Electronic transitions of the C5H- anion.

Electronic transitions of the triplet chain C5H- anion were studied using detachment spectroscopy. The system detected in the vicinity of the electron detachment threshold is assigned to the b3A ← a3A transition with a dipole bound state (DBS) character. The second system measured by autodetachment spectroscopy is attributed to the c3A ← a3A Feshbach electronic transition. Negative anharmonicity of the vibrational progression built upon the low frequency CCC in-plane bending mode ν12 is observed in both DBS and Feshbach states. This indicates a barrier to linearity on the potential energy surface in both excited states. The triplet chain C5H- anion exhibits similar electronic properties to C3H-. Renner-Teller and vibronic effects between the 3A DBS and Feshbach states are inferred and may explain the stabilisation of the DBS. This interaction is weaker in C5H- than C3H- leading to smaller barrier heights on the potential energy surface. [ABSTRACT FROM AUTHOR]