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Polarization quantum beat spectroscopy of HCF(A1A"). 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 A(1)A(")<--X(1)A(') system of the simplest singlet carbene, HCF, we report a detailed analysis of the K(a) = 1<--0 subband of 2(0) (4) 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 X(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 a(3)A("), as evidenced by large (19)F and (1)H 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 A(1)A(")-X(1)A(') 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 (19)F and (1)H hyperfine structure of the a(3)A(") state and the magnitude of the spin-orbit matrix elements.