Vibrational states of ArCO₂: Analysis of an internal dynamical transition using self-consistent field techniques

The van der Waals cluster molecule ArCO₂ is studied computationally by using the vibrational self-consistent field (SCF) approximation, with an approximate but reasonable potential function. Calculations are carried out both for the full six-dimensional motion and for a reduced two-dimensional problem in which the CO₂ is held rigid. An interesting dynamical transition is found in the motion of the Ar atom. Its equilibrium geometry is a symmetric T-shape, and for low excitations both the radial and the angular motions in the CO₂ plane resemble the states of anharmonic oscillators (smaller intervals with higher excitations). Above the sixth state of the bend in the angle θ, however, the bend spectrum changes to that of a rigid rotor, with spacings of 2Bn_θ for quantum number n_θ. The one-dimensional effective SCF potentials along the θ coordinate and plots of the wave function both show a dynamical transition, in which, above n_θ = 6, the motion of the Ar in the CO₂ plane is essentially that of a rigid rotor in the θ coordinate. Calculations of the principal moments of inertia support this interpretation.