Motion in One Dimension

Quantum Mechanics A

Schrödinger Equation The most fundamental equation of quantum mechanics; given a Hamiltonian ${\mathcal {H}}$, it describes how a state $|\Psi \rangle$ evolves in time.

Physical Basis of Quantum Mechanics

Basic Concepts and Theory of Motion UV Catastrophe (Black-Body Radiation) Photoelectric Effect Stability of Matter Double Slit Experiment Stern-Gerlach Experiment The Principle of Complementarity The Correspondence Principle The Philosophy of Quantum Theory

Schrödinger Equation

Brief Derivation of Schrödinger Equation Relation Between the Wave Function and Probability Density Stationary States Heisenberg Uncertainty Principle Some Consequences of the Uncertainty Principle

Operators, Eigenfunctions, and Symmetry

Linear Vector Spaces and Operators Commutation Relations and Simultaneous Eigenvalues The Schrödinger Equation in Dirac Notation Transformations of Operators and Symmetry Time Evolution of Expectation Values and Ehrenfest's Theorem

Motion in One Dimension

One-Dimensional Bound States Oscillation Theorem The Dirac Delta Function Potential Scattering States, Transmission and Reflection Motion in a Periodic Potential Summary of One-Dimensional Systems

Discrete Eigenvalues and Bound States

Harmonic Oscillator Spectrum and Eigenstates Analytical Method for Solving the Simple Harmonic Oscillator Coherent States Charged Particles in an Electromagnetic Field WKB Approximation

Time Evolution and the Pictures of Quantum Mechanics

The Heisenberg Picture: Equations of Motion for Operators The Interaction Picture The Virial Theorem

Angular Momentum

Commutation Relations Angular Momentum as a Generator of Rotations in 3D Spherical Coordinates Eigenvalue Quantization Orbital Angular Momentum Eigenfunctions

Central Forces

General Formalism Free Particle in Spherical Coordinates Spherical Well Isotropic Harmonic Oscillator Hydrogen Atom WKB in Spherical Coordinates

The Path Integral Formulation of Quantum Mechanics

Feynman Path Integrals The Free-Particle Propagator Propagator for the Harmonic Oscillator

Continuous Eigenvalues and Collision Theory

Differential Cross Section and the Green's Function Formulation of Scattering Central Potential Scattering and Phase Shifts Coulomb Potential Scattering

In this chapter, we will study the quantum mechaincs of one-dimensional systems. We study such problems for two reasons. Not only is it interesting to study the simplest cases to demonstrate some of the general properties of quantum mechanical systems, but it also turns out that there are two- and three-dimensional systems that can be reduced to effective one-dimensional problems, such as a particle in a central potential. Another example of such a problem is that of a particle in three dimensions in a potential that is a sum of terms that each depend on only one coordinate.

Let's consider the motion in 1 direction of a particle in the potential V(x). Supposing that V(x) has finite values when x goes to infinity:

$\lim _{x\to -\infty }V(x)=V_{-},\lim _{x\to +\infty }V(x)=V_{+}$

and assuming that: $V_{-}<V_{+}\!$

The Schrodinger equation becomes:

$\left[-{\frac {\hbar ^{2}}{2m}}{\frac {d^{2}}{dx^{2}}}+V(x)\right]\psi (x)=E\psi (x)$

$\rightarrow {\frac {d^{2}}{dx^{2}}}\psi (x)+{\frac {2m}{\hbar ^{2}}}(E-V(x))\psi (x)=0$

From this equation we can discuss some general properties of 1-D motion as follows:

If $E>V_{+}\!$:

$E-V(x)>0\!$ at both $-\infty$ and $+\infty$. Therefore, the solutions of Schrodinger equation can be express as linear combinations of trigonometric functions (sine or cosine). The wave function is oscillating at both $-\infty$ and $+\infty$. The particle is in an unbound state. The energy spectrum is continous. Both oscillating solutions are allowed, and the energy levels are two-fold degenerate.

If $V_{-}\leq E\leq V_{+}$:

$E-V(x)>0\!$ at $-\infty$ but $E-V(x)<0$ at $+\infty$. Therefore, the wave function is oscillating at $-\infty$ but decaying exponentially at $+\infty$. The energy spectrum is still continous but no longer degenerate.

If $E<V_{-}\!$:

$E-V(x)\!<0$ at both $-\infty$ and $+\infty$. Therefore, the wave function decays exponentially at both $-\infty$ and $+\infty$. The particle is in a bound state. The energy spectrum is discrete and non-degenerate.

Problem

Consider a particle of mass $m$ in a three dimensional potential: $V(x,y,z)=X(x)+Y(y)+Z(z)\!$

Using the Schroedinger's equation, show that we can treat the problem like three independent one-dimensional problems. Relate the energy of the three-dimensional state to the effective energies of one-dimensional problem.