Unraveling the Ã1B1 ← X̃1A1 Spectrum of CCl2: The Renner−Teller Effect, Barrier to Linearity, and Vibrational Analysis Using an Effective Polyad Hamiltonian
journal contributionposted on 13.11.2008, 00:00 by Craig Richmond, Chong Tao, Calvin Mukarakate, Haiyan Fan, Klaas Nauta, Timothy W. Schmidt, Scott H. Kable, Scott A. Reid
We report studies aimed at unraveling the complicated structure of the CCl2 Ã1B1 ← X̃1A1 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 Ã1B1 state. The derived Ã1B1 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 X̃1A1 state of ∼23 000−23 500 cm−1, in excellent agreement with ab initio theory.