The molecular structure and ionization potential of Si-2: The role of the excited states in the photoionization of Si-2

The ionization potentials (IP) of Si-2 (X (3)Sigma(g)(-)) to form the X (4) Sigma(g)(-) and a(2)Pi(u) states of Si-2(+) have been calculated at very hi gh levels of ab initio molecular orbital theory (CCSD(T) with augmented cor relation-consistent basis sets extrapolated to the complete basis set limit ). The calculated value of the IP to form the X (4)Sigma(g)(-) ground state of the ion is 7.913 eV as compared to an experimental value of 7.9206 eV. The a(2)Pi(u) stare is predicted to lie 0.52 eV above the X (4)Sigma(g)(-) ground state of Si-2(+). The 1 (3)Delta(u), 2 (3)Delta(u), H (3)Sigma(u)(-) , and K (3)Sigma(u)(-) excited states of Si-2, as well as the X (4)Sigma(g) (-), a(2)Pi(u), and 2(2)Pi(u) states of Si-2(+), have been calculated at th e multireference configuration interaction level. The agreement of the calc ulated positions of the states with the known experimental values is quite good (better than 0.1 eV. The calculated wave functions fur the excited sta tes of Si-2 show significant multireference character. This is especially t rue for the H state which has been used as an intermediate state in photoio nization experiments. The multireference character of the H state readily a llows the connection of this state to the ground X (4)Sigma(g)(-) electroni c state of Si-2(+) via a one electron photoionization process.