D.G. Leopold, K.K. Murray, W.C. Lineberger, Laser photoelectron spectroscopy of vibrationally relaxed CH2−: A reinvestigation of the singlet–triplet splitting in methylene, J. Chem Phys. 81 (1984) 1048–1050. doi:10.1063/1.447741.

CH2- photoelectron spectra following 2.54 eV (488 nm) excitation.
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 kcal/mol.

In view of the many questions concerning the photoelectron spectrum of CH2− we have reinvestigated this system using a new experimental apparatus which incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Results presented here enable several previously observed spectral features to be positively identified as vibrational hot bands, leading to a revised determination of the singlet-triplet splitting in methylene.

Wein filter magnet power supply
Negative ion photoelectron spectrometer in 2009; Wein filter magnet power supply
Negative ion photoelectron spectrometer in 2009
Negative ion photoelectron spectrometer in 2009