Second kind of shock

Energy scheme for a collision of the second kind

In physics, a collision of the second kind denotes a collision between two atoms or molecules , each of which has two energy levels with a similar distance and of which one atom is in the ground state and the other in the excited state . There is then a probability that the excited atom will go into the ground state and the other into the excited state. This effect was first described by Svein Rosseland and Oskar Klein in 1920, originally only on the basis of collisions between free electrons and atoms, but today the term is also used more generally for atomic and molecular collisions.

${\displaystyle A^{*}+B\ \rightarrow \ A+B^{*}+\Delta E,}$

where and represent the atoms in the ground state, and the atoms in the excited state, as well as the energy difference between the excited levels. must be smaller than the thermal energy , otherwise a collision of the second kind would not be possible and a "normal" collision (of the first kind) would always occur. ${\displaystyle A}$ ${\displaystyle B}$ ${\displaystyle A^{*}}$ ${\displaystyle B^{*}}$ ${\displaystyle \Delta E}$ ${\displaystyle \Delta E}$ ${\displaystyle kT}$

This effect is used, for example, in the helium-neon laser , where neon is the laser medium . The excited helium generates a population inversion of the neon through collisions of the second kind with neon atoms , which is the basis for the stimulated emission.

Further information

• F. Kneubühl, M. Sigrist: Laser . 7th edition, Zurich 2008, ISBN 9783835101456
• Website about laser technology ( Memento from August 12, 2007 in the Internet Archive )

Individual evidence

1. ↑ Fluorescence of organic compounds by Theodor Förster, page 91 on Google books
2. ↑ Klein, Rosseland: On Collisions Between Atoms and Free Electrons (November 20, 1920, published 1921)
3. ↑ F. Kneubühl, M. Sigrist: Laser . 7th edition, page 229