Knowledge Bank

To further investigate the Renner-Teller effect and excited state barriers to linearity and dissociation in the simplest singlet carbene, HCF, we measured fluorescence excitation spectra and lifetimes of the pure bending transitions $20n$ with $n=0-7$ and the combination bands $10120n$ with n = 1-6 and $20n301$ with $n=0-3$ in the HCF $A1A\prime \prime -X1A\prime$ system. 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. The derived $A1A\prime \prime$ state parameters are in excellent agreement with the predictions of ab initio electronic structure theory. The approach to linearity is evidenced in a sharp increase in the A rotational constant, a minimum in the bending vibrational intervals, and a pronounced fluorescence lifetime lengthening for levels with $Ka\prime >0$. A fit of the vibrational intervals for the pure bending levels yields a barrier to linearity of $6300cm-1$ above the vibrationless level. Our observation of the $Ka\prime =1$ level of (1,6,0) places a lower limit on the barrier to dissociation of $8555cm-1$ above the vibrationless level.