We report fixed-nuclei photoionization cross-sections and asymmetry parameters for photoionization leading to the X~2#PI#_u, A~2#SIGMA#_g~+, B~2#SIGMA#_u~+, and C~2#SIGMA#_g~+, and C~2#SIGMA#_g~+ states of acetylene with emphasis on the first four states. The magnitude of the photoionization cross-sections calculated here is in excellent agreement with recent experiment at both low and high photon energy. Further, as a result of the multichannel scattering methodology used to perform the calculation, the partial channel cross-sections and asymmetry parameters reported here resolve significant structure arising from indirect photoionization processes such as autoionization. Although vibrational degrees of freedom are not included within the fixed nuclei framework employed here, we find that, even without vibrational degrees of freedom, the present theoretical results generally exhibit the same detailed features as the experimental results, both for conventional photoionization spectra and, as a result of vibrational autoionization, for threshold photoionization spectra. This general agreement suggests that a large part often structure in the low energy or outer valence photoionization spectrum of acetylene is explicable solely I terms of Rydberg transitions. This study also predicts that dark states may cause some appreciable distortion of the profile of the photoionization asymmetry parameter in the photon region of approx 20-21.5 eV as a result of final state correlations with more intense states.
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