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

Abstract

Potential energy surfaces for all Born–Oppenheimer electronic states of IBr molecule correlating to the neutral ²P (²P_3/2 and ²P_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 ¹Σ⁺, A′ ³Π₂, A ³Π₁, 1 ³Π₀₋, B ³Π₀₊, B′ ³Σ⁻₀₊, and 2 ³Π₀₊) 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 (ω_e), and 0.05 Å (r_e). A new 1 ³Π₀₋ state correlating to ground-state atoms is predicted at T_e ≈ 14 000 cm⁻¹, ω_e ≈ 80 cm⁻¹, and r_e ≈ 3.0 Å. The second new state (2 ³Π₀₊) correlates to excited iodine atom, with T_e ≈ 20 000 cm⁻¹, ω_e ≈ 115 cm⁻¹, and r_e ≈ 3.3 Å. Non-adiabatic coupling parameters are calculated for the four avoided crossings, which arise due to electronic spin–orbit interaction. Estimated parameters of the B ³Π₀₊/B′ ³Σ⁻₀₊ crossing (R_c ≈ 3.32 Å; V ≈ 120 cm⁻¹) agree with experimental values. The previously unsuspected 2 ³Π₀₋/1 ¹Σ⁻ 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 ¹Π₁ and 2 ³Π₁) intersect the B/B′ system near the avoided crossing point, and may affect dynamics of IBr in strong laser fields.