New data on the ultra-violet absorption spectrum of ammonia and deuterated ammonias are reported and used, together with older data, in an interpretation of the spectrum. Five electronic transitions are particularly studied. Each is represented by a long progression of bands. The origins of four of the transitions lie at 2168 A, 1665 A, 1434 8, and 1330 8, ; the fifth is of unknown origin but is responsible for bands in the neighbourhood of 12688,. The first four transitions are each proved, by a vibrational analysis, to have planar upper states. It is concluded that the upper state of the fifth and the ground state of the NH; ion are also planar. The upper state of the 2168A transition is shown to have A; electronic symmetry and an N-H length greater than in the ground state. Some extra information concerning the ground-state vibrational levels is deduced. All the transitions studied are Rydberg in type, though the bands of the 2168 8, transition are predissociated through the influence of the lowest energy singlet excited intra-valency shell state ; the transitions lead to an ionization limit at 10.18 eV in good agreement with a photon impact value. Electronic and vibronic assignments of all the transitions are made. The v2A;I frequency in the ground state of NH$ is 920&10 cm-1.
Three band systems of Si2 have been found in absorption with a flash photolysis apparatus. Two of the band systems at 3200 and 2100 Å are new, whereas the third is an extension of the 3Σ–3Σ system observed by Douglas in emission. All three systems have the same lower state and arise from [Formula: see text] transitions. It is very probable that the [Formula: see text] state is the ground state of the Si0 molecule. Rotational and vibrational constants of all four 3Σ states have been determined. The dissociation energy of Si2 is estimated to be 3.0 ± 0.2 ev.
A widely spaced perpendicular band at 3440 Å observed in the flash photolysis of diazomethane is ascribed to the free HNCN radical. The study of the fine structure of this band for HNCN, DNCN, and HNC13N has yielded information about the geometrical structure of the molecule in both the upper and lower (ground) state. For the lower state[Formula: see text]The N—C—N group is very nearly linear, but the exact position of the C atom on this line could not be determined. The electronic transition is of the type 2A′–2A″, the transition moment being perpendicular to the plane of the molecule.
Four banded absorption systems of the C2C14 molecule are described. The vibrational patterns of all four are almost identical, qualitatively and quantitatively; from which it is deduced that all four transitions are Rydberg in type. They belong to a Rydberg series converging to a first ionization potential at 9.33 eV. The vibrational patterns are due to excitation of the three fundamental vibrations, v; v; and v; of the upper states, and to be in accord with simple theory based on symmetry considerations. The qualitative conclusions are that, relative to the ground state of C2C14, the upper states (and the ground state of the C2C14 ion) have an increased CC length, a less marked decrease in CC1 length and a small increase in Cl&l angle. These qualitative conclusions are then made quantitative by an application of the Franck-Condon principle ; the increase in CC length is 0.1 1 f 0.01 A, the decrease in CCl length is 0.03 f0.015 8, and the increase in &Cl angle is 3 i 1 O.One of us 1 has briefly reported the vacuum ultra-violet absorption spectrum of tetrachloro ethylene. We have now obtained improved spectrograms which we discuss below.
The electronic spectrum of tetrahydrofuran has been observed between 180 and 210nm. Two electronic transitions have been identified; a system of sharp bands with an origin at 199nm (50188 cm-l) and a system of diffuse bands beginning at about 195 nm (51400 cm-I). A vibrational analysis of the sharp system has been carried out, and the transition has been shown to be from the non-planar ground state to an upper state in which the ring is planar. From hot bands, six ground state ring puckering intervals have been identified. The corresponding upper state intervals are small and very unharmonic, ranging from 65 to 124 cm-'. The latter levels have been fitted to a potential function which is almost a two dimensional quartic oscillator. TABLE 1 .-FREQUENCIES AND ASSIGNMENTS * FOR THE 199 nm SYSTEM wavenumber/cm-l assignment t
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