technologies such as lasers and photodetectors.
  • Allowed transitions play a critical role in technologies like lasers and photodetectors because they dictate how efficiently these devices can operate. In lasers, allowed transitions ensure that electrons can be efficiently excited and cause stimulated emission, leading to coherent light generation. Similarly, in photodetectors, understanding allowed transitions allows for optimizing materials to absorb specific wavelengths of light effectively. As a result, the design and functionality of these devices hinge on a thorough grasp of allowed transitions and their governing principles.

Related terms

Selection rules: Guidelines that dictate whether a transition between quantum states is allowed or forbidden based on quantum mechanical principles.

Spontaneous emission: The process by which an excited electron in an atom or molecule falls back to a lower energy level, emitting a photon in the process.

Quantum states: Specific energy levels or configurations of a system, such as an electron in an atom, defined by quantum numbers.