The chemistry and physics of molecular surfaces

Configuration interaction calculations are carried out to study the potential energy surface for the system Ar-Ar ₂ ⁺ . An all-electron as well as a pseudopotential treatment is employed. It is found that in the perpendicular Ar approach the Ar ₂ ⁺ partner remains essentially unchanged and the potential can be characterized by an electrostatic ion-induced dipole interaction. In the collinear mode of Ar approach the Ar ₂ ⁺ bond separation increases considerably, the charge is redistributed and the interaction can be characterized as chemical bonding. The minimum on the surface is found to be the linear symmetric molecule with bond lengths of 2.62 Å. The optimum structure in the perpendicular approach lies 0.13 eV above the minimum and is the T-shaped molecule in which the Ar is 3.65 Å away from the midpoint of the Ar ₂ ⁺ (r=2.46 Å) system; the best equilateral triangle structure has a bond length of 2.99 Å but is found to lie 0.64 eV above the Ar ₃ ⁺ minimum. The dissociation energy into Ar ₂ ⁺ + Ar is calculated to be 0.16 eV in reasonable agreement with experimental values of 0.21 eV. The potential curves for the four lowest states of Ar ₂ ⁺ are also treated.