Precise Laboratory Observation of the [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]2[/TINF] ← [TSUP]3[/TSUP][ITAL]P[/ITAL][TINF]1[/TINF] Fine-Structure Transitions of [TSUP]12[/TSUP]C and [TSUP]13[/TSUP]C

We report measurements of the 3 P 2 ← 3 P 1 fine-structure transitions of both neutral atomic carbon species, 12 C and 13 C, near 809 GHz with the Cologne terahertz spectrometer. The carbon atoms were produced in a DC glow discharge of a CO and He mixture at liquid nitrogen temperature. The 13 C atoms were generated from an isotopically enriched 13 CO sample. The 3 P 2 ← 3 P 1 transition frequencies measured in absorption are determined to be These precise laboratory data are recommended for use as rest frequencies in future astronomical observations. The new value for the rest frequency of the 12 C (J=2←1) transition removes a long-standing uncertainty and small discrepancy between existing laboratory data and astronomical measurements. The 13 C rest frequencies have supported the first interstellar detection of this species, as reported by Keene et al.