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The electronic spectra of the C[sub 2n]H(D)[sup -], n = 2-4, anions have been observed in the gas phase using photodetachment spectroscopy. These are assigned to ¹Π ← X¹Σ[sup +] electronic transitions. The ¹Π 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 X²Π ground states of C[sub 6]H and C[sub 8]H whereas for C[sub 4]H, which has an X²Σ[sup +] ground state, the ²Π 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. [ABSTRACT FROM AUTHOR]

The origin band in the electronic transition to the dipole bound excited state of C5H(D)¯ anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X3∑¯ and excited A3∏ electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C5H¯: T00 = 19248.0(1), B′ = 0.0835(1), B″ = 0.0826(2), A′SO = -11.96(1), λSS′ = 1.97(8)λSS″ =0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C5H¯ is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C5H¯. [ABSTRACT FROM AUTHOR]

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 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 A [sup 2]Δ[sub u]←X [sup 2]Π[sub g], B [sup 2]Σ[sub u][sup -]←X [sup 2]Π[sub g], and C [sup 2]Σ[sub u][sup +]←X [sup 2]Π[sub g] electronic transitions of C[sub 3][sup -] were observed in a neon matrix and in the gas phase, although the energy of the excited electronic states involved in these transitions is 1-1.5 eV above the photodetachment threshold. The excited Feshbach states are sufficiently long-lived that some of the bands in the gas-phase photodetachment spectrum exhibit rotational structure. Assignment of the transitions is made on the basis of rotational analysis or profile simulations and theoretical calculations. The b [sup 4]Π[sub u]←X [sup 2]Π[sub g] transition is also weakly observed. The presence of such discrete bands, though in the continuum, provides a means of detection for anions in the interstellar medium. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]