Why can't there be an antisymmetric excited state in a delta function potential?

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Discussion Overview

The discussion centers on the existence of an antisymmetric excited state in a delta function potential, specifically in a potential that is zero everywhere except at five points on the x-axis where attractive delta potentials are located. Participants explore the implications of wavefunction characteristics and energy states in this context.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes the possibility of an antisymmetric excited state with a node at x=0 and four additional nodes between each delta potential.
  • Another participant questions the behavior of the first and second derivatives of the antisymmetric wave function at x=0.
  • A participant suggests that sketching the wavefunctions for varying numbers of delta potentials may clarify the situation.
  • One participant claims to have drawn an antisymmetric wavefunction that meets the proposed criteria but questions its physical validity.
  • Another participant argues that the proposed state would have positive energy and would not be a bound state, as it changes "too fast" between nodes.
  • It is noted that a bound state wave function must resemble a linear combination of exponentials in regions where the potential is zero.

Areas of Agreement / Disagreement

Participants express differing views on the feasibility of the antisymmetric excited state, with some arguing against its existence due to energy considerations while others maintain that it can be visualized. The discussion remains unresolved.

Contextual Notes

Participants highlight the need for careful consideration of wavefunction behavior at nodes and the implications for energy states, but do not reach a consensus on the validity of the proposed antisymmetric state.

dyn
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I have a potential which is zero everywhere except at -2a , -a , 0 , a , 2a on the x-axis where there is an attractive delta potential at each of the 5 points. I know there is a maximum of 5 bound states. I know there can be no nodes for |x| > 2a and a maximum of one node between each delta potential. My question is ; why can't there be one further excited state - an antisymmetric state with a node at x=0 and 4 further nodes ; one between each delta potential ?
 
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What would happen with the first and second derivative of the antisymmetric wave function at x=0?
 
Antisymmetric wavefunction would have the form of sine so 1st derivative would be cos(0) =1 , 2nd derivative would be sin(0)=0. I can't see how that helps and the same would happen for the 1st and 3rd excited states as well
 
I suggest you sketch the wavefunctions. If you encounter difficulty, sketch the wavefunctions for one delta potential, then two, then three, etc.
 
I tried that. I can manage to draw an antisymmetric wavefunction that goes through x=0 and has 4 other nodes !
 
That state will have positive energy. The wave function changes "too fast" between the node at x=0 and the nodes between 0 and +/-a to have negative energy. It is also not an energy eigenstate; it will have nonzero inner product with both positive and negative energy states.
 
dyn said:
Antisymmetric wavefunction would have the form of sine so 1st derivative would be cos(0) =1 , 2nd derivative would be sin(0)=0. I can't see how that helps and the same would happen for the 1st and 3rd excited states as well

As Avodyne says, a wave function that looks like the sine function has positive energy so will not be bound. A bound state wave function must look like a linear combination of exponentials in regions where V=0.
 

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