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Abstract
A new Rydberg state of acetylene has been identified as a three photon resonance in the four photon ionization spectra of both C2H2 and C2D2. In both isotopic species the zero point level of this new electronic state is sufficiently long lived to support rotational structure. Spectral analysis, including use of the appropriate three photon rotational linestrength theory, enables characterization of the excited state symmetry as 1Φ u . Quantum defect considerations, and comparisons with recent ab initio calculations, encourage assignment of this 1Φ u state in terms of the electronic promotion π3 u 3dδ g ← π4 u . Possible reasons for the dominance of this 1Φ u -1Σ+ g transition in the 3 + 1 MPI spectrum of acetylene are considered.
