Fast ion-molecule reactions in planetary atmospheres: a semiempirical capture approach

The description of planetary and interstellar chemistry relies strongly on ion-molecule reaction rate data collected at room temperature or above. However, the temperature in the ionospheres of planets and in the interstellar medium can decrease down to 100 K and 10 K, respectively. We present here a simple semiempirical method to extend available measurements towards those temperatures. Our approach is based on the long-range capture theory combined with room temperature data. Results are presented for cation-molecule and anion-molecule reactions. An overall good agreement is observed between our model and various experimental data in the temperature range 20-295 K. Deviations larger than a factor of 2 are found, however, with ion trap measurements below similar to 50 K. Predictions are also made for reactions of carbon chain and hydrocarbon ions with atomic hydrogen, of particular importance in Titan's atmosphere and in interstellar clouds.