To further investigate the Renner–Teller (RT) effect and barriers to linearity and dissociation in the simplest singlet carbene, we recorded fluorescence excitation spectra of bands involving the pure bending levels 2n0 with n = 0–9 and the combination states 1102n0 with n = 1–8 and 2n0310 with n = 0–5 in the Ã
1A′ system of CDF, in addition to some weak hot bands. The spectra were measured under jet-cooled conditions using a pulsed discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants; fluorescence lifetimes were also measured to probe for lifetime lengthening effects due to the RT interaction. The derived Ã state parameters are compared with previous results for CHF and with predictions of ab initio electronic structure theory. The approach to linearity in the Ã state is evidenced in a sharp increase in the A rotational constant with bending excitation, and a minimum in the vibrational intervals near 29. A fit of the vibrational intervals for the pure bending levels yields an Ã state barrier to linearity in good agreement both with that previously derived for CHF and ab initio predictions. From the spectra and lifetime measurements, the onset of extensive RT perturbations is found to occur at a higher energy than in CHF, consistent with the smaller A constant.
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