Spectroscopy of Carbon Vapor Condensed in Rare-Gas Matrices at 4°K. III

Abstract

For C₃, controlled warming of neon matrices containing carbon vapor caused the multiple sites for C₃ molecules to disappear. The assignments of vibronic transitions in the ¹II_u←X¹Σ_g⁺ system could then be made more definite using the parameters of Gausset, Herzberg, Lagerqvist, and Rosen. Two unaccounted‐for bands were attributed to transitions involving v₃′≅840 cm⁻¹. This low value was rationalized by comparison with other similar molecules. The ¹Σ_u⁺←X¹Σ_g⁺ transition of C₃ in a neon matrix was not observed in the ultraviolet out to 1950 Å. For C₂, the spectrum of carbon—neon matrices, before and after warming to 12°K, indicated that the absorption bands in the region of 4000–6000 Å are probably not the C₂ Swan system but may be due to a larger carbon molecule. A band at 2320 Å in neon was attributed to the (0, 0) band of the Mulliken system (¹Σ_u⁺←x¹Σ_g⁺ of C₂. These and other observations indicate that the trapping of C₂ in the excited ³II_u state in matrices is doubtful. CNN is formed in neon matrices containing nitrogen by the reaction of C atoms in carbon vapor with N₂. The proportion of C atoms in the vapor and the formation of CNN can be enhanced by vaporization of carbon from thin tungsten or tantalum cells because of the formation and vaporization of the solid carbide. For C_n, carefully controlled diffusion of carbon molecules relatively concentrated in a neon matrix led to the appearance of many new vibrational bands. These were analyzed in terms of two extreme linear chain models. It is concluded, for example, that C₄ is probably best written as ·C≡C–C≡C·, in agreement with theory.