More Scattering: the Partial Wave Expansion
Michael Fowler 1/17/08
Plane Waves and Partial Waves
We are considering the solution to Schrödinger’s equation for scattering of an incoming plane wave in the z-direction by a potential localized in a region near the origin, so that the total wave function beyond the range of the potential has the form
The overall normalization is of no concern, we are only
interested in the fraction of the
ingoing wave that is scattered. Clearly
the outgoing current generated by scattering into a solid angle 
at angle θ, φ is 
multiplied by a
velocity factor that also appears in the incoming wave.
Many potentials in nature are spherically symmetric, or nearly so, and from a theorist’s point of view it would be nice if the experimentalists could exploit this symmetry by arranging to send in spherical waves corresponding to different angular momenta rather than breaking the symmetry by choosing a particular direction. Unfortunately, this is difficult to arrange, and we must be satisfied with the remaining azimuthal symmetry of rotations about the ingoing beam direction.
In fact, though, a full analysis of the outgoing scattered waves from an ingoing plane wave yields the same information as would spherical wave scattering. This is because a plane wave can actually be written as a sum over spherical waves:
Visualizing this plane wave flowing past the origin, it is
clear that in spherical terms the plane wave contains both incoming and outgoing spherical waves. As we shall discuss in more detail in the next
few pages, the real function 
is a standing wave, made up of incoming and outgoing waves of
equal amplitude.
We are, obviously, interested only in the outgoing spherical waves that originate by scattering from the potential, so we must be careful not to confuse the pre-existing outgoing wave components of the plane wave with the new outgoing waves generated by the potential.
The radial functionsappearing in the above expansion of a plane wave in its
spherical components are the spherical
Bessel functions, discussed below.
The azimuthal rotational symmetry of plane wave + spherical potential
around the direction of the ingoing wave ensures that the angular dependence of
the wave function is just 
, not 
. The coefficient 
is derived in Landau and Lifshitz, §34, by comparing the
coefficient of 
on the two sides of
the equation: as we shall see below, 
does not appear in 
for l greater than n, and 
does not appear in for l less than n, so the combination only occurs once—in the nth
term, and the coefficients on both sides of the equation can be matched. (To get the coefficient right, we must of
course specify the normalizations for the Bessel function—see below—and the Legendre
polynomial.)
Mathematical Interval: The Spherical Bessel and
Neumann Functions
The plane wave 
is a trivial solution
of Schrödinger’s equation with zero potential, and therefore, since the form a linearly
independent set, each term 
in the plane wave
series must be itself a solution to the zero-potential Schrödinger’s
equation. It follows thatsatisfies the zero-potential radial Schrödinger equation:
The standard substitution 
yields
For the simple case l = 0 the two solutions are 
. The corresponding
radial functions R0(r) are (apart from overall constants)
the zeroth-order Bessel and Neumann functions respectively.
The standard normalization for the zeroth-order Bessel function is
and the zeroth-order Neumann function
Note that the Bessel function is the one well-behaved at the origin: it could be generated by integrating out from the origin with initial boundary conditions of value one, slope zero.
Here is a plot of 
from kr = 0.1 to
20:
For nonzero l,
near the origin is 
The well-behaved 
solution is the Bessel
function, the singular function the Neumann function. The standard normalizations of these
functions are given below.
Here are
:
Detailed Derivation of Bessel and Neumann Functions
This subsection is
just here for completeness. We use the
dimensionless variable r =
kr.
To find the higher l solutions, we follow a clever trick given in Landau and Lifshitz (§33).
Factor out the 
behavior near the
origin by writing
The function 
satisfies
The trick is to differentiate this equation with respect to r :
Writing purely formally 
, the equation becomes
But this is the
equation that 
must obey! So we have
a recursion formula for generating all the 
from the zeroth one: 
and 
, up to a
normalization constant fixed by convention.
In fact, the standard normalization is
Now
This is a sum of only even
powers of r. It is easily checked that operating on this
series with 
can never generate any negative powers of r. It follows
that
written as a power series in r, has leading term proportional to r l.
The coefficient of this leading term can be found by applying the
differential operator to the series for 
This r l behavior near the origin is the usual well-behaved solution to Schrödinger’s equation in the region where the centrifugal term dominates.
Note that the small r
behavior is not immediately evident
from the usual presentation of the ’s, written as a mix of powers and trigonometric functions,
for example
Turning now to the behavior of the 
’s for large r, from
it is evident that the dominant term in the large r regime (the one of order 1/r) is generated by differentiating only the trigonometric function at each step. Each such differentiation can be seen to be equivalent to multiplying by (-1) and subtracting p/2 from the argument, so
These , then, are the physical partial-wave solutions to the
Schrödinger equation with zero potential.
