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 IEad = (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(nsσg, n = 4–11), R2(ndσg, n = 4–7) and R3(ndδg, n = 4–6) converging to the A2Πu state of the cation, and to a new series R′1(nsσg, n = 3) converging to the state of the cation. The autoionization mechanisms and their consistence with specific selection rules are discussed.