Deuteration of positive hydrogen cluster ions H5+ to H17+ at 10 K Original Research Article

The dynamics of the sequential deuteration of hydrogen cluster cations Hn+, n odd, is studied in a 22-pole radio frequency ion trap at a nominal temperature of 10 K. Rate coefficients for the step by step replacement of hydrogen by deuterium in initially mass selected clusters, Hn+, for n = 3, /3., 17 are extracted from the measured temporal evolution of the product ion intensities. The rates for the exchange of a single proton or alternatively for the replacement of a hydrogen molecule are strongly cluster size dependent. This is additional evidence for a previously proposed dynamical shell structure [W. Paul et al., Intern. J. Mass Spectrom. Ion Processes 150 (1995) 373]. Measurements are carried out in normal deuterium and deuterium with admixtures of helium, para hydrogen or normal hydrogen in order to investigate thermalization and the influence of back reactions. Results are discussed on the basis of calculated equilibrium structures, zero point energies and simple dynamical considerations. Constraints for intracluster rearrangement processes are found.