  Absolute photodetachment cross sections of two anions of astrophysical
importance CN- and C3N- were measured to be (1.18 +- (0.03)_stat (0.17)_sys) *
10^-17 cm^2 and (1.43 +- (0.14)_stat (0.37)_sys) * 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 C3N- 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.

Solar and Stellar Astrophysics;Chemical Physics