Multichannel Schwinger study of C 1s photoionization of acetylene

We have studied the C 1s photoionization of acetylene using single- and mul tichannel scattering calculations including the core and selected shakeup c hannels. Our calculation found a shape resonance occurs in the 1 sigma(g)(- 1) channel. However, this resonance is not the dominant feature in photoabs orption spectrum due to strong shake-up intensities at about the same posit ion, in agreement with the high-resolution photoelectron spectra. The 1 sig ma(g)(-1) and 1 sigma(u)(-1) channels have been identified as having differ ent dynamics in the core ionization process. The calculated intensity ratio between these two channels agrees well with the recent measurement. Electr on correlation in the target states is found to have a great impact on the computed cross sections and asymmetry parameters. In the coupled-channel sc attering calculation, an insufficient treatment of target electron correlat ion leads to exaggerated channel coupling effects. A multi-electron resonan t process via the doubly excited state (1 sigma(g)(-1)1 pi(u)(-1)1 pi(g)(+2 )) was also found in the multichannel calculation. Results are also present ed for ionization leading to some shake-up states. (C) 2000 American Instit ute of Physics. [S0021-9606(00)00329-9].