Infrared spectrum and autodetachment dynamics of NH−

Abstract

The infrared vibration‐rotation spectrum of NH⁻ has been obtained by autodetachment spectroscopy in a coaxial laser–ion beam spectrometer. Transitions from the v=0 to v=1 vibrational levels were excited with an F‐center laser, and subsequent autodetachment from the v=1 levels was observed. The apparatus resolution was better than 20 MHz, allowing the resolution of the fine structure and Λ‐doubling transitions. The linewidths of the autodetachment resonances revealed some of the dynamics of the autodetachment process. The autodetachment rates were, in general, much greater for the upper Λ‐doublet levels of NH⁻(v=1) than for the lower levels. In addition, the increase of the autodetachment rate with rotational energy for the upper levels was much faster than would be predicted if vibrational autodetachment were the primary detachment mechanism. It therefore appears that rotational‐electronic coupling plays an important role in this system, and the differences in the Λ‐doublet autodetachment rates are explained in terms of this mechanism.