Ramsauer effect

from Wikipedia, the free encyclopedia

The Ramsauer effect , also known as the Ramsauer-Townsend effect , describes the extreme permeability of gases to slow electrons and was discovered by Carl Ramsauer in 1920. Today it is considered to be the first experimental indication that free electrons cannot be described with classical mechanics either .

The name Ramsauer-Townsend-Effect , also used for this effect, includes the research of the Irish physicist John Sealy Edward Townsend , who in 1901 discovered the dependence of the mean free path on the kinetic energy in the movement of free electrons in gases. The effect had already been discovered and published by Nils Akesson in the same institute in 1916, but was not recognized because of alleged weaknesses in his experiments. However, its results were gradually confirmed in the 1920s.

description

When electrons pass through a gas, they interact with the gas molecules. In order to determine this interaction, Ramsauer introduced the term effective cross-section (in contrast to the cross-section determined in kinetic gas theory ): the larger the effective cross-section of a gas molecule, the sooner it is hit by an electron, and the fewer electrons penetrate the gas . According to the ideas of classical mechanics, the smaller the cross-section, the greater the kinetic energy and thus the speed of the electron, since slower electrons are more strongly deflected by the electric field inside the atoms.

Ramsauer found, however, that the cross-section depends differently on the kinetic energy of the electrons. For example, at around one electron volt, many cross-sections show a minimum ( Ramsauer cross-section ), which is sometimes well below the gas kinetic cross-section. As the electron energy increases, the collision cross-section initially rises to a maximum, and then drops sharply again at values ​​above 20 electron volts, sometimes below the value of the minimum collision cross-section at lower energies.

At that time this effect could not be explained.

Explanation

Only with de Broglie's thesis of matter waves and thus through the wave-particle dualism was the minimum of the cross-section understood heuristically : the speed-dependent De Broglie wavelength of the electrons must move in the same order of magnitude as the size of the gas atoms cause the scattering of electrons. As a result, the gas atoms allow the electrons to be transmitted so that electrons of a sufficiently large wavelength can pass through the gas molecules with less hindrance.

The complete explanation of the Ramsauer effect is based on scattering theory and therefore requires the quantum mechanics developed from 1925 to 1927 . Due to the wave character of the electrons, the transmission leads to interference phenomena also behind the atoms, which, however, are not the object of observation.

Individual evidence

  1. Carl Ramsauer: About the cross-section of gas molecules against slow electrons . In: Annals of Physics . tape 64 , 1921, pp. 513-540 ( online [PDF; accessed July 7, 2020]).
  2. ^ Gyeong Soon Im: The formation and development of the Ramsauer effect . In: Historical Studies in the Physical and Biological Sciences . tape 25 , no. 2 , 1995, p. 269–300 , doi : 10.2307 / 27757746 ( https://online.ucpress.edu/hsns/article-pdf/25/2/269/149734/27757746.pdf online [PDF; accessed July 7, 2020]).