Photoionizationhyphen;efficiency curves for C2H2and C2D2have been remeasured in the wavelength region from onset of ionization to 600 Aring;, using a goldhyphen;coated grating and the Hopfield continuum as a photon source. The curve shapes of the molecule ions near threshold are compared with calculations of the Franckmdash;Condon factors. Nearly exact agreement between experiment and calculation is obtained when taking into account hot bands. A general discussion is given for the shape of the moleculehyphen;ion curve and for that of the fragment ions, C2H+and C2D+. The observed onsets of ionization for the latter ions are 17.22mdash; and 17.34eV, respectively.
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