  Recent ab initio calculations [Mattausch et al., Phys. Rev. B 70, 235211
(2004)] of carbon clusters in SiC reveal a possible connection between the
tricarbon antisite (C_3)_Si and the U photoluminescence center in 6H-SiC [Evans
et al., Phys. Rev. B 66, 35204 (2002)]. Yet, some of the predicted vibrational
modes were not observed experimentally. Here we report experiments which indeed
confirm the existence of a low-energy mode for the U-center (as well as for the
HT3- and HT4-centers with spectral details similar to the U-center). We
calculated the isotope splitting for the (C_3)_Si-defect and found near-perfect
agreement with our data. In addition, we discuss the carbon di-interstitial
(C_2)_Hex as a model for the Z- and HT5-centers. The isotope splitting is also
well reproduced, but the absolute values of the local mode energies show a
discrepancy of about 10 meV.
