Ab initio investigation of potential energy curves of the 23 electronic states of IBr correlating to neutral P-2 atoms

Potential energy surfaces for all Born-Oppenheimer electronic states of IBr molecule correlating to the neutral P-2 (P-2(3/2) and P-2(1/2)) iodine and bromine are calculated for the first time. Electric dipole and polarizability curves ( static and transition) are also determined. Calculations include scalar and spin-orbit relativistic effects within all-electron Douglas-Kroll two-component Hamiltonian. Electron correlation is treated with quasi-degenerate multi-reference second-order perturbation theory. Seven adiabatic electronic states (X (1)Sigma(+), A ' (3)Pi(2), A (3)Pi(1), 1 (3)Pi(0-), B (3)Pi(0+), B ' (3)Sigma(-)(0+), and 2 (3)Pi(0+)) exhibit significant covalent bonding, and can support vibrational states. Calculated spectroscopic parameters agree with experiment to better than 1000 cm(-1) (T-e), 10 cm(-1) (omega(e)), and 0.05 angstrom (r(e)). A new 1 (3)Pi(0-) state correlating to ground-state atoms is predicted at T-e approximate to 14 000 cm(-1), omega(e) approximate to 80 cm(-1), and r(e) approximate to 3.0 angstrom. The second new state (2 (3)Pi(0+)) correlates to excited iodine atom, with T-e approximate to 20 000 cm(-1), omega(e) approximate to 115 cm(-1), and r(e) approximate to 3.3 angstrom. Non-adiabatic coupling parameters are calculated for the four avoided crossings, which arise due to electronic spin-orbit interaction. Estimated parameters of the B (3)Pi(0+)/B ' (3)Sigma(-)(0+) crossing (R-c approximate to 3.32 angstrom; V approximate to 120 cm(-1)) agree with experimental values. The previously unsuspected 2 (3)Pi(0-)/1 (1)Sigma(-) crossing of two repulsive surfaces provides a new collisional deactivation channel for Br* atoms at relative velocities above 1000 m s(-1). Several repulsive states (including 1 (1)Pi(1) and 2 (3)Pi(1)) intersect the B/B ' system near the avoided crossing point, and may affect dynamics of IBr in strong laser fields.