Laser photoelectron spectroscopy of vibrationally relaxed CH−2: A reinvestigation of the singlet–triplet splitting in methylene

The photoelectron spectrum of CH−2 has been reinvestigated using a new apparatus that incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Earlier photoelectron studies [J. Chem. Phys. 74, 5460 (1981)] employing gas discharge and sputter ion sources were plagued by hot CH−2 ions whose vibrational population distribution, for reasons not yet explicable, could not be detectably altered by modification of source conditions. In contrast, the spectrum of cooled CH−2 displays a markedly changed vibronic band intensity profile for the CH−2 (2B1) →−e− CH2(3B1) transition. These results enable several previously observed spectral features to be positively identified as vibrational hot bands. The new CH−2 photoelectron spectrum is consistent only with the methylene singlet–triplet splitting of approximately 9 kcal/mol determined by numerous recent experimental and theoretical studies.