Electronic transition of C3H- in the vicinity of the lowest photodetachment threshold

The electronic spectrum of the C3H(D)(-) anion has been studied near the lo west electron detachment threshold associated with the chain isomer. On the basis of the vibrational and rotational analyses, the photodetachment spec trum is assigned to the (3)A " <--(X) over tilde (3)A " electronic transiti on of the cumulene-like C3H( D) anion. The spectrum is characterized by a r ich vibrational structure leading to the determination of almost all fundam ental frequencies. The vibrational progression involving the CCH (D) in pla ne bending mode (nu'(6))is fitted with a 1-dimensional potential energy cur ve along the CCH( D) angle. This rt and the rotational analysis enable the geometry of the upper state to be determined. Although this excited state i s expected to have dipole bound character, its neutral counterpart has a si gnificantly different structure. The observation of an excited triplet stat e near the electron detachment threshold and close to the the calculated tr ansition state for the electron catalysed isomerization reaction leads to n ew insight into this process.