Absolute photodetachment cross sections of two anions of astrophysical importance CN ^ { - } and C _ { 3 } N ^ { - } were measured to be ( 1.18 \pm ( 0.03 ) _ { stat } ( 0.17 ) _ { sys } ) \times 10 ^ { -17 } cm ^ { 2 } and ( 1.43 \pm ( 0.14 ) _ { stat } ( 0.37 ) _ { sys } ) \times 10 ^ { -17 } cm ^ { 2 } respectively at the ultraviolet wavelength of 266 nm ( 4.66 eV ) .
These relatively large values of the cross sections imply that photodetachment can play a major role in the destruction mechanisms of these anions particularly in photon-dominated regions .
We have therefore carried out model calculations using the newly measured cross sections to investigate the abundance of these molecular anions in the cirumstellar envelope of the carbon-rich star IRC+10216 .
The model predicts the relative importance of the various mechanisms of formation and destruction of these species in different regions of the envelope .
UV photodetachment was found to be the major destruction mechanism for both CN ^ { - } and C _ { 3 } N ^ { - } anions in those regions of the envelope , where they occur in peak abundance .
It was also found that photodetachment plays a crucial role in the degradation of these anions throughout the circumstellar envelope .