Relations of Pp′ Groups in Atoms of the Same Electronic Structure

Pp′ groups of spectral lines.—(1) For two-valence-electron atoms. A quintuplet of nearly equally spaced lines (really an unresolved sextuplet) has now been found for all the two-valence electron spectra which have been studied by the method of hot-spark spectroscopy, namely for the series ${\mathrm{Mg}}_{\mathrm{I}}$, ${\mathrm{Al}}_{\mathrm{II}}$, ${\mathrm{Si}}_{\mathrm{III}}$, ${\mathrm{P}}_{\mathrm{IV}}$, ${\mathrm{S}}_{\mathrm{V}}$, ${\mathrm{Cl}}_{\mathrm{VI}}$, and for ${\mathrm{Be}}_{\mathrm{I}}$, ${\mathrm{B}}_{\mathrm{II}}$, ${\mathrm{C}}_{\mathrm{III}}$, ${\mathrm{N}}_{\mathrm{IV}}$ and ${\mathrm{O}}_{\mathrm{V}}$. In both of these series, this $p{p}^{\prime }$ group has been definitely shown to arise from an electron jump from the lowest $p$ to the lowest $s$ orbit, combined with a simultaneous jump of the other electron between the $p_1$, $p_2$, $p_3$ orbits. The frequency differences between groups of successive atoms obey the irregular doublet law, showing that the jumps are between levels of the same total quantum number. (2) For three-valence-electron atoms. A new quadruplet $p{p}^{\prime }$ group has been discovered for all the atoms of this type studied, ${\mathrm{Al}}_{\mathrm{I}}$, ${\mathrm{Si}}_{\mathrm{II}}$, ${\mathrm{P}}_{\mathrm{III}}$, ${\mathrm{S}}_{\mathrm{IV}}$, ${\mathrm{Cl}}_{\mathrm{V}}$ and ${\mathrm{C}}_{\mathrm{II}}$, ${\mathrm{N}}_{\mathrm{III}}$, ${\mathrm{O}}_{\mathrm{IV}}$. The relative intensity of this group increases with the effective nuclear charge, from ${\mathrm{Al}}_{\mathrm{I}}$ to ${\mathrm{Cl}}_{\mathrm{V}}$. This group is proved to be due to the return of one of the $s$ electrons after displacement to a $p$ position combined with the simultaneous interchange of the $p$ electron between the $p_1$, $p_2$ orbits. This group also obeys the irregular doublet law. These groups are of exceptional theoretical interest because their existence proves that the energy changes due to the simultaneous jumping of two electrons between orbits, are integrated into monochromatic radiation. The mechanism by which such an integration takes place is at present a complete mystery.