When a potential is turned on, the wave function near the origin is
still 
(assuming, as we
always do, that the potential is negligible compared with the 
term sufficiently close to the origin). The wave function beyond the range of the
potential can be found numerically in principle by integrating out from the
origin, and in fact will be like above except that
there will be an extra phase factor, called the “phase shift” and denoted by δ) in the sine. The significance of this is that in the far
region, the wave function is a linear combination of the Bessel function and the Neumann function (the solution
to the zero-potential Schrödinger equation singular at the origin). It is therefore necessary to review the
Neumann functions as well.
As stated above, the l = 0 Neumann function is
the minus sign being the standard convention.
An argument parallel to the one above for the Bessel functions establishes that the higher-order Neumann functions are given by:
Near the origin
and for large r
so a function of the form 
asymptotically can be
written as a linear combination of Bessel and Neumann functions in that region.
Finally, the spherical Hankel functions are just the combinations of Bessel and Neumann functions that look like outgoing or incoming plane waves in the asymptotic region:
so for large r,
The Partial Wave Scattering Matrix
Let us imagine for a moment that we could just send in a (time-independent) spherical wave, with θ variation given by Pl(cosθ). For this l th partial wave (dropping overall normalization constants as usual) the radial function far from the origin for zero potential is
If now the (spherically symmetric) potential is turned on, the only possible change to this standing wave solution in the faraway region (where the potential is zero) is a phase shift δ:
This is what we would find on integrating the Schrödinger equation out from nonsingular behavior at the origin.
But in practice, the ingoing wave is given, and its phase cannot be affected by switching on
the potential. Yet we must still
have the solution to the same Schrödinger equation, so to match with the result
above we multiply the whole partial wave function by the phase factor 
. The result is to put
twice the phase change onto the
outgoing wave, so that when the potential is switched on the change in the
asymptotic wave function must be
This equation introduces the scattering matrix
which must lie on the unit circle |S| =1 to conserve probability—the outgoing current must equal the ingoing current. If there is no scattering, that is, zero phase shift, the scattering matrix is unity.
It should be noted that when the radial Schrödinger’s
equation is solved for a nonzero potential by integrating out from the origin,
with 
initially, the real function thus generated differs from the
wave function given above by an overall
phase factor .
Scattering of a Plane Wave
We’re now ready to take the ingoing plane wave, break it into its partial wave components corresponding to different angular momenta, have the partial waves individually phase shifted by l-dependent phases, and add it all back together to get the original plane wave plus the scattered wave.
We are only interested here in the wave function far away from the potential. In this region, the original plane wave is
Switching on the potential phase shifts factor the outgoing wave:
The actual scattering by the potential is the difference between these two terms. The complete wave function in the far region (including the incoming plane wave) is therefore:
The i l
factor cancelled the 
The -1
in 
is there because zero
scattering means 
. Alternatively, it
could be regarded as subtracting off the outgoing waves already present in the
plane wave, as discussed above. There is
no j-dependence since with
the potential being spherically-symmetric the whole problem is
azimuthally-symmetric about the direction of the incoming wave.
It is perhaps worth mentioning that for scattering in just
one partial wave, the outgoing current is equal to the ingoing current, whether
there is a phase shift or not. So, if
switching on the potential does not affect the total current scattered in any
partial wave, how can it cause any scattering?
The point is that for an ingoing plane
wave with zero potential, the ingoing and outgoing components have the
right relative phase to add to a component of a plane wave—a tautology,
perhaps. But if an extra phase is
introduced into the outgoing wave only,
the ingoing + outgoing will no longer
give a plane wave—there will be an extra outgoing part proportional to .
Recall that the scattering amplitude 
was defined in terms
of the solution to Schrödinger’s equation having an ingoing plane wave by
We’re now ready to express the scattering amplitude in terms of the partial wave phase shifts (for a spherically symmetric potential, of course):
where
is called the partial wave scattering amplitude, or just the partial wave amplitude.
So the total scattering amplitude is the sum of these partial wave amplitudes:
The total scattering cross-section
and the normalization of the Legendre polynomials
gives
So the total cross-section is the sum of the cross-sections for each l value. This does not mean, though, that the differential cross-section for scattering into a given solid angle is a sum over separate l values—the different components interfere. It is only when all angles are integrated over that the orthogonality of the Legendre polynomials guarantees that the cross-terms vanish.
Notice that the maximum possible scattering cross-section
for particles in angular momentum state l
is 
which is four times the classical cross section
for that partial wave impinging on, say, a hard sphere! (Imagine semiclassically particles in an
annular area: angular momentum L = rp, say, but 
and 
so l = rk. Therefore the annular area corresponding to
angular momentum “between” l and l + 1 has inner and outer radii 
and 
and therefore area 
.) The quantum result
is essentially a diffractive effect, we’ll discuss it more later.
