Photoionization and ion cyclotron resonance studies of the reaction C2H4+ + C2H4 → C3H5+ + CH3

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Abstract
High pressure photoionization mass spectrometry has been employed to study the ion molecule reaction C2H4+ + C2H4 → C3H5+ + CH3 at wavelengths between 700 and 1180 Å. Measurements have been made of the apparent ionization cross section of both C2H4+ and C3H5+ as a function of photon energy. The threshold energy for the reaction coincides with the ionization threshold of the parent ion at 10.51 eV. The reaction cross section decreases as the internal energy of the ionic reactant increases. Vibrational structure, observed in the C2H4+ photoionization efficiency curve immediately above threshold (10.51−11.7 eV) permits a rough estimate of the change in reaction cross section with vibrational excitation. For the first three observed levels the change in cross section is ? 10%. However, for the fourth and fifth levels the cross section has dropped by approximately 25%. In the energy region between 11.7 and 12.1 eV a slight increase in the C2H4+ photoionization efficiency curve appears which is not reflected in the C3H5+ curve. A large increase in the photoionization efficiency curve of C2H4+ occurs at 12.2 eV, the threshold of the 2B3 first excited electronic state of the ion. A dramatic decrease is observed in the reaction probability of C2H4+ formed at energies above 12.2 eV. The cross section for reaction in the excited 2B3 state is ? 13% of the cross section for reaction in the lowest vibrational level of the ground electronic state. The observed decrease in reactivity is independent of repeller voltage at ion exit energies below 3 eV. Pressure studies reveal that after one or two collisions, the reactivity of C2H4+ is re−established, suggesting that relaxation of internal excitation is an efficient process. The change in reactivity of C2H4+ with internal energy is compared to the effects of translational excitation as determined using the techniques of ion cyclotron resonance spectroscopy.