Rydberg states of C2H4and C2D4: assignments using the technique of low-energy electron energy-loss spectroscopy

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Abstract
Energy-loss spectra of C2H4 and C2D4 are presented covering the energy-loss range 6-11 eV, using incident electron energies in the range 5-100 eV above threshold, and scattering angles up to 60 degrees . Members of several optically assigned Rydberg series were identified, and their intensities measured at a number of scattering angles. Each transition was then classified as electric dipole or quadrupole allowed, and this information was used to assign the Rydberg orbitals involved. Four new triplet Rydberg states have been observed, and their assignments are also discussed. For two of the triplet states, more than one vibrational level was observed, and where possible the frequencies of the symmetric C=C stretch and CH2 torsional modes were obtained. For the lowest triplet Rydberg state, the barrier to internal rotation of the CH2 groups was estimated to be 0.08 eV.