High-resolution photoabsorption and photoionization spectra of C2H4 and C2D4

Abstract

The high-resolution photoabsorption and photoionization cross sections of C2H4 and C2D4 are measured and observed to exhibit both sharp and broad structures in the spectral region between the ionization threshold, 1180 Å, and 1075 Å. Members of several Rydberg series of the respective C2H4 and C2D4 molecules that converge to the first excited electronic state of the ethylene ion, Ã(2B3), have been identified. They have been tentatively assigned as transitions from the ground state to the s-, p-, and d-Rydberg states. In the wavelength region studied in this paper, the Rydberg states do not autoionize but, rather, appear to competitively compete with ionization from the continuum. The generally smaller photoionization yield in C2H4 relative to C2D4, plus the greater diffuseness of the absorption bands in C2H4 are interpreted to indicate that the Rydberg states of the light isotopomer have shorter lifetimes than those of the heavier isotopomer. The physical phenomena appear to be dominated by the nuclear dynamics. Thus, the absorbed photon energy is quickly transferred to the nuclei, shortening state lifetimes and curbing ionization. The efficiency for this to occur is higher in the lighter isotopic molecule.