Collisions of hydrogen cluster ions with a gas target at 250–900 eV energy

Abstract

Results of attenuation measurements of hydrogen cluster ion beams with N 2 as scattering partner are presented. The mass of the cluster ion is varied from H 2 + to H 27 +. Except for H 2 +, only the odd masses had sufficient intensity to be detected. The measured H 2 + ion current amounts to 10 −13 A at an ion energy of 890 eV. For the smallest odd cluster ions, e.g. H 5 +, the ion current is a factor of 100 lower and decreases slowly with increasing size of the ion. The measurements have been performed at three laboratory ion energies, 250, 510 and 890 eV, with and without mass analysis of the attenuated beam. The attenuation of the H 2 + signal is always substantially larger than the attenuation of the H 3 + signal, confirming a larger bonding distance at H 2 + compared to H 3 + [1]. A model for the cluster ions is suggested as a result of the measurements obtained with mass analysis of the attenuated beam. The parameter of this model is the density n 2 of H 2 molecules in the cluster ion, which is found to be n 2 = (0.025 ± 0.005) Å −3. Comparing this value with n = 0.02 Å −3, which is valid for the density of H 2 molecules in liquid hydrogen, a small contraction of the cluster ion, due to the charge in its centre, is found. Assuming the cluster ion to be scattered by a potential of the form V( r) = − C s / r s, s rises from s = 4 for H 3 + to s = 11 for cluster ions larger than H 9 +. Thus the “soft” H 3 + core is increasingly screened by H 2 molecules. Differences between measured attenuations with and without mass analysis of the attenuated beam are caused by the fragmentation of cluster ions upon collision. These fragments are mainly scattered in the forward direction and are detected if no mass analysis of the attenuated beam is applied. It is demonstrated that whether or not a cluster ion disintegrates into fragments upon collision depends sensitively on its laboratory energy and its mass. This behaviour can be understood in terms of a model.