Rotational Energy and Degeneracy

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SUMMARY

The discussion centers on the rotational energy equation for a rigid rotor, expressed as ε = kθ_r J (J+1), where k is the Boltzmann constant and J is the rotational quantum number. The degeneracy for a given J value is defined as 2J+1. For an energy level of 2 Joules, the only valid solution is J = 1, resulting in a degeneracy of 3. This three-fold degeneracy arises from the three linearly independent solutions to the Schrödinger equation corresponding to J = 1, specifically the spherical harmonics Y^1_m for m = -1, 0, and 1.

PREREQUISITES
  • Understanding of quantum mechanics, specifically rotational energy equations.
  • Familiarity with the rigid rotor model in quantum physics.
  • Knowledge of degeneracy in quantum systems.
  • Basic comprehension of spherical harmonics and their applications in quantum mechanics.
NEXT STEPS
  • Study the derivation of the rigid rotor model in quantum mechanics.
  • Learn about the implications of degeneracy in quantum systems.
  • Explore the properties and applications of spherical harmonics in quantum mechanics.
  • Investigate the Schrödinger equation and its solutions for different quantum states.
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Students and professionals in physics, particularly those focusing on quantum mechanics, molecular physics, and anyone interested in the mathematical foundations of rotational energy and degeneracy.

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I have a question regarding the rotational energy equation, derived based on the rigid rotor assumption:

##\epsilon = k\theta_r J (J+1)##
where k = Boltzmann constant, and J is the rotational quantum number.

The degeneracy is 2J+1.

Let's assume the constant quantity ##k\theta_r## = 1 and that the energy level is 2Joules.
Thus 2 = J(J+1) => J = +/- 1. The only physical solution is J = 1.
Thus degeneracy is 3 for J = 1.

Here is where I am confused. I thought degeneracy is when you have an energy level that DOES NOT consist of a unique set of quantum numbers. So if the degeneracy is 3, doesn't that mean I should have 3 different J values that can give me an energy level of 2 Joules?
I think I am understanding this incorrectly, because for 2 joules, the only solution is J = 1. Doesn't that mean the degenearcy is 1?
 
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There are three different and linearly independent solutions to the Schrödinger equation with ##J=1##. That's where the three-fold degeneracy comes from.

Google for "spherical harmonics" to see how this can be; the three solutions are multiples of the three functions ##Y^1_m## with ##m## equal to -1, 0, or 1.
 

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