Unraveling the ùB₁ ← X̃ ¹A₁ spectrum of CCl₂: The Renner-Teller effect, barrier to linearity, and vibrational analysis using an effective polyad hamiltonian

We report studies aimed at unraveling the complicated structure of the CCl₂ ùB₁ ← X̃ ¹A₁ system. We have remeasured the fluorescence excitation spectrum from ∼17 500 to 24 000 cm^-1 and report the term energies and A rotational constants of many new bands for both major isotopologues (C³⁵Cl₂, C³⁵Cl³⁷Cl). We fit the observed term energies to a polyad effective Hamiltonian model and demonstrate that a single resonance term accounts for much of the observed mixing, which begins ∼1300 cm^-1 above the vibrationless level of the ùB₁ state. The derived à ¹B₁ vibrational parameters are in excellent agreement with ab initio predictions, and the mixing coefficients deduced from the polyad model fit are in close agreement with those derived from direct fits of single vibronic level (SVL) emission intensities. The approach to linearity and thus the Renner-Teller (RT) intersection is probed through the energy dependence of the A rotational constant and fluorescence lifetime measurements, which indicate a barrier height above the vibrationless level of the X̃¹A ₁ state of ∼23 000-23 500 cm^-1, in excellent agreement with ab initio theory.