Radiative decay lifetimes of CH−2

Recently the presence and radiative decay of vibrationally excited CH−2, generated in a hot cathode discharge of methane, was established by measuring the time dependent photodetachment from excited states of CH−2 as it radiatively relaxed in a high vacuum ion trap. The time dependence of the photodetachment was found to be consistent with an electron affinity of 5250 cm−1 (0.65 eV) for ground state X̃ 3B1 methylene. The radiative decay lifetimes of the first three excited bending vibrations of CH−2 were also tentatively assigned. Here, we report a more refined analysis of the experimental data along with theoretical ab initio determinations of the radiative decay lifetimes of the first four excited bending vibrational levels of CH−2. There is some discrepancy between the ab initio values (431, 207, 118, and 68 ms for the v2=1, 2, 3, and 4 levels respectively) and the experimental values (525, 70, and 14 ms for v2=1, 2, and 3 respectively) for v2=2 and 3. Possible reasons for this discrepancy are discussed but none of the alternatives are entirely satisfactory.