The Abundance of C_7^- in Diffuse Clouds

It has recently been proposed that C_7^- is a carrier of five of the numerous diffuse interstellar bands. In this paper, we report an attempt to reproduce the required abundance of C_7^- relative to hydrogen of n(C_7^-) ~ 1 × 10^-9n_H through the use of various models for the chemistry in diffuse clouds. Our model network contains large neutral and charged linear hydrocarbons and incorporates the time-dependent desorption of seed molecules from grain surfaces. We find a limited set of physical and chemical conditions that result in the required fractional abundance of C_7^- but only for rather short periods of time