Frequency-modulation transient absorption spectrum of the HCCl à ¹A″(0,0,0)←X̃ ¹A′(0,0,0) transition

The rotationally resolved spectrum of the HCCl à ¹A″ (0,0,0)←X̃ ¹A′(0,0,0) transition between 12 230 and 12 410 cm^-1 was obtained for the first time at Doppler-limited resolution using a frequency-modulation (FM) transient absorption technique. Five major subbands (K_a=0→1, 1←0, 1←2, 2←1, and 2←3) of HC³⁵Cl and one subband (K_a=0←1) of HC³⁷Cl were observed and assigned. In addition, the forbidden subband (K_a=0←0) of HC³⁵Cl was also observed. Thus far, we have assigned most (more than 70%) of the strong lines. The analysis of these subbands determines the geometry for the HCCl Ã(0,0,0) level: 〈HCCl=134.7° and R_C-Cl=1.623 Å while R_C-H is fixed at the corresponding value of CH₂. The height of the barrier to linearity was found to be approximately 2170 cm^-1 above the vibrationless level of the à state. The rotational structure of this spectrum shows irregular perturbations. Both random and J-dependent (anomalous K-type doubling) effects were observed. As in methylene, the perturbations most likely arise from Renner-Teller effect between the two singlet states in combination with spin-orbit coupling between the singlet and triplet states.