Question

Using the momentum representation, calculate the bound-state energy eigenvalue and the corresponding eigenfunction for the potential $V(x)=-g \delta(x)($ for $g>0$ ). Compare with the results in Section 6.4.

    Using the momentum representation, calculate the bound-state energy eigenvalue and the corresponding eigenfunction for the potential $V(x)=-g \delta(x)($ for $g>0$ ). Compare with the results in Section 6.4.
Quantum mechanics
Quantum mechanics
Eugen Merzbacher 3rd Edition
Chapter 9, Problem 2 ↓

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Step 1: Start with the time-independent Schrödinger equation in momentum space: $$\frac{p^2}{2m}\phi(p) - g\phi(0) = E\phi(p)$$  Show more…

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Using the momentum representation, calculate the bound-state energy eigenvalue and the corresponding eigenfunction for the potential $V(x)=-g \delta(x)($ for $g>0$ ). Compare with the results in Section 6.4.
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Key Concepts

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Momentum Representation in Quantum Mechanics
The momentum representation is an alternative formulation of quantum mechanics in which states and operators are expressed in terms of momentum eigenstates rather than position eigenstates. In this framework, the Schrödinger equation is recast as an integral equation because the potential term, especially if it is localized as a delta function, becomes a convolution. This representation is particularly useful when dealing with potentials that have a simple structure in momentum space or when the Fourier transform of the potential is easier to handle than the original position-space form.
Bound States in Quantum Mechanics
Bound states refer to the quantum states where particles are confined to a finite region of space, characterized by discrete energy eigenvalues that are lower than the continuum of scattering states. These states typically result from attractive potentials, where the particle's energy is less than the potential at infinity. In the delta potential problem, the attractive nature of the potential (-g ?(x) for g > 0) leads to one unique bound state, whose energy is negative, indicating that the particle is indeed bound.
Delta Function Potential
The delta function potential is a highly localized potential that acts only at a single point in space. In quantum mechanics, it serves as an idealized model for short-range interactions and is mathematically represented by a Dirac delta function. Despite its simplicity, the delta potential captures the essential features of bound and scattering states and is exactly solvable. Its simplicity makes it an excellent example for teaching and understanding the key principles of potential scattering and bound state formation.
Fourier Transform and Its Role
The Fourier transform is a mathematical tool that is used to convert functions between position space and momentum space. In the context of the delta potential problem, the Fourier transform is applied to the Schrödinger equation to move from the position representation, where the delta potential is localized, to the momentum representation, where the potential appears as a constant. This transformation simplifies the analysis and allows one to derive the bound state conditions and eigenfunctions in momentum space more straightforwardly.
Energy Eigenvalue Equation
The energy eigenvalue equation arises from solving the Schrödinger equation for a given potential. In momentum space, this leads to an integral equation that must be solved self-consistently to find the discrete energy eigenvalues corresponding to bound states. For the delta potential, this process reveals a quantization condition that yields a single negative energy eigenvalue, confirming that there is only one bound state. This eigenvalue and the corresponding eigenfunction, when transformed back to position space, agree with the well-known results for the attractive delta function potential.

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Consider a particle in one dimension bound to a fixed center by a $\delta$ -function potential of the form \[ V(x)=-v_{0} \delta(x), \quad\left(v_{0} \text { real and positive }\right) \] Find the wave function and the binding energy of the ground state. Are there excited bound states?

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