Photodetachment spectroscopy of cold trapped molecular anions near threshold

Negatively charged molecular ions have drawn a lot of attention in recent years, in particular owing to the detection of several molecular anions in cold interstellar molecular clouds. Cryogenic radiofrequency ion traps are well suited tools to study bound-bound spectroscopy and photofragmentation processes with high resolution and high sensitivity. We have studied the photodetachment of the amide anion NH$_2^-$ and the interstellar anions CN$^-$ and C$_3$N$^-$ near threshold and have compared the spectra to the threshold behavior given by the Wigner threshold laws. These data reveal information about the role of the permanent electric dipole moment of the neutral product, which is important for the description of anion formation by electron attachment. For the NH$_2^-$ anion, the spectra have allowed us to probe the lowest two rotational transitions in this ion near 447 GHz and 934 GHz and the contribution of the different hyperfine states.