D.G. Leopold, K.K. Murray, A.E.S. Miller, W.C. Lineberger, Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH2and CD2J. Chem Phys. 83 (1985) 4849. doi:10.1063/1.449746.


Apparatus from Leopold, Murray, Miller and Lineberger,  J. Chem. Phys. 83, 4849 (1985).
Apparatus from Leopold, Murray, Miller and Lineberger, J. Chem. Phys. 83, 4849 (1985).

Photoelectron spectra are reported for the CH2(X̃ 3B1)+e←CH2 (X̃ 2B1) and CH2(ã 1A1)+e←CH2(X̃ 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 cm−1 above the zero point vibrational level of the 3B1 states, revealing a previously unexplored region of the quasilinear potential surface of triplet methylene. Approximately 20 new vibration‐rotation energy levels for CH2 and CD2 are measured to a precision of ∼30 cm−1 in the v2=2–7 region (bent molecule numbering). Bending vibrational frequencies in the methylene anions are determined to be 1230±30 cm−1 for CH and 940±30 cm−1 for CD2, and the ion equilibrium geometries are bracketed. The measured electron affinity also provides values for the bond strength and heat of formation of CH2, and the gas phase acidity of CH3. A detailed description of the new flowing afterglow photoelectron spectrometer is given.