UV-visible and infrared spectroscopy of carbon cluster molecules in solid argon.

Abstract

The UV-visible absorption spectrum of carbon vapor trapped in solid argon at low temperature contains many intense features, but only those of C₂ and C₃ have been identified. For this work, graphite vapor was produced under high vacuum conditions and condensed with argon onto a cold (∼10K) substrate. The resulting matrix-isolated carbon molecules were analyzed with both UV-visible and infrared absorption spectroscopy. Slight warming of the sample allowed formation of larger molecules and subsequent spectra traced the growth of their absorption features. The experiment associated infrared features to particular UV-visible features via their growth curves. The most reliable correlations are listed below. Theoretical calculations of equilibrium geometries and IR vibrational frequencies were performed on linear and nearly linear carbon chain molecules from C₃ to C₉ at the HF/SCF level of theory and from C₃ to C₇ at the MP2 level of theory, both using the 6-31G* basis set. Tentative assignment of the UV-visible features to molecules was made based on these considerations and on the following: the experimental IR/UV-visible correlations, previous experimental IR work, and modeling of the growth of the UV-visible features during matrix annealing. (UNFORMATTED TABLE FOLLOWS:) Molecule, UV-visible Feature (Å), IR Feature (cm⁻¹), Correlation Coefficient. C₃ (linear), 4100, 2040, .98. C₈ (linear), 3075, 1998, .97. C₈ (cyclic), 3900, 1804, .90. C₆ (linear), 2465, 1952, .86. C₇, 3480. C₉, 4480.