Excited atoms in decaying optically thick plasmas

With attention confined to decaying H-H + -e and He-He + -e plasmas which are optically thick towards lines ending on the ground level, the quasi-equilibrium number density of atoms in the first excited level n Q '(2) is computed using collisional deactivation coefficients K (2, 1) based on experiment and taking Penning ionization into account. This last is discussed in some detail. Quantal calculations are carried out on the rate at which the electronic transitions involved in 2H(2s-p) -> H(1s) + H + + e occur. It is concluded that the rate is probably fast enough for almost all close collisions to lead to ionization. This also seems to be the case for 2He(2 3 S) -> He(1 1 S) + He + + e provided collisions which can yield ionization only by violation of Wigner's spin conservation rule are excluded from consideration. Penning ionization is of little importance in hydrogen plasmas, but it can be significant in helium plasmas where it tends to reduce n Q '(2) and the apparent recombination coefficient.