Ionization photophysics and Rydberg spectroscopy of diacetylene

Abstract

Photoionization of diacetylene was studied using synchrotron radiation over the range 8–24 eV, with photoelectron-photoion coincidence (PEPICO) and threshold photoelectron–photoion coincidence (TPEPICO) techniques. Mass spectra, ion yields, total and partial ionization cross-sections were measured. The adiabatic ionization energy of diacetylene was determined as IE_ad = (10.17 ± 0.01) eV, and the appearance energy of the principal fragment ion C₄H⁺ as AE = (16.15 ± 0.03) eV. Calculated appearance energies of other fragment ions were used to infer aspects of dissociation pathways forming the weaker fragment ions , C₃H⁺, and C₂H⁺. Structured autoionization features observed in the PEPICO spectrum of diacetylene in the 11–13 eV region were assigned to vibrational components of three new Rydberg series, R1(nsσ_g, n = 4–11), R2(ndσ_g, n = 4–7) and R3(ndδ_g, n = 4–6) converging to the A²Π_u state of the cation, and to a new series R′1(nsσ_g, n = 3) converging to the state of the cation. The autoionization mechanisms and their consistence with specific selection rules are discussed.

Acknowledgements

The authors wish to thank Gustavo Garcia and Laurent Nahon for excellent support at the DESIRS beamline as well as the Soleil staff for running the facility.