Rare-Gas and Hydrogen Molecule Electronic States, Noncrossing Rule, and Recombination of Electrons with Rare-Gas and Hydrogen Ions

Recent experimental work on helium jets has indicated that excited atoms result usually from collisional-radiative electron capture by atom ions ($A^{+}+2e\to A^{*}+e$; etc.) rather than by dissociative recombination by molecule ions (${A_2}^{+}+e\to A^{*}+A$), while for the heavier rare gases, the latter process is apparently relatively important. It is shown in the present paper that the theoretically expected forms of the potential curves of excited ${\mathrm{He}}_2$ and other rare-gas molecules make dissociative recombination improbable for ${\mathrm{He}}_2^{+}$ but more probable for the other rare-gas diatomic ions. Available spectroscopic data and theoretical considerations make it fairly sure that many of the familiar excited states of ${\mathrm{He}}_2$ have high maxima in their potential curves, but that no pure repulsion curves, except the one for two normal He atoms, exist below the energy of ${\mathrm{He}}^{+}$ + He. Besides the stable states with stable ${\mathrm{He}}_2^{+}$ core ($1{{\sigma }_g}^{2}1{\sigma }_u$, ${{}_{}{}^2{}_{}{}^{}\Sigma _u^{}{}_{}{}^{}}^{+}$) plus a Rydberg electron, there must be others with a core ($1{\sigma }_{g}1{{\sigma }_u}^2$, ${{}_{}{}^2{}_{}{}^{}\Sigma _g^{}{}_{}{}^{}}^{+}$) which per se is unstable but which on addition of an electron in a sufficiently strongly bound Rydberg orbital is stable; consideration of these states makes possible a reasonable explanation of the Hornbeck-Molnar effect in helium. Statements analogous to those about potential curves and electron recombination in helium are applicable also to a large extent for hydrogen.