Weakly bound anionic systems present a new domain for negative ion spectroscopy. Here we report on a multifaceted study of the CHCN dipole-bound state, employing high-resolution photoelectron spectroscopy from 130 different wavelengths, velocity-map imaging at threshold, and laser scanning photodetachment experiments. This uncovers a wide variety of different vibrational and rotational autodetaching resonances. By examination of both sides of the problem, absorption from the anion to the dipole-bound state and vibrational/rotational autodetachment to the neutral, a complete model of the dipole-bound chemistry is formed. Precise values for the electron affinity EA = 12468.9(1) cm, dipole binding energy = 40.2(3) cm, and anion inversion splitting ω = 115.9(2) cm are obtained. This model is then employed to study possible astronomical implications, revealing good agreement between the = 1 ← 0 CHCN dipole transition and the λ8040 diffuse interstellar band.
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