Ionisation of fast Rydberg ions in collision with target atoms

Abstract

Rydberg ions with orbital dimensions up to approximately 1000 a,, are found to traverse thin gas targets fairly undisturbed, indicating relatively small ionisation cross sections ui. Theoretical estimates which take into account screening of target nuclei reveal that q(n) becomes asymptotically independent of principal quantum number n. In recent years a number of reports have dealt with Rydberg ions which travel through gaseous targets and residual gas in beam lines prior to being observed by field ionisation or radiative decays (Braithwaite et a1 1975, Kim and Meyer 1980, Betz et a1 1980, 1983). It is well understood that the long lifetime of Rydberg states, 7,~ a n312, where n and 1 denote principal and angular quantum number, respectively, allows detection long after creation of the state, but no explanation was given for the fact that these ions could be observed and were not destroyed in collisions with atoms along the flight path. For obvious reasons, a steep rise of electron-loss cross section, cri(n) a n2, is expected only for ionisation by charged particles. We estimate ionisation by neutral atoms and find that ui(n) deviates significantly from a n2 dependence even for the lowest n values and becomes asymptotically independent of n. The latter result is in agreement with expectations from Butler and May (1965) and estimates by Matsuzawa (1980). 125 MeV sulphur ions from the Munich Tandem van de Graaff accelerator were prestripped to obtain bare nuclei (charge 16') and directed through a gas cell of length 3 cm containing target gases (N2, CH4) at pressures up to approximately 1 Torr. Two cryogenic pumping systems aided by two turbomolecular pumps served to achieve high vacuum (about Torr) behind the gas target. At a distance of 10 cm behind the centre of the gas target a Si(Li) detector observed K x-rays due to decays of Rydberg states formed by electron capture in the gas target. Energy resolution was sufficient to resolve hydrogen-like transitions. Separate experiments with other lengths of the gas cell and variation of target-detector distance showed that the observed x-ray decays contain negligible contributions due to states formed by collisions along the flight path between gas target and the end of the detection region. Beam intensity was monitored by means of a Faraday cup. Further experimental details and dis- cussions of the technique to observe Rydberg states via radiative decay cascades are