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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]

The electronic spectra of the C 3 H - and C 3 D - anions have been studied above the lowest electron detachment threshold. On the basis of the vibrational, rotational analysis and ab initio calculations, the photodetachment spectrum is assigned to the d 3 A″←a 3 A″ Feshbach resonance in the bent chain C 3 H(D) - anion. The vibronic system is characterized by a long vibrational progression involving the CCH in plane bending mode ν 4 . The potential curves along this coordinate obtained from the spectral analysis and theoretical calculations reveal the importance of vibronic coupling in the electronic excited states. A strong Renner-Teller effect is thought to be the reason for the existence of the Feshbach resonance because the 4 Σ - neutral parent and the 3 Π anion excited states are close in energy. As for the neutral, ν 4 appears to be the active mode and drives the interaction between the Feshbach and the dipole bound states. [ABSTRACT FROM AUTHOR]

The rotational structure of the origin band for the 1 A′← X 1 Σ + electronic transition, lying just below the electron affinity of C 4 H, was recorded by means of a two-colour resonant photodetachment technique. This allowed a determination of the rotational constants in the X 1 Σ + ground and 1 A′ dipole bound excited state. The low lying A 2 Π excited state of C 4 H is inferred to be the parent of the dipole bound state. The excited electronic state is deduced to have a nonlinear planar structure whereas the ground is linear according to the spectral analysis. The rotational constants have been obtained: B ″ = 0.1552(2) cm -1 for the X 1 Σ + state, and A ′ = 30.73(1), B ′ = 0.1587(2), C ′ = 0.1581(2) cm -1} for the 1 A′ state. [ABSTRACT FROM AUTHOR]

The electronic spectrum of the C[sub 3]H(D)¯ anion has been studied near the lowest electron detachment threshold associated with the chain isomer. On the basis of the vibrational and rotationalanalyses, the photodetachment spectrum is assigned to the [sup 3]A ′′ ← X[sup ~3]A ′′ electronic transition of the cumulene-like C[sub 3]H(D)¯ anion. Thespectrum is characterized by a rich vibrational structure leading to the determination of almost all fundamental frequencies. The vibrational progression involving the CCH(D) in plane bending mode (ν′[sub 6])is fitted with a 1-dimensional potential energy curve along the CCH(D) angle. This fit and the rotational analysis enable the geometry of the upper state to be determined. Although this excited state isexpected to have dipole bound character, its neutral counterpart has a significantly different structure. The observation of an excited triplet state near the electron detachment threshold and close tothe the calculated transition state for the electron catalysed isomerization reaction leads to new insight into this process. [ABSTRACT FROM AUTHOR]