Spectroscopic studies of simple molecules

In the introduction to this thesis it is explained that there are two spectroscopic methods of obtaining information about the highly excited electronic energy levels of molecules. The first is by examining directly the transitions to these levels from the ground state of the molecule, the second by discovering transitions between the high levels themselves. In the molecule CO, given as an example, only one transition involving the ground state lies at wave-lengths greater than 2000 Å, end much information has been obtained at shorter wavelengths. However, for CO the second method has proved more fruitful, for many transitions of this molecule may be observed in the visible and near u-v regions of the spectrum. In this property CO is unusual, partly because it is a stable gaseous species of which a plentiful supply may be provided for a discharge tube, and partly because of the peculiar stability of its upper states and its high dissociation energy. On the other hand the majority of diatomic molecules do not possess so many emission systems above 200 Å and for these it was desired to find a general method for examining them at shorter wave-lengths. It is known that oxygen in the air absorbs light in this region, and so a vacuum spectrograph was designed and built around aim. glass grating ruled 30,000 lines/inch. In order that the identification of the observed spectra should be as simple as possible, it was desired to obtain absorption spectra rather than emission ones. Previous work in this direction has been confined largely to gaseous or volatile substances, but for , general method a long absorption tube surrounded by a furnace is necessary, so that involatile solids may be heated. Now the most feasible source of continuous radiation from 2000-400 Å is the Lyman source, a powerful condensed spark through a silica capillary. This is almost a point source and so the intensity falls off very rapidly as it is moved away from the slit; thus the introduction of a long absorption tube requires a very bright source end a very fast spectrograph. The intensity in this work hps been increased by the use of an aluminized mirror collecting light from the source and focussing it on the slit. In this manner the absorption spectrum of five diatomic molecules has been examined between 1300- 2000 Å, and some measure of the generality of the method is that in every case at least two new energy levels have been observed.
In Part I of the thesis, after details of the design and operation of the spectrograph have been given, the observed spectra are recorded for the monoxides of silicon, germanium and tin, and also for stannous sulphide. The new molecular energy levels are, in cm-1 :-
State ve we xewe
SiO ? 79650
H(1∑+)? 76427 1155 15
G [72011] 829 5
F(1∑+)? 68562.5 1114.6 7.2
E(3∑+)? 52863.2 675 4.15
GeO F 67474.4 809.0 5.66
E 49637.3 504.3 4.8
SnO F 58809.6 724 21
? 57491(vo) 462 -
Other fragmentary systems
SnS F 56886(vo) repulsive
? 55928 (vo) dissociates before v' = 1
Other fragmentary systems
For SiO the states observed are compared with those known for CO, in addition it is suggested that the highest level, together with the H and F levels form a Rydberg series leading to 10.5 e.v. as the ionization potential of SiO.
In a discussion of the dissociation energies of these molecules by comparison of the spectroscopic and thermochemical evidence, the following best values for Do" were obtained (in k.cal./mol.):-
SiO GeO SnO SnS
~180 154 &pm; 5 122 &pm; 2 110 &pm; 1
In Part II of the thesis the observed emission spectrum of aluminium monofluoride is described. The source used was a heated hollow cathode discharge tube where the cathode contained a mixture of aluminium and aluminium trifluoride. Later the absorption spectrum of this molecule was examined in the vacuum ultraviolet and six systems observed, together with a group of unanalysed bands at very short wave-lengths. The new states of AlF discovered are as follows:-
State ve we xewe zewe Observed Transitions
d(3∑)? 72350 ~940 d &rightarrow; a
c(3∑)? 69640 ~940 c &rightarrow; a
H ? 67312 958 7.0 H &leftarrow; X
G 1∑+ 66325 948 8.0 0.02 G &leftarrow; X and G &rightarrow; A
F(1Π)? 65785 955 5.0 0.01 F &leftarrow; X, F &rightarrow; A
E(1Π)? 63739(vo) E &leftarrow; X, E &rightarrow; A
b3∑ 61331.3 933.5 4.90 b &rightarrow; a
D1Δ 61225 902 6.5 D &rightarrow; A
C1∑+ 57687 938.4 5.18 C &leftarrow; X, C &rightarrow; A
B1∑+ 54241.5 862 7.0 0.012 B &leftarrow; X
a3Π2 33659.5
a3Π1 33612 830.9 4.55 a &rightarrow; X ?
a3Π0 33574.5
The A and X states referred to are the A1Π and X1∑+ discovered previously. The a &rightarrow; X transition is very weak, and this makes the absolute levels of the triplet states rather doubtful. Plausible dissociation products are assigned to these states on the basis of a dissociation energy of AlF of 147 k.cal./mol. This value is supported by a calculation of the Intent heat of sublimation of aluminium from vapour pressure data.