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Site-Selective Ionization in Nanoclusters Affects Subsequent Fragmentation Print

Understanding charge-transfer processes at the atomic level of nanoscale systems is of the utmost importance for designing nanodevices based on quantum-dot structures, nanotubes, or two-dimensional graphene sheets. Researchers from Western Michigan University, Berkeley Lab, and other international research facilities investigated charge-transfer processes and subsequent ion fragmentation dynamics in nanoclusters composed of argon (Ar) shells and xenon (Xe) cores. The clusters were site-selectively ionized (i.e, ionization took place either in the xenon core or in the argon shell). Using a high-resolution photoelectron–ion coincidence technique at ALS Beamlines 10.0.1 and 11.0.2, the researchers concluded that charge-transfer processes and fragmentation dynamics are strongly influenced by the environment of the initially ionized atoms.

Multidisciplinary Nanolabs

Smaller, faster and more efficient. That’s the ongoing trend in all fields of technology. The realization of these goals requires a new generation of materials and devices; therefore, new knowledge and the answers to many questions are needed. For example, the invention of nanotubes, quantum dot structures, and 2D graphene sheets herald a revolution in material science. Their properties like conductivity and mechanical strength make them promising candidates to become materials of the future.

Clusters also play an important role in the new materials race. The advantage of clusters is their simple malleability in size and composition. The ability of researchers to easily access and investigate the physical and chemical properties derived from a cluster’s structure makes them perfect model systems for nano objects with more direct technological relevance, such as nanomagnets for data storage and transport.

Clusters composed of two atoms to hundreds of thousand of atoms are easy to produce and serve as ideal test objects for the investigation of more complex samples. The physical and chemical properties of these nanometer-sized spheres can easily be tailored by varying composition and size, making them useful in a multitude of scientific disciplines. Varying composition enables the investigation of charge migration and fragmentation dynamics in a heterogeneous nanosample. Varying size allows the study of how physical and chemical properties change in going from a single atom to a macroscopic crystal.

For this research, clusters consisting of a Xe core, or bulk, surrounded by an Ar shell were used to investigate charge-transfer processes and subsequent fragmentation dynamics resulting from site-selective ionization within different parts of a cluster. The heterogeneous clusters were produced by a supersonic co-expansion of a 2% Xe in Ar gas mixture through a nozzle into the experimental vacuum chamber.

The different photoionization cross-sections of Xe 4d and Ar 2p electrons enable site-selective ionization of the cluster’s bulk (Xe) or surface (Ar). By measuring coincidences between the subsequent photoelectrons and the resulting ionic cluster fragments, charge transfer and fragmentation processes can be determined.

Due to the different photoionization probabilities of Ar and Xe, site-selective ionization of either the Ar shell or the Xe bulk was possible. Two excitation photon energies, 110 eV and 291.2 eV, were chosen for the reported measurements. Different numbers of neighboring atoms caused differences in the electron binding energies of a cluster’s bulk, interface, and surface. Therefore, the kinetic energy spectrum shows a separation between the cluster and residual gas (atom) peaks and even among the corresponding surface, interface, and bulk peaks. The fragmentation’s dependence on the site of initial photoionization can be determined by measuring the fragment ions in coincidence with the cluster photoelectrons. To trace the dependence of the charge-transfer process and fragmentation dynamics to the ionization site, the researchers set filters on the different cluster fragments and analyzed the corresponding coincident electron spectra.

Top: Ion time-of-flight spectra of Ar–Xe cluster fragments after photoexcitation at 110 eV measured in coincidence with the angle-resolved detection of an electron. The inset shows the corresponding Xe 4d photoelectron spectrum. The two sharp lines that partially overlap with the 4d3/2 cluster peak result from fast valence electrons produced by the subsequent photon bunch. The average cluster size is smaller than 8000. The inset at top presents the kinetic energy spectrum of Xe 4d electrons at 110 eV.
Bottom: Xe 4d5/2 (left) and Ar 2p3/2 (right) photoelectron spectra without selection of ionic coincidences (top) and in coincidence with Ar+2, Xe+2, and XeAr+ fragments (bottom panels).

The results showed that the fragmentation scenario of a nanosystem such as Ar–Xe clusters depends heavily on the site of the initial ionization (surface, interface, bulk). The specific environment of the ionized atom influences the decay and charge localization times. Surprisingly, coincidences with Ar 2p electrons measured after surface ionization did not show fragmentation channels as distinct as those from coincidences with Xe 4d electrons measured after ionization of the interface or bulk. The latter clearly shows preferred decay channels. After surface ionization, the system fragments very fast in a broad range of decay channels. Fragmentation takes longer, however, after bulk ionization due to inner rearrangement favoring a small number of energetically dominant decay channels. Additionally, strong hints for a very effective charge transfer from ionized Ar atoms to surrounding Xe atoms were measured, which was attributed to the electron transfer mediated decay (ETMD) mechanism.

To summarize, after inner-shell photoionization, charge is transferred to neighboring atoms; subsequent charge localization depends on the site of ionization. Ionizing the cluster bulk leads to more distinct fragmentation channels than surface ionization. This is attributed to different electronic decay, charge localization, and fragmentation times, leading researchers to conclude that charge-transfer processes and fragmentation dynamics are strongly influenced by the environment of the initially ionized atom.



Research conducted by M. Hoener (Western Michigan University, Lawrence Berkeley National Lab), D. Rolles (Western Michigan University, Max Planck Advanced Study Group at CFEL), A. Aguilar and E. Red (Lawrence Berkeley National Lab), R. Bildodeau (Western Michigan University, Lawrence Berkeley National Lab), D. Esteves (Lawrence Berkeley National Lab, University of Nevada, Reno), P. Olalde Velasco (Lawrence Berkeley National Lab, Universidad Nacional Autonoma de Mexico), Z.D. Pesic (Laboratory for Atomic Collision Processes, Institute of Physics, Belgrade), and N. Berrah (Western Michigan University)

Research funding: US Department of Energy, Chemical Sciences, Geosciences and Biosciences Division. M.H. and D.R. are grateful to the Alexander von Humboldt Foundation for support through the Feodor Lynen Program. D.E acknowledges support from the ALS Doctoral Fellow in Residence Program. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: M. Hoener, D. Rolles, A. Aguilar, R. C. Bilodeau, D. Esteves, P. Olalde Velasco, Z. D. Pesic, E. Red, and N. Berrah, “Site-selective ionization and relaxation dynamics in heterogeneous nanosystems,” Phys. Rev. A 81, 021201(R) (2010). doi: 10.1103/PhysRevA.81.021201

ALS Science Highlight #209


ALSNews Vol. 310