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

Neutral and anionic carbon clusters have been generated via a laser-induced graphite-based plasma and deposited in a solid argon matrix. Anionic clust ers were formed from neutral clusters by using crossed electron/carbon clus ter beams. Thermal annealing (to 36 K) resulted in the aggregation of the s maller carbon species, leading to the formation of long chain neutral and a nionic clusters. Spectroscopic measurements in the ultraviolet, visible, ne ar-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-number ed C-6(-)-C-36(-) anionic clusters. Good agreement is found for the band po sitions 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 u sing mass selection or laser vaporization, followed by neon matrix isolatio n. 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 ce rtain low frequency Raman stretching frequencies decrease in a predictable way with increasing chain length. Comparison of the 0(0)(0) absorption tran sitions of the even C-6(-)-C-6(-) anionic clusters with the 'unidentified' infrared (UIR) interstellar emission bands suggests that the electronic emi ssion from specific long chain carbon anions may contribute to the some of the UIR bands. (C) 2001 Elsevier Science B.V. All rights reserved.