First observation of autodetachment lifetimes of methide, CH3−

Abstract

The methide ion, CH3−, has been studied in a mass-selected ion beam. Two autodetachment lifetimes were observed that differ by three orders of magnitude. Infrared laser excitation near 3 micrometers reveals a vibrationally excited state with an autodetachment lifetime of 9–12 ns. This is the first resonant vibrational–rotational transition observed in methide. In addition, metastable ions were observed with a much longer autodetachment lifetime. For simplicity in the data analysis, all metastable ions are assumed to have a single autodetachment lifetime. This is an oversimplification, because the metastable ions decay by rotational autodetachment, and the metastable lifetime will therefore vary with rotational quantum number. The data analysis yields two possible values for the autodetachment lifetime: either 1.59±0.35 μs or 0.42±0.28 ms. The longer lifetime is much more plausible. The metastable states are attributed to rotationally excited states, which can only decay by channels with ΔJ⩾5. This interpretation is supported by modeling the energy manifold and populations of ions and neutrals. In contrast, the nanosecond lifetime arises from a vibrationally excited state. There are presently no theoretical calculations of the autodetachment mechanism or lifetimes for the methide ion.