To further investigate the Renner-Teller (RT) effect and spin-orbit mixing in the (A) over tilde (1)A"<--(X) over tilde (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) over tilde (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) over tilde (3)A', as evidenced by large F-19 and H-1 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) over tilde (1)A"-(X) over tilde (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 F-19 and H-1 hyperfine structure of the a(3)A(') state and the magnitude of the spin-orbit matrix elements. (C) 2004 American Institute of Physics.