Stimulated Emission = Stimulated Absorption

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Discussion Overview

The discussion revolves around the relationship between stimulated emission and stimulated absorption in the context of Einstein's coefficients, particularly focusing on their roles in maintaining equilibrium in a system. Participants explore the conceptual understanding of these processes as described in Griffith's text, touching on mathematical proofs and underlying principles.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about why stimulated emission would not be less than stimulated absorption in a system at equilibrium, despite mathematical proofs suggesting equivalence.
  • Another participant questions the basis for the assumption that stimulated emission would be less, asking for clarification on the reasoning behind this thought.
  • A different participant critiques the proof in Griffith's book as circular, arguing that it relies on the Bose-Einstein distribution to establish the equality of the coefficients, suggesting that the principle of detailed balance is a more fundamental explanation derived from quantum theory.
  • Another participant clarifies that while stimulated emission equals stimulated absorption, normal absorption also occurs, and that the total absorption includes both stimulated and normal absorption, leading to a more complex relationship in equilibrium.

Areas of Agreement / Disagreement

Participants express differing views on the conceptual understanding of stimulated emission and absorption, with some agreeing on the mathematical equivalence while others challenge the assumptions and proofs presented. The discussion remains unresolved regarding the conceptual implications of these processes.

Contextual Notes

Limitations include potential circular reasoning in proofs, dependence on specific distributions for equilibrium, and the complexity introduced by normal absorption processes, which may not be fully addressed in the discussion.

zuukr
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I'm having a bit of trouble understanding this. I am going through the section in Griffith's regarding Einstein's Coefficients. For a system in equilibrium, the rate of particles undergoing emission needs to equal the rate of particles undergoing absorption in order to maintain equilibrium. When spontaneous emission is considered as a factor of emission as well, the rate of stimulated emission is still equivalent to the rate of stimulated absorption. I can see that this is proven mathematically, but I am trying to conceptualize this. Why wouldn't stimulated emission be less than stimulated absorption for a system in equilibrium?

Thank you
 
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I'm curious as to why you would think stimulated emission would be less.
If it makes sense mathematically what is making you think it should be any different?
 
Well, the proof in Griffiths book is circular, because he assumes the Bose-Einstein distribution as the equilibrium distribution to prove ##B_{ab}=B_{ba}##. What's really behind it is the principle of detailed balance, which can be derived from the unitarity of time evolution in quantum theory.
 
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Stimulated emission (B10) will equal stimulated absorption, but there is also normal absorption by unexcited molecules. The Einstein coefficient of absorption B01 is for both stimulated and normal absorption. In equilibrium total absorption equals total emission (B10 = A01 + B01, where A[10] is the coefficient of spontaneous emission.)

Note, letters A and B were chosen by the German speaking Einstein who chose A for spontaneous Ausgang (emission), not to be confused with A for absorption.
What I have called normal absorption N01 will equal spontaneous emission A10.
 

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