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

The values of the absorption cross section $\Sigma$ and the photoionization yield $\Gamma$ (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 $\Gamma$ and in the photoionization cross section, ${\Sigma }_i{\mathrm{(\Sigma }}_i\mathrm{\hspace{0.17em}=\hspace{0.17em}\Gamma \Sigma )}$ . This increased ionization is discussed in terms of the Platzman competitive ionization mechanism, and the effects of methyl substitution on $\Sigma$ and ${\Sigma }_i$ are also discussed.