Electronic spectra of some diatomic molecules

The work prevented in this thesis is divided into two chapters, each being devoted to a study of the band system of a diatomic molecule of Group VI elements. The molecules studied were Se2 and TeO, and their choice depended on the feasibility of the work and on some current trends in theoretical thinking. To gain useful information about these molecules from their spectra it was essential that rotational analyses should be done and thus the experimental work was directed towards achieving high dispersion and maximum resolution, using isotropically pure samples. The theoretical interest in Se2 and TeO arises from the differences to be found in the molecules formed from the Group VI elements with increasing charge. Thus much of the earlier work on Se2 has been shown to be wrong because of the misunderstanding of the nature of a 3Σ state for a heavy molecule.
1. Se2
Earlier work by the author on the emission apparatus has shown that the main system arises from the transition 3Σ-u - 3Σ-g, in which the spin-splitting constants λ are so large that two widely separated components are given, F1 and F2,F3 for each state. The present work gives the results of a complete study of the absorption spectrum of the F1-F1 components of the separated isotopic species 78Se2 and 80Se2. The numerous upper state perturbations confuse the analysis but, by studying the bands of both isotopes arising from all the stable upper state levels, 0 to 21, it has been found possible to derive accurate rotational and vibrational constants. The upper state has a potential maximum and the observation of several predissociations prompted a reinvestigation of the emission spectrum. Two limiting curves of predissociation are found and the atomic products have been identified. The value of the ground state dissociation energy, obtained from these, is in fair agreement with the latest mass spectroscopic figure but is much more precise.
The derived constants, in cm.-1, for 80Se2 are as follows: F1
Tee = 25980.36
ω′e = 246.291 ω″e = 385.3028
xeω′e = 1.016 xeω″e = 0.96363
yeω′e = -5.49 × 10-3 yeω″e = -8.184 × 10-4
B′e = 0.07048 B″e = 0.08992
α′e = 0.000345 α″e = 0.000288
γ″e = 0.610 × 10-7
D = 4 × 10-8 Dv = 2.4 × 10-8 - 0.023 × 10-8(v+½)
r′e = 2.4465 Å r″e = 2.1161 Å
D00 = 24974 ± 20 cm.-1 = 71.39 ± 0.03 k.cal.mole-1
At the same time the observation of some F2,F3 absorption bands has enabled the vibrational numbering, previously unknown, to be confirmed by the isotope effect. The splitting between the three components of 3Σ- is such that the ground state appears to be in case-c while the upper state retains some case-b character.
The derived constants, in cm.-1, for 80Se2 are as follows: F2,F3
Tee = 25545.85 + 2λ″   2λ″-2λ′ = 434.51
ω′e = 246.42 ω″e = 386.43
xeω′e = 1.225 xeω″e = 0.9058
yeω″e = -13.13 × 10-4
B′e(F2) = 0.07048 B″e(F2) = B″e(F3) = 0.09012
B′e(F3) = 0.07105 α″e(F2) = α″e(F3) = 0.000297
α′(F2) = α′(F3) = 0.000553
r′e(F2,F3) = 2.4400 Å r″e(F2,F3) = 2.1631 Å
2. TeO
The ground states, 3Σ- of the Group VI elements so far analysed, O2, SO, S2, SeO, have been described in terms of Hund's case-B with an increasing value of the spin-splitting constant λ″ as the nuclear charge increases. For Se2 the transition to case-c is almost complete and the study of the main system of TeO was designed to determine whether the ground state is 3Σ- and, if so, now the increase in nuclear charge has affected the value of λ″.
As in the case of Se2, the problems involved in the analysis of the spectrum of TeO have been largely due to the presence of numerous isotopic species. In the present work the use of highly separated isotopes has again so clarified the spectrum that the analysis is indisputable, and it shows that the spin-splitting in the 3Σ- states of TeO has become so large that they have gone over into case-c, that is into 0+ and 1 states. The difference in the behaviour of a 3Σ- state in case-b with a large value of λ (for example, in SeO) and in case-c (for example, in TeO) is discussed.
The derived constants, in cm.-1, for 128TeO
are as follows:
T00 ωe xeωe
A 1 27641.83 + 2λ″ - -
A 0+ 28036.48 ΔG½ = 445.25 X 1 2λ″ 798.70 4.00 X 0+ 0 797.69 4.00
2λ″ ~ 2750
B 103α r, Å
A 1+ B0 = 0.2779 - r0 = 2.066
1- B0 = 0.2775 -
A 0+ Be = 0.2763 5.5 re = 2.071
X 1+ Be = 0.3575 2.379 re = 1.821
1- B0 = 0.3572 2.368
X 0+ Be = 0.3560 2.375 re = 1.825
The upper state is perturbed and is also predissociated in AO+ at v=0. The dissociation energy D″0 is estimated to be about 90 k.cal.mole-1.
In Appendices are given details of (A) a reinvestigation of the emission spectrum of XeO, in which a new assignment of the diffuse rotational lines, based on the isotope effect, is proposed; (B) the theoretical performance of an Ebert spectrograph and of an order-sorter; (C) Algol computer programmes for the settings of a Jerrell-Ash 3.4 metre grating spectrograph, and for the determination of potential curves by the Klein method.