Probing spin–orbit mixing and the singlet–triplet gap in dichloromethylene via Ka-sorted emission spectra
Abstract
The magnitude of the singlet–triplet gap in dichloromethylene (CCl2) has been a point of controversy in the recent literature. In this study, we report single vibronic level 1A1 system of the
1A1 state up to ≈10 000 cm−1 above the vibrationless level. By the careful selection of bands where complete isotope and Ka′ selectivity in excitation was possible, we measured Ka′-sorted
″) rotational constant, which is significantly larger for pure triplet levels due to the larger equilibrium bond angle. In the region between 3500 and 9000 cm−1 above the vibrationless level of the
1A1 state, we find only a very modest increase in (A″
−
″), and ∼86% of the lines observed between 5000 and 9000 cm−1 can be assigned to
1A1 levels within 3 standard deviations of our Dunham expansion fit, which included more than 140 levels in total. A nearly complete set of Dunham parameters was determined for the C35Cl2 isotopomer, and the
1A1 state term energies up to 4000 cm−1 are in excellent agreement with recent variational calculations of Tarczay, et al. [G. Tarczay, T. A. Miller, G, Czakó and A. G. Császár, Phys. Chem. Chem. Phys., 2005, 7, 2881]. Finally, the implication of our results for the singlet–triplet gap in dichloromethylene is discussed.