CH2 b̃1B1−ã1A1 Band Origin at 1.20 μm

The origin band in the b̃1B1−ã1A1 transition of CH2 near 1.2 μm has been recorded at Doppler-limited resolution using diode laser transient absorption spectroscopy. The assignments of rotational transitions terminating in upper state levels with Ka = 0 and 1, were confirmed by ground state combination differences and extensive optical−optical double resonance experiments. The assigned lines are embedded in a surprisingly dense spectral region, which includes a strong hot band, b̃(0,1,0) Ka = 0  ã(0,1,0) Ka = 1 sub-band lines, with combination or overtone transitions in the ã1A1 state likely responsible for the majority of unassigned transitions in this region. From measured line intensities and an estimate of the concentration of CH2 in the sample, we find the transition moment square for the 000 ← 110 transition in the b̃1B1(0,0,0)0−ã1A1(0,0,0)1 sub-band is 0.005(1) D2. Prominent b̃1B1(0,1,0)0−ã1A1(0,1,0)1 hot band lines were observed in the same spectral region. Comparison of the intensities of corresponding rotational transitions in the two bands suggests the hot band has an intrinsic strength approximately 28 times larger than the origin band. Perturbations of the excited state Ka = 0 and 1 levels are observed and discussed. The new measurements will lead to improved future theoretical modeling and calculations of the Renner−Teller effect between the ã and b̃ states in CH2.