Electronic absorption and resonance Raman spectra of large linear carbon clusters isolated in solid argon

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Neutral and anionic carbon clusters have been generated via a laser-induced graphite-based plasma and deposited in a solid argon matrix. Anionic clusters were formed from neutral clusters by using crossed electron/carbon cluster beams. Thermal annealing (to 36 K) resulted in the aggregation of the smaller carbon species, leading to the formation of long chain neutral and anionic clusters. Spectroscopic measurements in the ultraviolet, visible, near-infrared and infrared regions revealed a series of bands attributable to a homologous set of odd-numbered C 5–C 29 neutral clusters and even-numbered C 6 −–C 36 − anionic clusters. Good agreement is found for the band positions of carbon chains containing odd C 15–C 21 neutrals and even C 6 −–C 22 − anions, with species previously identified by Maier and coworkers using mass selection or laser vaporization, followed by neon matrix isolation. Resonance Raman frequencies for the neutral C 17, C 21 and C 23 species are shown to be consistent with the above attributions. Density functional theory calculations agree well with the observed bands. It is found that certain low frequency Raman stretching frequencies decrease in a predictable way with increasing chain length. Comparison of the 0 0 0 absorption transitions of the even C 18 −–C 36 − anionic clusters with the ‘unidentified’ infrared (UIR) interstellar emission bands suggests that the electronic emission from specific long chain carbon anions may contribute to the some of the UIR bands.