Photodetachment spectroscopy of the C2nH− (n=2–4) anions in the vicinity of their electron detachment threshold

The electronic spectra of the C2nH(D)−, n=2–4, anions have been observed in the gas phase using photodetachment spectroscopy. These are assigned to Π1←X 1Σ+ electronic transitions. The Π1 excited states possess a dipole bound character indicated by the energetic proximity between the origin of the transitions and electron affinities. The dipole bound states are related to the X 2Π ground states of C6H and C8H whereas for C4H, which has an X 2Σ+ ground state, the Π2 is an excited state. Vibronic coupling through a bending motion of the carbon skeleton is inferred to be the reason of the stabilization of this state.