A photoionization mass spectrometric study of acetylene and ethylene in the VUV spectral region

The molecular and dissociative photoionization of C2H2 and C2H4 has been studied using synchrotron radiation within the energy range 11-30 eV. Positive and negative photoion mass spectra have been obtained. Appearance thresholds for the formation of fragment ions have been determined and are compared with the thermochemical values. Above threshold it is shown that superexcited states, involving intravalence transitions and shape resonances which can decay through ion pair formation, play an important role in the photodissociative ionization process. The positive photoion yield curves display significant hitherto unobserved structures, which are unrelated to those present in the photoelectron spectra. (C) 2002 Elsevier Science B.V. All rights reserved.