The original interpretation of the method of molecular orbitals as a theory of valency, in which the single electron possesses bonding power was based on the assumption that non-premoted electrons are bonding and premoted ones are anti-bonding or non-bonding. This assumption is not the outcome of any requirements of theory but is an empirical postulate, which seeks justification in experimental facts. Earlier attempts at correlation between the electronic levels of molecules and the energy states of the constituent atoms indeed appeared to lend support to this assumption. Later investigations have, however, shown that completely different correlations are possible and are absolutely necessary to satisfy the experimental facts. For example, in a molecule like BeO the stable triplet term, which should arise from unexcited atoms according to the older correlation, is not found and by the correlation of certain excited terms to anomalous terms of the metal atom it is shown, that a non-premoted odd electron in the configuration of the ground level lowers its dissociation energy. Similar remarks apply to the types BeF and NO and the new correlation of molecular terms to atomic states from the pair-bond view is shown to be satisfactory without exception in a larger number of molecules of these three classes, recently.

These three types represent molecules with free valencies and indeed just in such molecules the difference of the two view-points must become apparent. For other such molecules with free valencies, whose band spectrum is investigated in detail,i.e., for the oxides of the third and the halides of the fourth group of the periodic system, only one example of each,i.e., AlO and SnCl, was originally available. We have now shown in the present paper, that the correlation according to the pair-bond theory of valency holds also for other molecules of this type and appears to be generally valid.All this evidence, concerning the band spectra of oxides and halides of the second, third, fourth, and fifth groups of the periodic systemis definitely against the postulate of the identification of premoted and non- premoted with non-bonding and bonding electrons. On the other hand, such a satisfactory correlation from the pair-bond view necessitates a revision of the theory of valency, based on the results of band spectroscopy.

A few new band heads belonging to the triplet carbon band system of CO are reported. These necessitate an increase of two units in the existingv’ numbering. The origin of the system may be still further in the region of longer waves but is definitely shifted at least tov=13310.7 cm.⁻¹ according to these new results, thus locating thed³II₁ level at 61785·5 cm⁻¹. from the ground level of the CO molecule.

From the experimental conditions under which the spectra are obtained it is assumed that they are due to molecular radiation. Thermochemical and spectrascopic data enable one to illustrate the relation among the energy levels of the various systems in the formation of the multi-atomic molecules from their constituent atoms. Such simplified Franck-Condon diagrams are constructed for SnCl₄ (which includes SnCl₂) and SiCl₄. An interpretation, uniform in all cases, of the observed continuous bands in terms of the transitions among the energy levels, transitions, which of course involve a repulsive state for their final level, is suggested.

A discharge tube containing air at low pressure is excited by h.f. frequency oscillations from a modified Hartley circuit. It is observed that there is a critical frequency of exciting oscillations, namely 735 k.c.|sec. below which the colour of the discharge is pinkish red and above which the colour is orange yellow. The spectrum of the discharge in the visible region consists of the first positive and second positive bands of nitrogen. A study of some of these bands as regards their intensity variation depending upon the frequency of exciting oscillations is made. It is found that the first positive bands at 5371, 5406 and 5441 Å decrease in intensity as the frequency of exciting oscillations is reduced from 790 to 680 k.c.|sec. The second positive bands at 4355, 4417, 4490, 4649, 4724, 4815 and 4917 Å on the other hand, increase in intensity under the same conditions. The results are in harmony with the theoretical conceptions, regarding such phenomena and allow us to conclude that in the present experiments an energy of 15 e.v. is available for excitation when oscillations of frequency 735 k.c.|sec. are used.

A single crystal of 5% Pr³⁺ in LaCl₃ was grown by Stockbarger’s method and its absorption spectrum at 77° K. recorded in the region 4200–5400 Å on a grating spectrograph having a dispersion of 5 Å/mm. in the first order. Polarization of the absorption lines has also been obtained. In the group of lines arising from¹I₆ ←³H₄ transition, six more lines in addition to the four previously known have been observed. Satisfactory analysis of all these ten lines is given. One additional Stark level in¹I₆ is established at 21407 cm.⁻¹ withμ=1. For a few lines the polarization results obtained here do not agree with those of previous workers but fit in satisfactorily in the analysis.

The paper deals with an extension towards shorter wavelengths of the absorption spectrum of Nd³⁺ employing a 5% Nd³⁺ doped LaCl₃ crystal. Five new groups of lines have been recorded and the analysis and interpretation of one of these groups,viz., group O between 3010 and 3023 Å are given. The lines observed in this group are shown to be due to transitions from the ground level (⁴I_9/2) stark components (μ=5/2, 1/2 and 3/2) to the two stark components (μ=3/2, 1/2) of²D_3/2 level at about 33200 cm.⁻¹

The observed fluorescence and absorption spectra of single crystals of KCl and KBr doped with praseodymium (Pr) show that the rare earth enters substitutionally as Pr³⁺ in these lattices. However, the site symmetry of the ion has at most a twofold axis. This is shown by the fact that thej-degeneracy of its energy levels is completely removed. Fluorescence is observed to occur only from³P₀ level to³F_{2, 3, 4} and³H₆ levels. The overall Stark splitting shows that the Pr³⁺ ion experiences a stronger field in these matrices than in LaCl₃ or LaBr₃.