Ramsauer Effect

in the narrow sense, a high degree of “transparency” of the molecules or atoms of a gas to low-energy electrons; in a more general sense, the anomalous—from the standpoint of classical physics—character of the interaction of electrons with the neutral atoms or molecules of some gases. The effect was discovered in 1921 by the German physicist C. Ramsauer in a study of the scattering of electrons in argon. It was later detected in other, heavier inert gases and was also indirectly confirmed in the investigation of electron mobility in gases.

The Ramsauer effect is expressed in the presence of a pronounced minimum of the effective cross section σ for collisions of electrons with atoms at electron energies of about 1 electron volt (eV) and in the subsequent increase of σ until it reaches a maximum at about 12 eV (Figure 1). This phenomenon is inexplicable within the framework of classical particle mechanics. The reason for the effect is that in processes of interaction with atoms low-energy electrons behave like waves (seeDE BROGUE WAVES and WAVE-PARTICLE DUALITY). When the wavelength corresponding to the moving electron and the characteristic dimensions of the atom are in a certain relation, particularly favorable conditions are created for the unhindered passage of the wave through the atom.

Figure 1. Effective cross section for the elastic scattering of electrons σ_s in different gases as a function of electron energy W

In the more rigorous quantum mechanical explanation of the Ramsauer effect, the electron wave is decomposed into harmonics associated with the different values of the quantized angular momentum of the electron with respect to the scattering atom, and the phase shifts introduced into these harmonics by the field of the atom are studied. The Ramsauer effect results if a phase shift corresponding to an integral number of wavelengths is introduced into the harmonic that provides the principal contribution to the effective cross section. Calculations have substantiated the presence of the Ramsauer effect in Ar and other heavy inert gases and its absence in H₂, He, and Ne. The quantum mechanical character of the scattering of electrons in gases is also evidenced, in addition to the Ramsauer effect, in the angular distribution of the scattered electrons, where alternating maxima and minima are observed (the Ramsauer effect is in some respects analogous to wave interference).

The Ramsauer effect plays an important role in processes that accompany electrical discharges in gases. It determines, for example, the dependence of the characteristics of the discharge plasma on gas pressure and is responsible for many phenomena associated with electron mobility.