Visible absorption and magnetic-rotation spectroscopy of 1CH2: The analysis of the b̃ 1B1 state

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Abstract
The b̃1B1←ã 1A1 spectrum of CH2 radical has been recorded with Doppler-limited resolution in the 15 600–18 650 cm−1 wavelength region by laser flash-kinetic absorption spectroscopy. Singlet methylene is produced by photolysis of ketene at 308 nm. Assignments for 477 transitions originating from 1A1(0,0,0) and (0,1,0) levels with J≤8 and Ka≤5 are reported from some 10 000 lines observed. Term values are given for these 1A1 levels and 1B1(0,v2,0) levels with 13≤v2≤17. Magnetic-rotation signals were observed for 60% of the 424 lines studied. This anomalous Zeeman effect is explained by singlet–triplet mixing in both ã and b̃ states. Extensive singlet–triplet mixing occurs for 1B1 levels with Ka≠0; this mixing is allowed in second order by Renner–Teller coupling of b̃ 1B1 with ã 1A1 and by spin-orbit coupling via ã 1A1 to X̃ 3B1. The consequent shifts prevent determination of accurate rovibrational structural parameters for the 1B1 state.

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