# 14.6: Hard-Sphere Scattering

- Page ID
- 15818

Let us test out this scheme using a particularly simple example. Consider scattering by a *hard sphere*, for which the potential is infinite for \(r<a\), and zero for \(r>a\). It follows that \(\psi({\bf r})\) is zero in the region \(r<a\), which implies that \(u_l =0\) for all \(l\). Thus, \[\beta_{l-} = \beta_{l+} = \infty,\] for all \(l\). Equation ([e17.82]) thus gives \[\label{e17.90} \tan \delta_l = \frac{j_l(k\,a)}{y_l(k\,a)}.\]

Consider the \(l=0\) partial wave, which is usually referred to as the \(S\)-wave. Equation ([e17.90]) yields \[\tan\delta_0 = \frac{\sin (k\,a)/k\,a}{-\cos (k\,a)/ka} = -\tan (k\,a),\] where use has been made of Equations ([e17.58a]) and ([e17.58b]). It follows that \[\label{e17.92} \delta_0 = -k\,a.\] The \(S\)-wave radial wave function is [see Equation ([e17.80])] \[\begin{aligned} {\cal R}_0(r) &= \exp(-{\rm i}\, k\,a)\, \frac{[\cos (k\,a) \,\sin (k\,r) -\sin (k\,a) \,\cos (k\,r)]}{k\,r}\nonumber\\[0.5ex] &= \exp(-{\rm i}\, k\,a)\, \frac{ \sin[k\,(r-a)]}{k\,r}.\end{aligned}\] The corresponding radial wavefunction for the incident wave takes the form [see Equation ([e15.49])] \[\tilde{\cal R}_0(r) = \frac{ \sin (k\,r)}{k\,r}.\] Thus, the actual \(l=0\) radial wavefunction is similar to the incident \(l=0\) wavefunction, except that it is phase-shifted by \(k\,a\).

Let us examine the low- and high-energy asymptotic limits of \(\tan\delta_l\). Low energy implies that \(k\,a\ll 1\). In this regime, the spherical Bessel functions reduce to: \[\begin{aligned} j_l(k\,r) &\simeq \frac{(k\,r)^l}{(2\,l+1)!!},\\[0.5ex] y_l(k\,r) &\simeq -\frac{(2\,l-1)!!}{(k\,r)^{l+1}},\end{aligned}\] where \(n!! = n\,(n-2)\,(n-4)\cdots 1\) . It follows that \[\tan\delta_l = \frac{-(k\,a)^{2\,l+1}}{(2\,l+1) \,[(2\,l-1)!!]^{\,2}}.\] It is clear that we can neglect \(\delta_l\), with \(l>0\), with respect to \(\delta_0\). In other words, at low energy, only \(S\)-wave scattering (i.e., spherically symmetric scattering) is important. It follows from Equations ([e15.17]), ([e17.73]), and ([e17.92]) that \[\frac{d\sigma}{d{\mit\Omega}} = \frac{\sin^2 k\,a}{k^{\,2}} \simeq a^{\,2}\] for \(k\,a\ll 1\). Note that the total cross-section \[\sigma_{\rm total} = \int\frac{d\sigma}{d{\mit\Omega}}\,d{\mit\Omega} = 4\pi \,a^{\,2}\] is four times the geometric cross-section \(\pi \,a^{\,2}\) (i.e., the cross-section for classical particles bouncing off a hard sphere of radius \(a\)). However, low energy scattering implies relatively long wavelengths, so we would not expect to obtain the classical result in this limit.

Consider the high-energy limit \(k\,a\gg 1\). At high energies, all partial waves up to \(l_{\rm max} = k\,a\) contribute significantly to the scattering cross-section. It follows from Equation ([e17.75]) that \[\label{e17.99} \sigma_{\rm total} \simeq \frac{4\pi}{k^{\,2}} \sum_{l=0,l_{\rm max}} (2\,l+1)\,\sin^2\delta_l.\] With so many \(l\) values contributing, it is legitimate to replace \(\sin^2\delta_l\) by its average value \(1/2\). Thus, \[\sigma_{\rm total} \simeq \sum_{l=0,k\,a} \frac{2\pi}{k^{\,2}} \,(2\,l+1) \simeq 2\pi \,a^{\,2}.\] This is twice the classical result, which is somewhat surprising, because we might expect to obtain the classical result in the short-wavelength limit. For hard-sphere scattering, incident waves with impact parameters less than \(a\) must be deflected. However, in order to produce a “shadow” behind the sphere, there must also be some scattering in the forward direction in order to produce destructive interference with the incident plane-wave. (Recall the optical theorem.) In fact, the interference is not completely destructive, and the shadow has a bright spot (the so-called “Poisson spot” ) in the forward direction. The effective cross-section associated with this bright spot is \(\pi \,a^{\,2}\) which, when combined with the cross-section for classical reflection, \(\pi \,a^{\,2}\), gives the actual cross-section of \(2\pi \,a^{\,2}\) .

# Contributors

Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)

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