It’s easy to prove the optical theorem for a spherically-symmetric potential: just take the imaginary part of each side of the equation
at θ = 0,
using 
from which the optical theorem 
follows immediately.
It’s also worth noting what the unitarity of the lth partial wave scattering
matrix
implies for the partial wave amplitude . Since it follows that
From this, gives:
This can be put more simply:
In fact,
Phase Shifts and Potentials: Some Examples
We assume in this section that the potential can be taken to be zero beyond some boundary radius b. This is an adequate approximation for all potentials found in practice except the Coulomb potential, which will be discussed separately later.
Asymptotically, then,
This expression is only exact in the limit 
but since the
potential can be taken zero beyond r
= b, the wave function must have the
form
for r > b.
(The - sign comes from the standard convention for Bessel and Neumann functions—see earlier.)
The Hard Sphere
The simplest example of a scattering potential:
The wave function must equal zero at r = R, so from the above
form of 
,
For l = 0,
so 
This amounts to the wave
function being effectively moved over to begin at R instead of at the origin:
for r > R, of course 
for r
< R.
For higher angular momentum states at low energies (kR << 1),
Therefore at low enough energy, only l =
0 scattering is important—as is obvious, since an incoming particle with
momentum and angular momentum 
is most likely at a
distance l/k from the center of the
potential at closest approach, so if this is much greater than R, the phase shift will be small.
The Born Approximation for Partial Waves
From the definition of
and
recall the Born approximation amounts to replacing the wave
function 
in the integral on the right by the incoming plane wave,
therefore ignoring rescattering.
To translate this into a partial wave approximation, we first
take the incoming 
to be in the z-direction, so in the integrand we replace by
Labeling the angle
between 
and 
by γ,
Now is in the direction 
and in the direction 
, and γ is the
angle between them. For this situation, there is an addition theorem for
spherical harmonics:
On inserting this
expression and integrating over 
, the nonzero m
terms give zero, in fact the only nonzero term is that with the same l as the term in the expansion, giving
and remembering
it follows that for small phase shifts (the only place it’s valid) the partial-wave Born approximation reads
Low Energy Scattering: the Scattering Length
From
the l = 0 cross section is
At energy 
the radial Schrödinger equation for 
away from the potential
becomes 
, with a straight line solution 
This must be the 
limit of 
, which can only be correct if δ0 is itself linear in k for sufficiently small k,
and then it must be 
a being the point at which the extrapolated external wavefunction
intersects the axis (maybe at negative r!) So, as k
goes to zero, the cot term dominates in the denominator and
The quantity a is called the scattering length.
Integrating the zero-energy radial Schrödinger equation out from u(r) = 0 at the origin for a weak (spherical) square well potential, it is easy to check that a is positive for a repulsive potential, negative for an attractive potential.
Repulsive potential, zero-energy wave function (so it’s a straight line outside of the well!):
Attractive potential:
On increasing the strength of the repulsive potential, still solving for the zero-energy wave function, a tends to the potential wall—here’s the zero-energy wavefunction for a barrier of height 6:
For an infinitely high barrier, the wave function is pushed out of the barrier completely, and the hard sphere result is recovered: scattering length a, cross-section 4π a2.
On increasing the strength of the attractive well, if there is a phase change greater that π/2 within the well, a will become positive. In fact, right at π/2, a is infinite!
And a little more depth to the well gives a positive scattering length:
In fact, a well deep enough to have a positive scattering
length will also have a bound state. This becomes evident when one considers
that the depth at which the scattering length becomes infinite can be thought
of as formally having a zero energy bound state, in that although the wave
function outside is not normalizable, it is equivalent to an exponentially
decaying function with infinite decay length.
If one now deepens the well a little, the zero-energy wave function
inside the well curves a little more rapidly, so the slope of the wave function
at the edge of the well becomes negative, as in the picture above. With this slightly deeper well, we can now lower the energy slightly to negative
values. This will have little effect on
the wave function inside the well, but make possible a fit at the well edge to
an exponential decay outside—a genuine bound state, with wave function 
outside the well.
If the binding energy of the state is really low, the
zero-energy scattering wave function inside the well is almost identical to
that of this very low energy bound state, and in particular the logarithmic
derivative at the wall will be very close, so 
, taking a to be
much larger than the radius of the well.
This connects the large scattering length to the energy of the weakly bound state,
(Sakurai, p 414.)
Wigner was the first to use this to estimate the binding energy of the deuteron from the observed cross section for low energy neutron-proton scattering.