Mass-Spectrometric Study of Photoionization. IV. Ethylene and 1,2-Dideuteroethylene

Abstract

Photoionization‐efficiency curves for ions of the molecule and of selected fragments of C₂H₄ and CHDCHD are reported for the wavelength region from onset of ionization to 600 Å. The minimum threshold of ionization is 10.50 eV for both molecules. In addition, certain features of the ion‐yield curves are discussed and identified as resulting from vibrationally excited states or from autoionization from Rydberg levels leading to electronically excited states. Threshold measurements of the acetylene ions permit calculation of heats of dissociation and indicate that the C₂H₂⁺ ion is formed without excess activation energy. Of the vinyl ions, the threshold for the C₂H₃⁺ ion is the most clearly defined and leads to a C₂H₃–H bond‐dissociation energy of 100 kcal·mole⁻¹.