G e r a d e Rydberg states of acetylene studied by multiphoton ionization and photoelectron spectroscopy

The lowest few gerade Rydberg states of C2H2, C2HD, and C2D2 have been observed as two-photon resonances in the multiphoton ionization (MPI) spectra of these isotopic species. Analysis of the two-photon excitation spectra of these molecules, and of the accompanying MPI-photoelectron spectra for C2H2, enable characterization of the 1Σ+g and 1Δg Rydberg states arising from the electronic promotion π3u3pπ1u←π4u and reveal extensive vibronic interaction in this energy region. A search was made for higher gerade Rydberg states at energies up to 90 000 cm−1. Weak, diffuse signal is observed for the 4p complex. No ionization signal was observed in the energy range where the np (n=5–7) Rydberg states are expected to appear. This is taken as evidence for opening of a facile nonradiative decay channel at high excitation energies.