Ultra-violet absorption spectrum of 1,1-dichloro ethylene

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Abstract
The electronic spectrum of 1,1-dichloro ethylene has been re-investigated. The bands in the vacuum ultra-violet are divided into two types, a and b. Among the a bands, six transitions have qualitatively and quantitatively, almost identical vibrational patterns. The transitions are deduced to be Rydberg in type and their vibrational origins to belong to several Rydberg series all converging to the first ionization limit. An earlier values of the first ionization potential is shown to be incorrect; the true value is 9.85 eV. The Rydberg states (and hence the ground state of the positive ion) have the same shape as the ground state of the neutral molecule, but a longer CC bond and shorter CCl bonds. Simple theoretical arguments explain this. The b bands appear to be separate electronic transitions unaccompanied by vibrational energy changes; this is compatible with their representing Rydberg transitions leading to the second ionization potential, but it has not been possible to obtain a reliable value of the second limit.

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