CH2- photoelectron spectra following 2.54 eV (488 nm) excitation. (a) Previously reported spectrum (Zittel 1976, Engelking 1981) obtained with a gas discharge ion source. (b) Spectrum of vibrationally and rotationally cooled CH2- prepared in a flowing afterglow ion source. The instrumental resolution is 60 meV in (a) and 10 meV in (b). The absence of peaks A, B, and C in (b) establishes their hot band nature and gives II singlet-triplet splitting of 9 kcaVmol.