On Renner-Teller effect and barriers to linearity and dissociation in HCF(A1A′′)

further investigate the Renner-Teller effect and barriers to linearity and dissociation in the simplest singlet carbene (HCF), we recorded fluorescence excitation spectra of the pure bending transitions 2(0)(n) with n = 0-7 and the combination bands 1(0)(1) 2(0)(n) with n = 1-6 and 2(0)(n) 3(0)(1) with n = 0-3 in the HCF (A) over tilde (1)A" <-- (X) over tilde (1)A' 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 (A) over tilde -state parameters are in excellent agreement with the predictions of ab initio electronic structure theory. The approach to linearity in the (A) over tilde -state is evidenced in a sharp increase in the rotational constant (A) over tilde, as first reported by Kable and co-workers, and a minimum in the vibrational intervals near the 2(7) level. A fit of the vibrational intervals for the pure bending levels yields a barrier to linearity of 6300 270 cm(-1) above the vibrationless level. Our observation of the K-a' = 1 level of 1(1)2(6) places a lower limit on the (A) over tilde state barrier to dissociation of similar to 8555 cm(-1) above the vibrationless level.