Isotropic Harmonic Oscillator

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

We now solve the isotropic harmonic oscillator using the formalism that we have just developed. While it is possible to solve it in Cartesian coordinates, we gain additional insight by solving it in spherical coordinates, and it is easier to determine the degeneracy of each energy level.

The radial part of the Schrödinger equation for a particle of mass $M\!$ in an isotropic harmonic oscillator potential $V(r)={\frac {1}{2}}M\omega ^{2}r^{2}$ is given by:

$-{\frac {\hbar ^{2}}{2M}}{\frac {d^{2}u_{nl}}{dr^{2}}}+\left({\frac {\hbar ^{2}}{2M}}{\frac {l(l+1)}{r^{2}}}+{\frac {1}{2}}M\omega ^{2}r^{2}\right)u_{nl}=Eu_{nl}.$

Let us begin by looking at the solutions $u_{nl}\!$ in the limits of small and large $r.\!$

As $r\rightarrow 0\!$, the equation reduces to

$-{\frac {\hbar ^{2}}{2M}}{\frac {d^{2}u_{nl}}{dr^{2}}}+{\frac {\hbar ^{2}}{2M}}{\frac {l(l+1)}{r^{2}}}u_{nl}=Eu_{nl}.$

The only solution of this equation that does not diverge as $r\rightarrow 0$ is $u_{nl}(r)\simeq r^{l+1}.$

In the limit as $r\rightarrow \infty ,$ on the other hand, the equation becomes

$-{\frac {\hbar ^{2}}{2M}}{\frac {d^{2}u_{nl}}{dr^{2}}}+{\frac {1}{2}}M\omega ^{2}r^{2}u_{nl}=Eu_{nl}$

whose solution is given by $u_{nl}(r)\simeq e^{-M\omega r^{2}/2\hbar }.$

We may now assume that the general solution to the equation is given by

$u_{nl}(r)=r^{l+1}e^{-M\omega r^{2}/2\hbar }f_{nl}(r).$

Substituting this expression into the original equation, we obtain

${\frac {\partial ^{2}f_{l}(r)}{\partial r^{2}}}+2\left({\frac {l+1}{r}}-{\frac {Mw}{\hbar }}r\right){\frac {\partial f_{l}(r)}{\partial r}}+\left[{\frac {2ME}{\hbar ^{2}}}-(2l+3){\frac {Mw}{\hbar }}\right]f_{l}(r)=0$

Now we try the power series solution

$f_{l}(r)=\sum _{n=0}^{\infty }a_{n}r^{n}=a_{0}+a_{1}r+a_{2}r^{2}+a_{3}r^{3}+...+a_{n}r^{n}+...$

Substituting this solution into the reduced form of the equation,

$\sum _{n=0}^{\infty }\left[n(n-1)a_{n}r^{n-2}+2\left({\frac {l+1}{r}}-{\frac {Mw}{\hbar }}\right)na_{n}r^{n-1}+\left[{\frac {2ME}{\hbar ^{2}}}-(2l+3){\frac {Mw}{\hbar }}\right]a_{n}r^{n}\right]=0$

which reduces to the equation

$\sum _{n=0}^{\infty }\left[n(n+2l+1)a_{n}r^{n-2}+\left(-{\frac {2Mw}{\hbar }}n+{\frac {2ME}{\hbar ^{2}}}-(2l+3){\frac {Mw}{\hbar }}\right)a_{n}r^{n}\right]=0$

For this equation to hold, the coefficients of each of the powers of r must vanish seperately.

So,when $n=0\!$ the coefficient of $r^{-2}\!$ is zero, $0.(2l+1)a_{0}=0$ implying that $a_{0}\!$ need not be zero.

Equating the coefficient of $r^{-1}\!$to be zero, $1.(2l+2)a_{1}=0\!$ implying that $a_{1}\!$ must be zero.

Equating the coefficient of $r^{-n}\!$ to be zero, we get the recursion relation which is:

$\sum _{n=0}^{\infty }(n+2)(n+2l+3)a_{n+2}=\left[-{\frac {2ME}{\hbar ^{2}}}+(2n+2l+3){\frac {Mw}{\hbar }}\right]a_{n}$

The function $f_{l}(r)\!$ contains only even powers in n and is given by:

$f_{l}(r)=\sum _{n=0}^{\infty }a_{2n}r^{2n}=\sum _{n^{'}=0,2,4}^{\infty }a_{n^{'}}r^{n^{'}}$

Now as $n\rightarrow \infty \!$ , $f_{l}(r)\!$ diverges so that for finite solution, the series should stop after $r^{n^{'}+2}\!$ leading to the quantization condition:

${\frac {2M}{\hbar ^{2}}}E_{n^{'}l}-{\frac {Mw}{\hbar }}(2n^{'}+2l+3)=0$

$E_{n^{'}l}=\left(n^{'}+l+{\frac {3}{2}}\right)\hbar w,n^{'}=0,1,2,3,...$

As a result, the energy of the isotropic harmonic oscillator is given by:

$E_{n}=\left(n+{\frac {3}{2}}\right)\hbar w,n=0,1,2,3,...$ with $n=n^{'}+l\!$

The degeneracy corresponding to the nth level is:

$g={\frac {1}{2}}(n+1)(n+2)$

The total wavefunction of the isotropic Harmonic Oscillator is given by:

$\psi _{nlm}(r,\theta ,\phi )=r^{l+1}f_{l}(r)Y_{lm}(\theta ,\phi )e^{-}{\frac {Mw}{2\hbar }}r^{2}=R_{nl}(r)Y_{lm}(\theta ,\phi )$