The 5s-4d Rydberg states of C2H2 and C2D2 studied by resonant multiphoton ionization and synchrotron radiation:: Structure and stability

The components of the 5s + 4d Rydberg Complex of C2H2 and C2D2 have been studied by (3 + 1) photon ionization spectroscopy and fragment fluorescence excitation spectroscopy following synchrotron radiation VUV excitation. Thanks to the remarquable spectral resolution of the new synchrotron beam line SU5 at Super-ACO (12 m Angstrom in the range 248-65 nm), rotationally resolved spectra of Rydberg states of acetylene could be observed through their photofragment visible fluorescence. Unlike the lower 4s + 3d Rydberg complex, all Rydberg components of this complex exhibit rotational line widths typical of a 1-10 ps predissociation. The (1)Phi (u) 4d delta (g) and (1)Delta (u) 4d pi (g) components have been characterized for both isotopic species. The relative electronic band intensities of the REMPI spectra have been interpreted within a semi-united atom model and by taking into account predissociation.