Free Particle in Spherical Coordinates

Quantum Mechanics A

Schrödinger Equation The most fundamental equation of quantum mechanics; given a Hamiltonian Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \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

A free particle is a specific case when $V_{0}=0\!$ of the motion in a uniform potential $V(r)=V_{0}\!$. So it's more useful to consider a particle moving in a uniform potential. The Schrödinger equation for the radial part of the wave function is:

$\left(-{\frac {\hbar ^{2}}{2m}}{\frac {\partial ^{2}}{\partial r^{2}}}+{\frac {\hbar ^{2}}{2m}}{\frac {l(l+1)}{r^{2}}}+V_{0}\right)u_{l}(r)=Eu_{l}(r)$

let $k^{2}={\frac {2m}{\hbar ^{2}}}|E-V|$. Rearranging the equation gives

$\left(-{\frac {\partial ^{2}}{\partial r^{2}}}+{\frac {l(l+1)}{r^{2}}}-k^{2}\right)u_{l}(r)=0$

Letting $\rho =kr\!$ gives the terms that ${\frac {1}{r^{2}}}={\frac {k^{2}}{\rho ^{2}}}$ and ${\frac {\partial ^{2}}{\partial r^{2}}}=k^{2}{\frac {\partial ^{2}}{\partial \rho ^{2}}}$. Then the equation becomes:

$\left(-{\frac {\partial ^{2}}{\partial \rho ^{2}}}+{\frac {l(l+1)}{\rho ^{2}}}\right)u_{l}(\rho )=u_{l}(\rho )=d_{l}d_{l}^{+}u_{l}(\rho )$

where $d_{l}\!$ and $d_{l}^{\dagger }\!$ become the raising and lowering operators:

$d_{l}={\frac {\partial }{\partial \rho }}+{\frac {l+1}{\rho }},$

$d_{l}^{\dagger }=-{\frac {\partial }{\partial \rho }}+{\frac {l+1}{\rho }}$

Being $d_{l}^{\dagger }d_{l}=d_{l+1}d_{l+1}^{\dagger }$, it can be shown that

$d_{l}^{\dagger }u_{l}(\rho )=c_{l}u_{l+1}(\rho )$

For $\ell =0$, $-{\frac {\partial ^{2}}{\partial \rho ^{2}}}u_{0}(\rho )=u_{0}(\rho )$, gives the solution as:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle u_0(\rho)=A\sin(\rho)-B\cos(\rho)\!}

The raising operator can be applied to the ground state in order to find high orders of Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \ u_0(\rho)} ;

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle d_0^\dagger u_0(\rho)=\left(-\frac{\partial}{\partial\rho}+\frac{l+1}{\rho}\right)u_0(\rho)=c_0 u_1(\rho)}

By this way, we can get the general expression:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle f_l(\rho)=\frac{u_l(\rho)}{\rho}=A_lj_l(\rho)+B_ln_l(\rho)} ,

where Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j_l(\rho) \!} is spherical Bessel function and Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n_l(\rho) \! } is spherical Neumann function.

Explicit Forms of the Spherical Bessel and Neumann Functions

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j_0(z) = \frac{\sin(z)}{z} }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j_1(z) = \frac{\sin(z)}{z^2} - \frac{\cos(z)}{z} }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j_2(z) = \left( \frac{3}{z^3} - \frac{1}{z}\right) \sin(z) - \frac{3}{z^2}\cos(z) }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n_0(z) = -\frac{\cos(z)}{z} }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n_1(z) = -\frac{\cos(z)}{z^2} - \frac{\sin(z)}{z} }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n_2(z) = - \left( \frac{3}{z^3} - \frac{1}{z}\right) \cos(z) - \frac{3}{z^2}\sin(z) }

The spherical Hankel functions of the first and second kind can be written in terms of the spherical Bessel and spherical Neumann functions, and are defined by:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h_{\ell}^{(1)} = j_{\ell}(z) + in_{\ell}(z) }

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle h_{\ell}^{(2)} = j_{\ell}(z) - in_{\ell}(z) }

The asymptotic form of the spherical Bessel and Neumann functions (as z Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \rightarrow} large) are given by:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j_{\ell}(z) = \frac{\sin(z-\frac{\ell \pi}{2})}{z} }

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n_{\ell}(z) = \frac{\cos(z-\frac{\ell \pi}{2})}{z} }

The first few zeros of the spherical Bessel function:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \ell = 0: 3.142, 6.283, 9.425, 12.566 }

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \ell = 1: 4.493, 7.725, 10.904, 14.066 }

The derivatives of the spherical Bessel and Neumann functions are defined by:

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle j'_{\ell}(z) = \frac{\ell}{z}j_{\ell}(z) - j_{\ell+1}(z) }

and

Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle n'_{\ell}(z) = \frac{\ell}{z}n_{\ell}(z) - n_{\ell+1}(z) }