Laser photoelectron spectroscopy of CH2−, and the singlet–triplet splitting in methylene

Abstract

Photodetachment of an electron from CH₂⁻ ions using visible light produces CH₂ in both its ground ³B₁ and lowest excited ¹A₁ states. In an earlier letter, this process was utilized to determine directly a singlet–triplet splitting of 19.5 kcal/mol. In this paper are reported the details of the early measurement and a number of additional investigations designed to test critically the earlier assignments. Data reported here indicate that one suggested problem with the early results, a misassignment of the ³B₁ origin resulting from negative ion hot bands, is not likely. These new results do reveal the presence of an additional peak in the photoelectron spectrum which could possibly be the lowest level of the ³B₁ state, allowing a singlet–triplet splitting of 19.5 or 23.2 kcal/mol. The results reported here do not indicate any other inconsistency in the original assignment. Finally, these data are discussed in light of the formidable body of theoretical and indirect thermochemical determinations suggesting a singlet–triplet splitting near 8 kcal/mol.