Valence one-electron and shake-up ionization bands of carbon clusters. II.The C-n (n=4,6,8,10) rings

The 1h (one-hole) and 2h-1p (two-hole; one-particle) shake-up bands in the valence ionization spectrum of small carbon rings (C-4, C-6, C-8, C-10) are investigated up to 40 eV, using the one-particle Green's Function approach . Calculations have been performed using both the third-order algebraic dia grammatic construction [ADC(3)] scheme and the outer-valence Green's functi on (OVGF) approximation. The obtained ADC(3) results indicate a major fragm entation of lines into complex sets of 2h-1p satellites, even for outer-lev els of Pi-character. The simulated ionization spectra provide striking stru ctural signatures for the carbon rings vs. the carbon chains, that could be usefully exploited to discriminate these two types of clusters in plasma c onditions. In general and in spite of the extent of the shake-up spreading, one can rather easily trace from our convolutions the energy degeneracies, and in the outer-valence region, the Sigma-Pi near-energy degeneracies whi ch characterize the electronic structure of these doubly conjugated rings. In relationship to its doubly antiaromatic (i.e., polyynic) nature and lowe r symmetry, C-8 is subject to stronger many-body effects than the other rin gs. ADC(3) calculations on first-order saddle point forms of C-6 and C-10 i ndicate that thermal averaging of conformations could have a substantial ef fect on the spectral bands, in particular the shake-up ones. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)30510-4].