An electron impact investigation of the 3p-Rydberg transitions of acetone

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Abstract
High resolution (15 meV), gas phase, electron impact energy loss spectra of acetone, and acetone‐d6 measured as a function of incident energy and scattering angle have been used to study the anomalously weak 7.4 eV 3p‐Rydberg transition in acetone. Assignments made on the basis of isotope shifts and differential electron scattering cross sections show that the weak transition observed optically and in the high energy electron impact spectrum is a forbidden transition to the out‐of‐plane 3p‐Rydberg orbital. The band system is built on a vibronically allowed false origin enabled by the ν23 (CO bend) vibration. The analogous transition in the less symmetric molecule methyl ethyl ketone was observed to be optically allowed. All observed bands could be assigned to the one electronic transition to the out‐of‐plane 3p‐Rydberg orbital. The other two 3p‐Rydberg transitions do not appear to be active in acetone or methyl ethyl ketone. An unusual feature of this investigation was the experimental investigation of the relative differential scattering cross sections of both true and false vibronic origins within a single electronic transition. The high energy resolution allowed isotopic substitution, a traditional technique of optical spectroscopy, to be used to establish assignments in an electron impact experiment.