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Carbon molecules were produced by evaporation of graphite and matrix-isolated in solid neon and argon. Using synchrotron radiation. the absorption spectra of the carbon clusters were recorded from 1100 to 5600 Angstrom, and the fluorescence or phosphorescence spectra were recorded from 1200 to 9000 Angstrom. We observed an intense, broad absorption band system centered at around 1600 Angstrom in neon and 1700 Angstrom in argon. By measuring the excitation spectrum of the (3)Pi(u) --> X (1)Sigma(g)(+) phosphorescence and checking the intensity correlation with the known (1)Pi(u) <-- X (1)Sigma(g)(+) absorption band, we could show that the observed VUV band system is the allowed (1)Sigma(u)(+) <-- X (1)Sigma(g)(+) electronic transition of the C Molecule. At the blue and the red side of the band system, distinct progressions can be observed which most likely correspond to a symmetric stretch of about nu(1) approximate to 1100 cm(-1) and a bending mode of about nu(3) approximate to 550 cm(-1), respectively. In the band center, however, a complicated superposition of several vibrational progressions appears indicating that besides the (1)Sigma(u)(+) state also other states seem to contribute to the absorption. Quantum chemical MR-AQCC calculations suggest that these contributing states are (1)Pi(g) states which are close in energy to the (1)Sigma(u)(+) state and can interact via vibronic coupling, a conjecture supported by preliminary calculations of synthetic spectra in which such coupling was included. Furthermore, the calculations show that the (1)Sigma(u)(+) energy decreases upon bending, leading to a complex landscape of energy surfaces which include avoided crossing type features, and rendering more detailed spectral calculations difficult.
