Spectroscopy of methylene: Einstein coefficients for CH2(b̃ 1B1–ã 1A1) transitions

Abstract

The b̃ 1B1→ã 1A1 emission spectrum of the CH2 radical has been studied in the 13 330–18 180 cm−1 region. Methylene (ã 1A1) is produced by photolysis of ketene at 308 nm and excited to single rovibrational levels of the b̃ 1B1 state by a tunable pulsed dye laser. The emission from the b̃ 1B1 state is spectrally dispersed with a resolution of 0.25 Å. Term values and transitions are reported for new levels of the b̃ 1B1(0,v2,0) (v2=14 and 15) and ã 1A1(0,0,0) states with J≥6 and 0≤Ka≤6. Unusually intense ΔKa=3 transitions and several ΔKa=5 transitions are observed. Analysis of the emission spectra gives the relative intensities of transitions from a selected rovibrational level in the upper b̃ 1B1 state to rovibrational levels in the ã 1A1 state. These relative intensities together with the fluorescence lifetimes of the b̃ 1B1 levels determine the absolute Einstein emission coefficients for rovibronic transitions in the visible spectrum of methylene. These data permit accurate measurement of rovibrational state populations for CH2(ã 1A1) by laser-induced fluorescence spectroscopy.