Vacuum-ultraviolet absorption spectra of propanone, butanone and the cyclic ketones CnH2n–2O (n= 4, 5, 6, 7)

Abstract

The high-resolution vacuum-ultraviolet absorption spectrum of propanone is reported and analysed in terms of Rydberg series of s, p and d type. The longest series is the s-series observed to n= 17 allowing the ionisation potential to be accurately determined at 78 295 ± 5 cm^–1. A number of extensions and corrections to previous work are reported. The spectra of butanone and the cyclic ketones are similarly analysed and extensive Rydberg structure is identified in all spectra except that of cyclobutanone. For cyclopentanone and cyclohexanone long d-series allow accurate determination of the ionisation potentials at 74 800 and 73 794 ± 5 cm^–1, respectively. Comparison of the spectra exposes systematic trends in the number of series of p- and d-types and in their relative intensities. The trends are related to the symmetry of the molecules and the location of the geometric origin of the Rydberg orbital set at the centre of charge of the molecular cation. The n= 3 transitions are distinct from all others in showing extensive vibronic structure. The absolute intensities of these transitions are discussed and the vibronic structure analysed. The spectra are discussed in terms of a general description of molecular Rydberg states.