15min:

H. FAN, I. IONESCU, C. ANNESLEY AND S. A. REID, Department of Chemistry, Marquette University, Milwaukee, WI 53233; J. XIN, Department of Physics and Engineering Technologies, Bloomsburg University, Bloomsburg, PA 17815.

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''-X¹A' 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'' 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 K_a' > 0. A fit of the vibrational intervals for the pure bending levels yields a barrier to linearity of 6300 cm^-1 above the vibrationless level. Our observation of the K_a' = 1 level of (1,6,0) places a lower limit on the barrier to dissociation of 8555 cm^-1 above the vibrationless level.