Rotationally Resolved Near-Infrared Spectrum of the HCCI ùA″ ← X̃¹A′ Transition

Near-infrared spectra of the HCCl ùA″(0,1,0) ← X̃¹A′(0,0,0) and ùA″(0,0,1) ← X̃¹A′(0,0,0) transitions between 13 100 and 13 330 cm^-1 were obtained at Doppler-limited resolution using a combination of frequency-modulation (FM) and transient absorption techniques. For the Ã(0,1,0) ← X̃(0,0,0) vibronic band, we have rotationally resolved and assigned five subbands (K_a = 0 ← 1, 1 ← 0, 1 ← 2, 2 ← 1, and 2 ← 3) of HC³⁵Cl and the K_a = 0 ← 1 subband of HC³⁷Cl. The rotational structure of the Ã(0,1,0) level shows strong perturbations. Both random perturbations and anomalous K-type doublings were observed. A detailed analysis qualitatively reveals the roles of the Renner-Teller effect and spin-orbit interaction in perturbation mechanisms. In addition, the Ã(0,0,1) ← X̃(0,0,0) transition was observed for the first time. The determined C-Cl stretching frequency in the à state indicates an increase of 20% on electronic excitation and is consistent with the determined C-Cl bond shortening.