Effect of Collisions on the Intensities of Nebular Lines

Abstract

MENZEL¹ has recently stated that “There appears to be a widespread misconception concerning the effect of electron collisions on the intensities of forbidden lines”. In the conventional treatment to which the above statement objects, the intensity is given by the formula where M is the number of atoms excited to the upper state per unit of time, A₂₁ is the probability of spontaneous emission and b₂₁ is the probability of removal from the upper state by collision with electrons, atoms, molecules, etc. b₂₁ in general increases with the density of these particles and is proportional to the density if the composition and velocity distribution are not altered. In this case, I remains approximately at the value Mhv as long as the density is so low that b₂₁ « A₂₁. When b₂₁ » A₂₁, however, I = MA₂₁hv/b₂₁, that is, I falls off inversely as the density if M, the rate of excitation of atoms excited to the upper state, is held constant.