Direct observation of the ionization threshold of triplet methylene by photoionization mass spectrometry

Abstract

The photoionization spectrum of the ionization threshold region of methylene has been recorded for the first time. The CH2 radical was produced in situ by successive hydrogen abstractions from methane precursor. The observed steplike onset corresponds to the vibrationless transition CH2+X̃ 2A1←CH2X̃ 3B1 and leads to the adiabatic ionization energy of CH2 of 10.393±0.011 eV. This value is slightly higher than the nominal midrise of the threshold step structure, which is depressed by rotational autoionization effects. In a separate set of experiments, the threshold region of the CH2+ fragment from CH3 was recorded at room temperature. The fragment appearance energy was accurately determined by fitting to be 15.120±0.006 eV at 0 K. The combination of these two measurements provides the best current experimental value for the bond dissociation energy of the methyl radical, D0(H–CH2)=4.727±0.012 eV=109.0±0.3 kcal/mol (corresponding to 110.4±0.3 kcal/mol at 298 K), and yields ΔHf0⊖(CH2,X̃ 3B1)=93.2±0.3 kcal/mol (93.3±0.3 kcal/mol at 298 K) and ΔHf0⊖(CH2,ã 1A2)=102.2±0.3 kcal/mol (102.3±0.3 kcal/mol at 298 K). The latter makes the reaction CH2 (ã 1A2)+H2O→CH3+OH essentially thermoneutral, ΔHr0⊖=0.0±0.3 kcal/mol.