Assignment of the electronic spectrum of allene by a theoretical configuration interaction approach

Early assignments of the vacuum UV photoabsorption spectrum of allene (1951-1972) gave a number of Rydberg series, but these series had limiting values for the ionization potential, which are incompatible (by up to 0.4 eV) with adiabatic IPs from molecular beam and photoelectron spectra (UV-PES). Following our recent re-analysis of the VUV spectrum of allene by systematic overlay of the VUV and UV-PES spectra, a number of new experimental assignments were obtained; specifically the following assignments provide a target for this theoretical study: 3s, 6.180; 3p, 6.960; 3d, 7.910; 4s, 8.076 eV. We now use multi-root configuration interaction calculations to determine the theoretical envelopes of both valence and Rydberg states. The CI calculations give a consistent picture of the Rydberg assignments as well as giving a clear indication of the positions of valence states. The second moment expectation values and oscillator strengths are used to determine the nature of the electronic states observed experimentally.