Sudden approximation cross sections for ionisation of H atoms by energetic C6+ and He2+ impact

Cross sections for the collisional ionisation of atomic hydrogen by energetic C6+ and He2+ ions have been calculated in the sudden approximation (simplified form of the first Magnus approximation). An impact parameter approach was used in which the projectile was assumed to follow a classical rectilinear trajectory. The problem associated with the anomalous large impact parameter behaviour of the ionisation probabilities in this approximation was dealt with by merging the sudden approximation calculations at small impact parameters with the Bang and Hansteen (K. Danske Vidensk. Selsk., Mat. -Fys. Meddr.; 31: 13 (1959)) semiclassical approximation results at larger impact parameters where perturbation theory becomes applicable. The cross sections obtained in this manner are in reasonable agreement with classical trajectory calculations and available Glauber theory estimates for collision velocities above about 3 au. (author)