Vibrational Spectroscopy of Small Matrix-Isolated Linear Carbon Cluster Anions

Carbon cluster anions have been generated, deposited in an argon matrix, and studied spectroscopically. A new method involving a dual laser beam-induced Ar plasma using Y or W metal and graphite targets is introduced for the efficient production of matrix-isolated carbon cluster anions. New bands found in the infrared spectra of the deposited plasma mixture have been assigned to asymmetric stretching modes of the C 3, C 5, C 6, C 7, and C 9 anions. Density functional theory calculations (B3LYP/6-31G* level) support the attribution of these bands to linear carbon cluster anions. Calculational results also indicate that reactions between linear anionic clusters and linear neutral clusters are highly exothermic and that aggregation between anions and even-membered neutrals are more highly exothermic than between anions and odd-membered neutrals. The anion infrared bands are found to be photosensitive at irradiation wavelengths shorter than a certain threshold value. These values are ca. 0.5 eV higher than the corresponding vertical detachment energies of the vapor phase C n - species. The possibility that longer chain carbon anions are present in the carbon-rich circumstellar envelopes of carbon stars is discussed.