Previous reports of threshold resonances occurring in the photodetachment spectra of molecular anions have provided detailed information about the nature of dipole‐supported states and the dynamics of autodetachment from the vibrationless level. In this paper we report the first observation and analysis of rotational band structure in an excited vibrational level of a dipole‐supported state. The 1 cm1 resolution laser photodetachment spectrum of cyanomethyl anion (CH2CN), the conjugate base of acetonitrile, was recorded in the 12 500–13 700 cm1 region using ion cyclotron resonance spectrometry. Rotational assignment of the resonances occurring in this region provides evidence for vibrational‐to‐electronic coupling in the autodetachment process.

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