MPI photoelectron spectroscopy of ungerade excited states of acetylene: Intermediate state mixing and ion state selection

Three photon resonant, four photon (3 + 1) ionization spectroscopy and photoelectron spectroscopy have been used to study the ungerade excited states of acetylene in the energy range from 74 500 to 90 000 cm^-1. Sharp bands from the nR (π_u³nsσ_g) and ¹Φ_u (π_u³ndδ _g) Rydberg series dominate the MPI spectrum. A large number of Rydberg and valence states which are prominent in VUV absorption spectra are absent or weak in MPI studies. These weak bands are only observable under high power conditions, which suggests that nonradiative decay is rapid enough to depopulate these states before ionization occurs. The photoelectron results provide further insight into the nature of the excited states. Ionization through the sharp bands occurs via Δv = 0 Franck-Condon transitions, resulting in ions in a single vibrational state. Ionization through bands which are mixed results in complicated ion vibrational distributions including excitation of both cis and trans bends.