Photoionisation mass spectrometric study of acetylene in the VUV region

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 C 2 H 2 + and also for fragment ions C 2 H + , C 2 + , CH 2 + and CH + . To assign the structures of C 2 H 2 + , 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 C 2 + and CH + , two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C 2 + is estimated from the appearance potential to be 1980 kJ mole -1 .