Collisions of hydrogen cluster ions with a gas target, at 200–850 eV energy

Abstract

Results of attenuation and fragmentation measurements of hydrogen cluster ion beams are presented with He as scattering partner. The mass of the cluster ions is varied from H 2 + to H 41 +. Except for H 2 +, only the odd masses had sufficient intensity to be detected. The ion energy ranges from 200–850 eV. Only results of attenuation measurements obtained with mass analysis of the attenuated beam are presented. Contrary to the N 2 results [1], it is found that for the H 2 + signal the attenuation is smaller than for the H 3 + signal. The attenuations of the cluster ion signals show a linear behaviour as a function of the mass of the ions up to H 41 +. Thus, within the experimental accuracy, little screening [1] occurs between the H 2 molecules in the cluster ions. Assuming that the ions are scattered by a potential of the form V( r) = - C s / r s, s rises from s = 5 for H 2 + and H 3 + to s = 7 for ions larger than H 17 +, demonstrating the influence of the H 2 molecules upon the attenuation. Head-on or nearly head-on collisions between particles in the cluster ion with a helium atom cause fragmentation of the ion. All possible fragment ions larger or equal than H 3 + occur with a variation in intensity of about a factor 2 × 10 3 in the mass range investigated. The acceptance angle of our apparatus is ca. 10 -4 sterad. The velocity of the fragment ions approximately equals the velocity of the original cluster ion. Effects of the binding energy of the H 2 molecules to the cluster ion upon the fragment velocity could not be resolved. It appears that fragmentation towards H 9 +, H 15 + and H 27 + has a larger cross-section than to their respective neighbours. We infer that these cluster ions are particularly stable.