ROTATION VIBRATION INTERACTIONS IN FORMALDEHYDE - RESULTS FOR LOW VIBRATIONAL EXCITATIONS

We have carried out large-scale variational calculations on formaldehyde including all six vibrational modes and the rotational mode associated with K (the projection of total angular momentum on a body-fixed axis). A complete form of the Watson Hamiltonian and a realistic potential function based on ab initio data are used. Emphasis in this paper is placed on the low vibrational energy regime and the effect of Coriolis interaction. Comparison is made with other theoretical calculations and experiment, where available. Analysis of some of our full mode rovibrational states shows instances of simultaneous vibration and rotation mixing due to a strong a-axis Coriolis interaction. We show that such mixing can be understood from the perspective of a classical nonlinear resonance between the rotational frequency and the difference in two bending mode (out-of-plane and in-plane) frequencies. This confirms previous full dimension classical and reduced dimension quantum studies and represents an alternative way of understanding Coriolis interaction in the low vibrational energy regime.