Experimental and Theoretical Studies of Photoionization-Efficiency Curves for C2H2 and C2D2

Abstract
Photoionization‐efficiency curves for C2H2 and C2D2 have been remeasured in the wavelength region from onset of ionization to 600 Å, using a gold‐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 Franck—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 molecule‐ion curve and for that of the fragment ions, C2H+ and C2D+. The observed onsets of ionization for the latter ions are 17.22— and 17.34 eV, respectively.