Near-infrared spectroscopy of CH₂ by frequency modulated diode laser absorption

A diode laser spectrometer incorporating a multi-pass Herriott type cell and frequency modulation detection was used to record a previously unaccessed region of the near-infrared singlet←singlet absorption spectrum of methylene between 10000 cm^-1 and 10600 cm^-1. With this spectrometer, signal-to-noise ratios close to the quantum noise limit have been attained. Identification of rovibronic transitions to five previously unobserved levels, K=1 ã(0,9,0), K=2b̃(0,1,0), K=2ã(1,6,0), K=3b̃(0,1,0) and K=3ã(0,10,0), was made. Despite the fact that the present spectra access levels within approximately 1300 cm^-1 of the barrier to linearity, the spectrum is dense and perturbed, characteristics in common with spectra recorded in many previous studies at shorter wavelengths. Recent spectroscopic observations of halomethylenes [J. Mol. Spectrosc. 188, 68 (1998)] had suggested that the CH₂ spectrum might become simpler at longer wavelengths, but this was not evident in the observed spectra. The mixed nature of the singlet states is evidenced by the assignment of rovibronic transitions to levels containing primarily ã¹A₁ state character. The new measurements provide a stringent test for modern theoretical models for CH₂ and will enable refinement of the electronic potential surfaces.