Polarization quantum beat spectroscopy of HCF(Ã ¹A″). II. Renner-Teller and spin-orbit mixing in the simplest singlet carbene

To further investigate the Renner-Teller (RT) effect and spin-orbit mixing in the à ¹A″ ← X̃ ¹A′ system of the simplest singlet carbene, HCF, we report a detailed analysis of the K _a= 1 ← subband of 20 using polarization quantum beat spectroscopy in combination with fluorescence excitation spectroscopy and lifetime measurements. This subband is perturbed both by RT and spin-orbit interactions, which are clearly differentiated due to the order-of-magnitude difference in matrix elements. We show that RT induced mixing with a high vibrational level of X̃ ¹ A′ leads to a splitting of this subband, and while the higher energy member is rotationally unperturbed, every line in the lower energy member is perturbed by spin-orbit mixing with background levels of ã ³A″, as evidenced by large ¹⁹F and ¹H hyperfine constants and Lande g factors. In contrast, the higher energy subband exhibits very small Lande g factors and hyperfine constants, which is explained within a model that incorporates only the à ¹A″-X̃ ¹A′ interaction. We thus demonstrate that polarization quantum beat spectra provides efficient discrimination between RT and spin-orbit interactions. Analysis of the lower energy subband in concert with ab initio electronic structure calculations has yielded the first information on the ¹⁹F and ¹H hyperfine structure of the ã ³A″ state and the magnitude of the spin-orbit matrix elements.