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 C-4(+), C3H+, C-3(+) 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(ns sigma(g), n = 4-11), R2(nd sigma(g), n = 4-7) and R3(nd sigma g, n = 4-6) converging to the Lambda(2)Pi(u) state of the cation, and to a new series R'1(ns sigma(g), n = 3) converging to the B'2 Sigma(+)(u) state of the cation. The autoionization mechanisms and their consistence with specific selection rules are discussed.