Photoionisation mass spectrometric study of acetylene in the VUV region

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Abstract
Photoionisation mass measurements on acetylene have been made between 1100 and 400 AA by using synchrotron radiation as a light source. Photoionisation efficiency curves are obtained for the molecular ion C2H2+ and also for fragment ions C2H+, C2+, CH2+ and CH+. To assign the structures of C2H2+, ab initio calculations have been performed for the excitation from the second (3 sigma g) and third (2 sigma u) occupied molecular orbitals. Two broad maxima at 930 and 810 AA between the first and second ionisation thresholds are assigned to the autoionisation from valence-type excited states. New Rydberg series (np pi u) converging to the second ionisation threshold were found. The appearance potentials of fragment ions by photoexcitation are given and discussed in terms of the heats of formation of molecular ions. For each of C2+ and CH+, two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C2+ is estimated from the appearance potential to be 1980 kJ mole-1.