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Physics LibreTexts

14.6: Ramsauer-Townsend effect

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This is the name given to the fact that electrons with energy about 1 eV can pass almost freely through Xe, Kr, and Ar:- there is a sharp minimum in electron scattering cross-section for these noble gases.

Due to polarization of these atoms by the incoming electron the potential appears to increase as K increases (more localized electrons are better able to polarise the atom). Thus δ0(k0)=nπ, in accordance with Levinson’s theorem, and δ0 initially increases as k increases, before eventually decreasing. Thus at a certain value of k, the phase shift is again δ0(k)=nπ, and the total scattering cross section σT has an abrupt minimum. Although there are subsequent s-wave minima at e.g. δ0(k)=(n1)π, these occur at sufficiently large values of k that s-wave scattering is no longer dominant.

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Figure 14.6.1: Minimum in scattering cross section in Ar due to δ0=3π; No such effect in Ne due to weaker polarization.

By contrast, neon and helium have lower polarisability, due to fewer bound electrons. Thus the phase shift δ0 decreases monotonically with k from nπ at k=0 at there is no low-energy minimum.

Higher l phase shifts may increase with k because higher k implies smaller impact parameter (classically, more chance of hitting the atom). The cross section increases more slowly due to the additional K2 dependence. The maximum in the Ar cross section at about 13 eV is mainly due to the d-wave δ2=π/2.

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Figure 14.6.2: More-localized electrons polarise atoms and thus increase the attractive potential

This page titled 14.6: Ramsauer-Townsend effect is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Graeme Ackland via source content that was edited to the style and standards of the LibreTexts platform.

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