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SPHERICAL BESSEL FUNCTIONS

When the Helmholtz equation is separated in spherical coordinates, the radial equation has the form

This is Eq. (9.65) of Section 9.3. The parameter k enters from the original Helmholtz equation, while n(n + 1) is a separation constant. From the behavior of the polar angle function (Legendre’s equation, Sections 9.5 and 12.5), the separation constant must have this form, with n a nonnegative integer. Equation (11.139) has the virtue of being selfadjoint, but clearly it is not Bessel’s equation. However, if we substitute

Equation (11.139) becomes

which is Bessel’s equation. Z is a Bessel function of order  (n an integer). Because of the importance of spherical coordinates, this combination, that is,

occurs quite often.

Definitions

It is convenient to label these functions spherical Bessel functions with the following defining equations:

These spherical Bessel functions (Figs. 11.13 and 11.14) can be expressed in series form by using the series (Eq. (11.5)) for J_n , replacing n with

         (11.142)

Using the Legendre duplication formula,

              (11.143)

we have

This yields

The Legendre duplication formula can be used again to give

              (11.147)

These series forms, Eqs. (11.144) and (11.147), are useful in three ways: (1) limiting values as x → 0, (2) closed-form representations for n = 0, and, as an extension of this, (3) an indication that the spherical Bessel functions are closely related to sine and cosine.
For the special case n = 0 we find from Eq. (11.144) that

            (11.148)

whereas for n₀ , Eq. (11.147) yields

          (11.149)

From the definition of the spherical Hankel functions (Eq. (11.141)),

              (11.150)

Equations (11.148) and (11.149) suggest expressing all spherical Bessel functions as combinations of sine and cosine. The appropriate combinations can be developed from the power-series solutions, Eqs. (11.144) and (11.147), but this approach is awkward. Actually the trigonometric forms are already available as the asymptotic expansion of Section 11.6.
From Eqs. (11.131) and (11.129a),

(11.151)

Now, P_n+1/2   and Q_n+1/2 are polynomials. This means that Eq. (11.151) is mathematically exact, not simply an asymptotic approximation. We obtain

(11.152)

and so on.

Limiting Values

For x ≪ 1,26 Eqs. (11.144) and (11.147) yield

The transformation of factorials in the expressions for n_n (x ) employs Exercise 8.1.3. The limiting values of the spherical Hankel functions go as ±in_n (x ).
The asymptotic values of j_n ,n_n , , and  may be obtained from the Bessel asymptotic forms, Section 11.6. We find

The condition for these spherical Bessel forms is that x ≫ n(n + 1)/2. From these asymptotic values we see that j_n (x ) and n_n (x ) are appropriate for a description of standing spherical waves;  (x ) and  (x ) correspond to traveling spherical waves. If the time dependence for the traveling waves is taken to be e^−iωt , then  (x ) yields an outgoing traveling spherical wave,  (x ) an incoming wave. Radiation theory in electromagnetism and scattering theory in quantum mechanics provide many applications.

Recurrence Relations

The recurrence relations to which we now turn provide a convenient way of developing the higher-order spherical Bessel functions. These recurrence relations may be derived from the series, but, as with the modified Bessel functions, it is easier to substitute into the known recurrence relations (Eqs. (11.10) and (11.12)). This gives

Rearranging these relations (or substituting into Eqs. (11.15) and (11.17)), we obtain

Here f_n may represent  

The specific forms, Eqs. (11.154) and (11.155), may also be readily obtained from Eq. (11.164).
By mathematical induction we may establish the Rayleigh formulas

Orthogonality

We may take the orthogonality integral for the ordinary Bessel functions (Eqs. (11.49) and (11.50)),

and substitute in the expression for j_n to obtain

Here α_np and α_nq are roots of j_n .
This represents orthogonality with respect to the roots of the Bessel functions. An illustration of this sort of orthogonality is provided in Example 11.7.1, the problem of a particle in a sphere. Equation (11.169) guarantees orthogonality of the wave functions j_n (r ) for fixed n. (If n varies, the accompanying spherical harmonic will provide orthogonality.)

Example 11.7.1 PARTICLE IN A SPHERE

An illustration of the use of the spherical Bessel functions is provided by the problem of a quantum mechanical particle in a sphere of radius a . Quantum theory requires that the wave function ψ , describing our particle, satisfy

and the boundary conditions (1) ψ(r ≤ a) remains finite, (2) ψ(a ) = 0. This corresponds to a square-well potential V = 0, r ≤ a , and V =∞, r> a .Here h¯ is Planck’s constant divided by 2π, m is the mass of our particle, and E is, its energy. Let us determine the minimum value of the energy for which our wave equation has an acceptable solution.
Equation (11.170) is Helmholtz’s equation with a radial part (compare Section 9.3 for separation of variables):

We choose the orbital angular momentum index n = 0, for any angular dependence would raise the energy. The spherical Neumann function is rejected because of its divergent behavior at the origin. To satisfy the second boundary condition (for all angles), we require

            (11.172)

where α is a root of j₀ , that is, j₀ (α) = 0. This has the effect of limiting the allowable energies to a certain discrete set, or, in other words, application of boundary condition (2) quantizes the energy E . The smallest α is the first zero of j₀ ,

α = π,

and

      (11.173)

which means that for any finite sphere the particle energy will have a positive minimum or zero-point energy. This is an illustration of the Heisenberg uncertainty principle for Δp with Δr ≤ a .
In solid-state physics, astrophysics, and other areas of physics, we may wish to know how many different solutions (energy states) correspond to energies less than or equal to some fixed energy E₀ . For a cubic volume (Exercise 9.3.5) the problem is fairly simple.
The considerably more difficult spherical case is worked out by R. H. Lambert, Am. J.
Phys. 36: 417, 1169 (1968).
The relevant orthogonality relation for the j_n (kr ) can be derived from the integral given in Exercise 11.7.23.

Another form, orthogonality with respect to the indices, may be written as

The proof is left as Exercise 11.7.10. If m = n (compare Exercise 11.7.11), we have 

Most physical applications of orthogonal Bessel and spherical Bessel functions involve orthogonality with varying roots and an interval [0,a ] and Eqs. (11.168) and (11.169) and Exercise 11.7.23 for continuous-energy eigenvalues.
The spherical Bessel functions will enter again in connection with spherical waves, but further consideration is postponed until the corresponding angular functions, the Legendre functions, have been introduced.