We report the first detection of the submm emission from the ^ { 13 } C isotope of atomic carbon in the ISM .
The F = \text@frac { 5 } { 2 } - \text@frac { 3 } { 2 } component of the { { } ^ { 3 } P _ { 2 } } - { { } ^ { 3 } P _ { 1 } } transition was observed with the CSO in a region \sim 4 ′ S of Orion IRc2 , near the western end of the Orion Bar .
The ^ { 12 } C to ^ { 13 } C isotopic abundance ratio is 58 \pm 12 corrected for opacity of the ^ { 12 } C i line and the fractional intensity of the ^ { 13 } C i hyperfine component ( 60 % ) .
This is in agreement with the value for the equivalent ratio in C ^ { + } .
In comparison , our measurement of the C ^ { 18 } O to ^ { 13 } C ^ { 18 } O ratio from observations of 2–1 and 3–2 lines toward the same position gives a value of 75 \pm 9 .
PDR models predict that the ^ { 12 } C to ^ { 13 } C abundance ratio is particularly sensitive to chemical fractionation effects .
If ^ { 13 } C ^ { + } is preferentially incorporated into ^ { 13 } CO at cloud edges there will be a dramatic reduction in the abundance of ^ { 13 } C. This is contrary to our observations , implying that the importance of chemical fractionation is small or is compensated for by isotopic-selective photo-dissociation of ^ { 13 } CO in this region with a large UV illumination .