Single- versus multi-state DIM model for RgX2 systems: On the influence of spin–orbit coupling on Ar–I2 potentials Original Research Article

A diatomics-in-molecule model is developed for rare gas–dihalogen systems with spin–orbit coupling included within the atoms-in-molecule scheme using the LS-representation for the basis set. This approach is found to provide good agreement with both direct ab initio results and experimental data for the iodine molecule. Potential energy surfaces (PES) for the interaction of Ar(1S0) with I2 in the ground and lowest-lying excited states (including B0u+, A1u, A′2u) are calculated. Because of relatively large separations of the associated nonrelativistic I2 states at their equilibrium distances, both their spin–orbit coupling and mixing by Ar perturb rather weakly the corresponding Ar–I2 PES, which can thus be represented by simple analytic expressions. Perturbative corrections are proposed for the case of the I2 states mixed strongly. Topologies of both the ground and lowest excited state PES are found to differ from the predictions of previous models, particularly concerning the existence of a well for the linear geometry. Such a well with a depth comparable to that in the T-shaped geometry is obtained for Ar–I2(X0g+) and a rather shallow well is obtained for Ar–I2(B0u+), in particular. The remaining deviations of the calculated ground state Ar–I2 equilibrium parameters from their experimental values are assigned to distortions of the electronic structure of the I atoms within the I2 molecule, which are not taken into account by the traditional DIM treatment. The effect of these distortions on the PES topology is discussed.