Isotope Effects in the Photoionization Yields and the Absorption Cross Sections for Acetylene, Propyne, and Propene

The values of the absorption cross section Σ and the photoionization yield Γ (the probability that photon absorption produces ionization) have been experimentally determined for C2H2 and C2D2, C3H4 and C3D4, and C3H6 and C3D6 at various photon energies in the energy range of 11.25 to 11.78 eV, 10.23 to 11.78 eV, and 9.63 to 11.78 eV, respectively. In both propyne and propene there is an increase in ionization for the heavier isotopic molecules, which is evident both in Γ and in the photoionization cross section, Σi (Σi = ΓΣ). This increased ionization is discussed in terms of the Platzman competitive ionization mechanism, and the effects of methyl substitution on Σ and Σi are also discussed.