We observed the upper fine structure line of neutral carbon , C I ( ^ { 3 } P _ { 2 } \to ^ { 3 } P _ { 1 } ) 
( \nu _ { rest } = 809 GHz ) , the ^ { 12 } CO ( J =3 \to 2 ) line ( \nu _ { rest } = 345 GHz ) and the 1.2 mm continuum emission from H1413+117 ( Cloverleaf quasar , z = 2.5 ) using the IRAM interferometer .
Together with the detection of the lower fine structure line ( Barvainis et al .
1997 ) , the Cloverleaf quasar is now only the second extragalactic system , besides M 82 , where both carbon lines have convincingly been detected .
Our analysis shows that the carbon lines are optically thin and have an excitation temperature of T _ { ex } \approx 30 K. CO is subthermally excited and the observed line luminosity ratios are consistent with n ( H _ { 2 } ) \approx 10 ^ { 3 - 4 } { cm } ^ { -3 } at T _ { kin } = 30–50 K. Using three independent methods ( C I , dust , CO ) we derive a total molecular gas mass ( corrected for magnification ) of M ( { { H } _ { 2 } } ) \approx 1.2 \pm 0.3 \times 10 ^ { 10 } { { M } _ { \odot } } .
Our observations suggest that the molecular disk extends beyond the region seen in CO ( 7–6 ) to a zone of more moderately excited molecular gas that dominates the global emission in C I and the low J CO lines .