The outer valence photoionization of acetylene

We report fixed-nuclei photoionization cross-sections and asymmetry paramet ers for photoionization leading to the X (2)Pi(u), A (2)Sigma(g)(+), B (2)S igma(u)(+), and C (2)Sigma(g)(+) states of acetylene with emphasis on the f irst four states. The magnitude of the photoionization cross-sections calcu lated here is in excellent agreement with recent experiment at both low and high photon energy. Further, as a result of the multichannel scattering me thodology used to perform the calculation, the partial channel cross-sectio ns and asymmetry parameters reported here resolve significant structure ari sing from indirect photoionization processes such as autoionization. Althou gh 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 detaile d features as the experimental results, both for conventional photoionizati on spectra and, as a result of vibrational autoionization, for threshold ph otoionization spectra. This general agreement suggests that a large part of the structure in the low energy or outer valence photoionization spectrum of acetylene is explicable solely in terms of Rydberg transitions. This stu dy also predicts that dark states may cause some appreciable distortion of the profile of the photoionization asymmetry parameter in the photon region of similar to 20-21.5 eV as a result of final state correlations with more intense states. (C) 1999 American Institute of Physics. [S0021-9606(99)004 37-7].