Spectroscopic properties of lead trimer (Pb3 and Pb3+):: Potential energy surfaces, spin-orbit and Jahn-Teller effects

Spectroscopic properties of the low-lying electronic states of neutral, cationic, and anionic lead trimer (Pb-3) are investigated. We have obtained the bending potential energy surfaces of several electronic states of Pb-3 and Pb-3(+) both with and without spin-orbit coupling. These computations were carried out using high level techniques that included electron correlation effects and spin-orbit coupling simultaneously using a multireference relativistic configuration interaction (RCI) scheme in the double group, subsequent to complete active-space-multiconfiguration self-consistent-field (CAS-MCSCF) computations. We have computed the equilibrium geometries, vibrational frequencies, excitation energies, atomization energies, ionization potentials, and adiabatic electron affinities. Our computations facilitated the assignment of the anion photodetachment spectra of Pb-3(-) and explained the closed-shell singlet like structures in the observed photodetachment spectra. Our computations show that spin-orbit coupling has a substantial impact on the geometries (bond angles changing up to 20 degrees) and the potential energy surfaces, which exhibit multiple minima separated by barriers due to avoided crossings and substantial spin-orbit mixings. The interplay between the Jahn-Teller effect and spin-orbit coupling was considered, and it was shown that the Jahn-Teller coupling is quenched by spin-orbit effect for Pb-3(+), but for the neutral trimer the bending potential energy surface of the ground state exhibits multiple minima due to a combination of these effects. The spin-orbit effect was also shown to reduce the strength of the Pb-Pb bonding and the atomization energy of Pb-3. Our computed atomization energy of Pb-3 including spin-orbit coupling is 224 KJ/mole in full agreement with the experimental value of 224 KJ/mole. We have shown that the spin-orbit coupling enhances the stability of Pb-3(+) while it weakens Pb-3 dramatically compared to lighter analogs such as Si-3. (C) 2001 American Institute of Physics.