Physical Processes in Gaseous Nebulae. X. Collisional Excitation of Nebulium.

Target areas for collisional excitation of 0 iii by electron impact are calculated by wave-mechanical methods. Coulomb wave functions are used for the colliding electron. The resulting target areas are large, particularly for excitation between the levels of 3P* Application to the nebulae leads to the result that coffisional de-excitations from 3P2 predominate over the spontaneous transitions. In consequence, the levels of 3P2 attain a high population, suited to the mechanism proposed by Bowen: resonance ab- sorption of X 303 of He ii by the coincident line 3P2 - 3P2 of 0 in. The figures also sug- gest, for the lower electron densities of interstellar space, a high population of atoms in the ground level, a result consistent with Dunham's observations of the interstellar lines of Till