Positive ion formation in doped helium droplets by electrons

In the presently proposed studies it is planned to probe positive ion formation upon collisions of free electrons with doped He droplets, utilizing cutting edge techniques. At high enough energies the dominant processes are electron ionization of one of the He atoms, followed by charge hopping and charge localization on the dopant, or Penning ionization of the dopant by an initially formed electronically excited He atom. Below 19.8eV direct ionization of the dopant is the only possible mechanism, but little attention has been given to this process so far. Furthermore, the electron energy resolution of previous experiments has been insufficient to answer some fundamental questions concerning direct ionization of the dopant. Free electrons require a minimum energy of about 1eV to penetrate bulk He or He nanodroplets. In the present studies we plan to determine the energy shift of the appearance energies of cations formed inside doped He nanodroplets compared to free atoms or molecules. If both outgoing electrons are liberated into the conduction band of the surrounding He one can expect an energy shift of more than 2eV. But also the main ionization channels are far from being completely understood. For iodine we recently discovered the formation of complexes of the form HenI2+ (H. Schöbel et al. Phys. Rev. Lett. 105 (2010) 243402). The energy required to from I2+ is way above the first ionization energy of He and thus a new mechanism has been proposed via sequential Penning ionization that will be investigated in detail in this project. Double ionization of dopant molecules and dopant clusters via collisions with a single He* or He+ and via sequential Penning ionization will be investigated. The careful analysis of the fragmentation patterns as a function of the electron energy is expected to provide information about Coulomb explosion of multiply charged intermediates. Concerning multiply charged cluster ions it is planned to investigate the critical size at cold temperatures. Evaporation of He may lead to the stabilization of multiply charged clusters below the critical size obtained conventionally. Magic numbers of cluster ions have been discussed in the literature extensively. Pickup atoms or molecules into He nanodroplets definitely will not lead to any intensity anomalies in the cluster size distribution of the neutral dopants. In a recent paper we could demonstrate that ionization of Ar clusters inside He nanodroplets via He+ charge transfer leads to the same intensity anomalies as electron ionization of pristine argon clusters. Direct electron ionization of a rare gas cluster embedded in helium, however, might be a method to prevent fragmentation of the dopant cluster and to preserve the neutral size distribution. In addition, fragmentation patterns of mass selected ions of the form HemX+ (with X+ an atomic, molecular or cluster ion) upon photon absorption and collisions with gas targets will be measured to investigate the evolution of magic numbers and so called snow balls, i.e., a layer of He tightly bound to the ionic impurity.

Besides the well-established mechanisms of charge transfer and Penning ionization for doped He nanodroplets a third one has been discovered within this project. At electron energies of 22eV, 23 eV and 26eV a metastable He anion is efficiently formed and with much less abundance at the two higher electron energies also formation of He2* has been observed. Both anions are metastable in the gas phase but exhibit very different properties with respect to the interaction with impurities or dopants in He nanodroplets. According to recent calculations He* is heliophilic (likes to reside in the center of the droplet) and very mobile. Via ion-induced dipole interaction this anion approaches quickly any polarizable impurity. Several novel interaction mechanisms for this exotic anion have been discovered, including a double electron transfer to fullerene clusters, leading to a cationic He ion and a doubly charged fullerene anion, and the formation of salt clusters. Both processes were published in 2014 in Angewandte Chemie International Edition. Furthermore, He* provides also for the first time a sound explanation for He+ formation at electron energies between 20eV and 24.6eV (the ionization potential of gas phase He). This unexpected resonance in the ion efficiency curve of He+ is known since 1990. A second major topic of this project has been the physisorption of atoms and molecules to fullerene ions and cluster ions of fullerenes. Polar molecules such as water and ammonia interact much less with the fullerene(s) compared to the each other which results in compact adsorbate clusters that are very little affected by the presence of the charged surface. In contrast apolar dopants exhibit the formation of mono-molecular layers often registered with the hexagonal and pentagonal rings of the fullerenes. In most cases the completion of the first layer results in pronounced intensity anomalies, i.e. magic numbers, of the cluster size distribution. For C60 with 20 hexagonal and 12 pentagonal faces this shell closure arises at a number of 32 and for C70 (25 hexagons and 12 pentagons) at 37 dopant molecules. For clusters of fullerenes preferential occupation of positions in dimples and grooves has been observed. Currently we perform molecular dynamics calculations to complement several measurements and to obtain a deeper understanding for the experimentally observed shell closures and magic numbers. We expect to finish these investigations in 2015 and plan to publish these results in refereed scientific journals.