PRECISE LABORATORY OBSERVATION OF THE P-3(2)[-P-3(1) FINE-STRUCTURE TRANSITIONS OF C-12 AND C-13

We report measurements of the P-3(2) <-- P-3(1) fine-structure transit ions of both neutral atomic carbon species, C-12 and C-13, near 809 GH z with the Cologne terahertz spectrometer. The carbon atoms were produ ced in a DC glow discharge of a CO and He mixture at liquid nitrogen t emperature. The C-13 atoms were generated from an isotopically enriche d (CO)-C-13 sample. The P-3(2) <-- P-3(1) transition frequencies measu red in absorption are determined to be C-12: J = 2 <-- 1: 809341.97 (5 ) MHz, C-13: J = 2 <-- 1, F = 3/2 <-- 1/2: 809125.5 (4) MHz, F = 5/2 < -- 3/2: 809493.7(2) MHz, F = 3/2 <-- 3/2: 809121.3 (4) MHz (calculated ). These precise laboratory data are recommended for use as rest frequ encies in future astronomical observations. The new value for the rest frequency of the C-12 (J = 2 <-- 1) transition removes a long-standin g uncertainty and small discrepancy between existing laboratory data a nd astronomical measurements. The C-13 rest frequencies have supported the first interstellar detection of this species, as reported by Keen e et al.