Striations in glow discharges are often used as an educational tool for demonstrating the energy quantization in atoms. However, it has apparently gone unnoticed so far that the generally accepted cross sections for collisional excitation of atomic levels lead to mean free paths for the electrons that are much too large in order to explain the sharply defined striations (see http://www.plasmaphysics.org.uk/research/striatn.htm and http://www.plasmaphysics.org.uk/research/striapot.htm ). It is obvious that only a resonance type excitation centered at the transition frequency is able to explain the observed features (see http://www.plasmaphysics.org.uk/research/reschem.htm ). For a more detailed treatment of this see chapter 2 of my page http://www.plasmaphysics.org.uk/papers/airglow2.htm where a resonance type cross section is shown to be also required to explain the enhancement of airglow by high power radio waves. Many phenomena where collisional excitation is relevant need therefore to be re-intrepreted in this sense (see for instance http://www.plasmaphysics.org.uk/research/sun.htm for an application to solar physics). Thomas
