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 st udied by (3 + 1) photon ionization spectroscopy and fragment fluorescence e xcitation spectroscopy following synchrotron radiation VUV excitation. Than ks 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 resol ved spectra of Rydberg states of acetylene could be observed through their photofragment visible fluorescence. Unlike the lower 4s + 3d Rydberg comple x, all Rydberg components of this complex exhibit rotational line widths ty pical of a 1-10 ps predissociation. The (1)Phi (u) 4d delta (g) and (1)Delt a (u) 4d pi (g) components have been characterized for both isotopic specie s. The relative electronic band intensities of the REMPI spectra have been interpreted within a semi-united atom model and by taking into account pred issociation.