The molecular structure and ionization potential of Si2:: The role of the excited states in the photoionization of Si2

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 high levels of ab initio molecular orbital theory (CCSD(T) with augmented correlation-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 the multireference configuration interaction level. The agreement of the calculated positions of the states with the known experimental values is quite good (better than 0.1 eV. The calculated wave functions fur the excited states of Si-2 show significant multireference character. This is especially true for the H state which has been used as an intermediate state in photoionization experiments. The multireference character of the H state readily allows the connection of this state to the ground X (4)Sigma(g)(-) electronic state of Si-2(+) via a one electron photoionization process.