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

In summary: A state with negative energy cannot have a nonzero inner product with both positive and negative energy states.
  • #1
dyn
773
61
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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  • #2
What would happen with the first and second derivative of the antisymmetric wave function at x=0?
 
  • #3
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
 
  • #4
I suggest you sketch the wavefunctions. If you encounter difficulty, sketch the wavefunctions for one delta potential, then two, then three, etc.
 
  • #5
I tried that. I can manage to draw an antisymmetric wavefunction that goes through x=0 and has 4 other nodes !
 
  • #6
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.
 
  • #7
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.
 

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

1. What is a delta function potential?

A delta function potential is a mathematical concept used in physics and mathematics to describe a localized potential energy that is infinitely strong at a single point and zero everywhere else. It is represented by the Dirac delta function, which is zero everywhere except at the point of interest where it is infinite.

2. How is a delta function potential used in physics?

Delta function potentials are commonly used in quantum mechanics to model point-like interactions between particles. They can also be used in classical mechanics to model point-like forces acting on a system. In both cases, the delta function potential is used to represent a strong, localized interaction at a specific point.

3. What are the properties of a delta function potential?

One of the main properties of a delta function potential is that it is infinitely strong at the point of interest and zero everywhere else. It is also an odd function, meaning that it is symmetric about the origin and its integral over the entire real line is equal to 1.

4. How is a delta function potential represented mathematically?

A delta function potential is represented by the Dirac delta function, which is denoted as δ(x). It is defined as zero for all values of x except at x = 0, where it is infinite. It is also important to note that the Dirac delta function is not a proper function in the traditional sense, but rather a distribution or generalized function.

5. What are some real-life examples of delta function potentials?

Delta function potentials can be observed in many physical systems, such as the interaction between an electron and a nucleus in an atom, where the electron experiences a strong, localized potential at the nucleus. They can also be used to model the force between two particles in a molecule, where the particles are treated as point-like objects. Other examples include the interaction between a particle and a boundary surface, and the behavior of electrons in a crystal lattice.

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