On a Class of One-Valence-Electron Emitters of Band Spectra

Bands emitted by one-valence-electron molecules.—Series A: $\mathrm{BeF}$, $\mathrm{BO}$, ${\mathrm{CO}}^{+}$, $\mathrm{CN}$, $N_2^{}{}_{}{}^{+}$. These molecules each contain, like the Na atom, 8+1 electrons outside the nuclei and K electrons. The bands of C${\mathrm{O}}^{+}$ (the comet-tail, negative Deslandres and new quadruple-headed bands reported by Baldet and Johnson) are particularly discussed and analogies to the bands of BO and of CN are pointed out, as well as analogies between the bands of ${\mathrm{N}}_2^{+}$ and of CN. The data for BeF bands are less complete. The lines of the known band systems of all these molecules can in each case be expressed as combinations of three electronic terms (each of course with a variety of vibrational and rotational states). These are a singlet term $N$ probably belonging to the normal state, a doublet term $A$ corresponding to the first excited state, and a singlet term $B$ corresponding to the second excited state. There is evidence that the average angular momentum of the emitting electron—which is presumably the odd ninth electron—is the same for the $N$ and $B$ states, but different for the doublet $A$ state; presumably the former resemble $s$ states, the latter resemble (inverted doublet) $p$ states. In the cases of BO and C${\mathrm{O}}^{+}$, all of the three conceivable combinations between $N$, $A$, and $B$ are known; two are known in the case of CN, and one in that of ${\mathrm{N}}_2^{+}$; the systems not yet observed probably lie in the infrared. There is a remarkable parallelism between the spectra of the several molecules mentioned in respect to structure of analogous band systems and to molecular constants of corresponding electronic states (cf Table I). Series B: $\mathrm{MgF}$, $\mathrm{AlO}$, $\mathrm{SiN}$. These molecules contain an additional electron shell. They are the probable emitters of band spectra which show marked analogies to those of BeF, BO and CN. Additional series of emitters are represented by CaCl, SrBr, etc.

Suggested grouping of bands according to the number of valence electrons of the emitters.—Molecules with no unused valence electrons—HF, LiF, BN, NaCl, etc.—show analogy to the rare gases; they do not emit electronic band spectra. Molecules with two excess electrons—CO, N${\mathrm{O}}^{+}$, ${\mathrm{N}}_2$, SiO—should show analogies to alkaline earth metals. The odd multiplicity of ${\mathrm{N}}_2$ agrees with this suggestion. Molecules with three excess electrons, e.g., NO, should resemble Al; in fact the bands probably due to NO show even multiplicity.

Nature of multiplicity in band spectra.—Two causes of multiplicity are suggested: (a) connected with the interaction of the emitting electron with the rest of the molecule, and (b) attributed by Kratzer to the interaction of the angular momentum of the emitting and other electrons with that of the nuclei. Examples of each are discussed.

Emitters of certain bands.—Second negative $N_2$ bands of Duffendack may be due to neutral nitrogen molecules in each of which two electrons are initially excited. Evidence is presented for attributing the familiar aluminum bands to AlO.