Vibrational states of argon-carbon dioxide molecule: 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 theta, however, the bend spectrum changes to that of a rigid rotor, with spacings of 2Bn(theta) for quantum number n(theta). The one-dimensional effective SCF potentials along the theta coordinate and plots of the wave function both show a dynamical transition, in which, above n(theta) = 6, the motion of the Ar in the CO2 plane is essentially that of a rigid rotor in the theta coordinate. Calculations of the principal moments of inertia support this interpretation.