In quantum mechanics, a particle in a spherically symmetric potential is a system where a particle's potential energy depends only on its distance from a central point, not on the direction. This model is fundamental to physics because it can be used to describe a wide range of real-world phenomena, from the behavior of a single electron in a hydrogen atom to the approximate structure of atomic nuclei. The particle's behavior is described by the Time-independent Schrödinger equation. Because of the spherical symmetry, the problem can be greatly simplified by using spherical coordinates ( r {\displaystyle r} , θ {\displaystyle \theta } and ϕ {\displaystyle \phi } ) and a mathematical technique called separation of variables. This allows the solution (the wavefunction) to be split into a radial part, depending only on the distance r {\displaystyle r} , and an angular part. The angular solutions are universal for all spherically symmetric potentials and are known as spherical harmonics. The radial part of the solution is specific to the shape of the potential V ( r ) {\displaystyle V(r)} and determines the allowed energy levels of the system. In the general time-independent case, the dynamics of a particle in a spherically symmetric potential are governed by a Hamiltonian of the following form: H ^ = p ^ 2 2 m 0 + V ( r ) {\displaystyle {\hat {H}}={\frac {{\hat {p}}^{2}}{2m_{0}}}+V({r})} Here, m 0 {\displaystyle m_{0}} is the mass of the particle, p ^ {\displaystyle {\hat {p}}} is the momentum operator, and the potential V ( r ) {\displaystyle V(r)} depends only on the radial distance r {\displaystyle r} from the origin. This mathematical setup leads to an ordinary differential equation for the radial part of the wavefunction, which can be solved for important potentials like the Coulomb potential (for atoms) and the spherical square well (for nuclei).
Structure of the eigenfunctions If solved by separation of variables, the eigenstates of the system will have the form: ψ ( r , θ , ϕ ) = R ( r ) Θ ( θ ) Φ ( ϕ ) {\displaystyle \psi (r,\theta ,\phi )=R(r)\Theta (\theta )\Phi (\phi )} in which the spherical angles θ {\displaystyle \theta } and ϕ {\displaystyle \phi } represent the polar and azimuthal angle, respectively. Those two factors of ψ {\displaystyle \psi } are often grouped together as spherical harmonics, so that the eigenfunctions take the form: ψ ( r , θ , ϕ ) = R ( r ) Y ℓ m ( θ , ϕ ) . {\displaystyle \psi (r,\theta ,\phi )=R(r)Y_{\ell m}(\theta ,\phi ).} The differential equation which characterises the function R ( r ) {\displaystyle R(r)} is called the radial equation.
Derivation of the radial equation
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