Electronic Spectra of the Carbon Chain Anions C2 n - 1H- (n = 5−8) in the Gas Phase

The electronic absorption spectra of the monohydrogenated odd carbon chain anions C2 n - 1H- (n = 5−8) have been measured in the gas phase using resonant two-color electron detachment spectroscopy. As for the even series C2 n H- (n = 5−10) electronic transitions of two isomeric species are observed. Ab initio calculations on C9H- support the assignment of one band system to a 3Σ- ← X3Σ- transition of a linear molecule and the other to a 1A‘ ← XA‘ transition of a bent isomer. The wavelength of the band systems shows an approximately linear dependence on the chain length. This plot is offset by 60 nm compared to the one for the linear C2 n H- anions, while the wavelengths for the even and odd carbon series of the cumulene-like isomer fall on the same line. These trends can be rationalized in terms of their electronic structure.