San José State University


appletmagic.com Thayer Watkins Silicon Valley & Tornado Alley U.S.A. 

The Quantum Mechanical Analysis of a Two Body System 

The successful application of Schrödinger's wave mechanics to the hydrogen atom was a key to the acceptance of wave mechanics as the proper model of atomic reality. It was recognized that that analysis equally applied to any hydrogenic atom; i.e., any atom or ion with but a single electron. This is a further generalization to any system consisting of two objects of opposite electrostatic charges. This would include hydrogenic atoms but also positronium (an electron/positron pair), muonium and tauonium hydrogenic atoms, and any atom involving an antiproton and a positively charged lepton.
Let the two subatomic objects and their properties be labeled 0 and 1. The force between them is proportional to the product of their charges q_{0} and q_{1} and inversely proportional to the square of their separation distance s; i.e.,
where J is a constant. Since q_{0} and q_{1} are of opposite sign the force is an attraction. The force can be expressed as
In the subsequent analysis it is only Q=−Jq_{0}q_{1} that is relevant. The potential energy V is given by
The kinetic energy K of the system is given by
The Hamiltonian (total energy) function for the system is then
The timeindependent Schrödinger equation for the system is then
where ψ is the wave function and E is the energy. For stable systems E is negative. Thus to emphasize the negativity of E, it is expressed as −E.
But the above equation is not a convenient form for analysis. What is needed is a coordinate system suited to the geometry of the system.
Let the origin of the coordinate system be located at the center of mass for the two objects. Then
It is easily shown that
Thus there are not two "radial" variable but only one, s. Likewise there are not four angle variables but only two.
The revolutions of the two bodies about their center of mass takes place in a plane. Therefore a polar coordinate system is appropriate rather than a spherical one.
The two radial momenta are
The reduced mass μ of the system is defined by
Thus
Likewise p_{s1} = μ(ds/dt).
Therefore the radial component of kinetic energy is
Define p_{s} as μ(ds/dt) and thus
First of all note that
The kinetic energy due to the tangential velocities is
This can be represented as
Now define p_{θ} as
This means that
From the previous material, the Hamiltonian is given by
The Laplacian operator ∇² for polar coordinates (r, θ) is
Thus the equation to be satisfied by ψ is:
At this point it will be assumed that ψ(r, θ) is equal to R(r)Θ(θ). This is the separation of variables assumption. This is a mathematical convenience that is fraught with danger of precluding the physically relevant solutions. In this case it is alright because only circular orbits will be dealt with later.
When R(s)Θ(θ) is substituted into the equation it can be reduced to
This equation may be put into the form
The LHS of the above is a function only of s and the RHS a function only of θ. Therefore their common value must be a constant. Let this constant be denoted as n².
Thus
This equation has solutions of the form
where A and θ_{0} are constants. Through a proper orientation of the polar coordinate system θ_{0} can made equal to zero. So Θ(θ) = A·cos(n·&theta). In order for Θ(θ+2π) to be equal to Θ(θ) n must be an integer. The probability density is the squared magnitude of the wave function. Therefore the probability density is proportional to cos²(nθ).
Below is the shape of this function for n=6.
The radial equation is then
For large values of s the above equation approaches the equation
Let
The solutions to the above equation for S are exp(+λs) and exp(−λs). Only the negative exponential is relevant for physical situations.
On the other hand at s goes to zero the radial equation approaches the solution to this equation
This equation has the solution
where l(l−1)=n².
This has the problem that l would not be an integer. It is not necessary that all parameters in QM solutions be integral but it is a desirable simplification of the analysis. Therefore a slightly different approach will be used to establish the radial component of the wave function for the electron.
From the Laplacian it can be shown that −(h²/2μ)∇²ψ is equivalent to
the operator for p_{s}²ψ+p_{θ}²ψ. Furthermore it is shown at
that p_{θ}²ψ is quantized to h²l(l+1).
The function R(s) must satisfiy the equation
The first step toward a solution is to let sR(s) be denoted as u(s). The resulting equation for u is
The analysis can be further simplified by introducing some nondimensional variables; i.e.,
The equation for u(s) then simplifies to
As σ increases without bound the equation for u asymptotically approaches the equation
The solution to this equation is of the form
where A and B are constants. The only solutions of this form that are bounded as σ→∞ are those for which B=0.
As σ goes to zero the equation for u asymptotically approaches the equation
If V(σ) is of the form σ^{β} then
This suggests a solution for u(σ) of the form
where F(σ) is a polynomial in σ that is finite everywhere.
When the proposed solution is substituted into the equation for u the result is that F(σ) must satisfy the following equation.
If
then the coefficients C_{j} must satisfy the condition
The polynomial F(σ) will be of finite order if and only if there is an integer q such that the numerator of the fraction in the above condition is equal to zero; i.e.,
This means that λ must be an integer and that q=λ−(l+1). Usually λ is called the principal quantum number and it is denoted as n. The definitions of of κ and λ imply a quantifization of energy; i.e.,
The quantization condition for energy is then
The polynomials represented by F(σ) are known as Laguerre polynomials. They depend upon an integral parameter q, which is equal to (n−(l+1)). The first few are:
q  L_{q}(σ) 
0  (1/0!)1 
1  (1/1!)(1σ) 
2  (1/2!)(23σ+σ²) 
3  (1/3!)(618σ+9σ²σ³) 
The radial function is then
The probability density function is proportional to the square of this radial function. The shapes of R and R² for several values of n and l are shown in the display below. For historical reasons the values of l are coded as letters: S=0, P=1, D=2.
Below is a depiction of the probability bumps in the plane of the electron's motion for a principal quantum number of 6.
The electron quantum mechanically moves relatively slowly in a probability bump, otherwise known as a state, and then relatively rapidly to the next state (bump).
The expected value of the radial dimension is the same for any angle θ. The probability density as a function of θ is as shown below. The red horizontal is the classical probability density ; i.e., .the proportion of time spent at the different locations. As can seen the QM PDF averaged over a range of angles would be equal to classical PDF.
What came out of the quantum mechanical analysis of the two body problem is a probability distribution for the states of the system; i.e., the separation distance of the bodies and the angle in the plane of their revolutions about their center of mass. This probability distribution corresponds to the proportion of the time the system spends in its various states. This sort of probability distribution for the bodies may be derived from the probability distribution of the system, but there is no probability distribution corresponding to an intrinsic indeterminancy of the bodies.
(To be continued.)
Reference:
Richard L. Liboff, Introductory Quantum Mechanics, HoldenDay Inc., San Francisco, 1980.