· “Towards a complete characterization of molecular dynamics in ultra-short laser fields”, B. Feuerstein, T. Ergler, A. Rudenko, K. Zrost, C.D. Schroeder, R. Moshammer, J.Ullrich, T. Niederhausen, and U. Thumm, Phys. Rev. Lett. 99, 153002 (2007).
· “Controlled vibrational quenching of nuclear wave packets in D2+”, T. Niederhausen and U. Thumm, Phys. Rev. A 77, 013407 (2007.
· “Capture and ionization in laser-assisted proton-hydrogen collisions”, T. Niederhausen and U. Thumm, Phys. Rev. A 73, 041404(R) (2006).
· "Mapping of coherent and decohering nuclear wave packet dynamics in D2+ with ultrashort laser pulses", B. Feuerstein and U.Thumm, Phys. Rev. A 67, 063408 (2003).
· "Fragmentation of H2+ in strong 800nm laser pulses: initial vibrational state dependence", B. Feuerstein and U. Thumm, Phys. Rev. A 67, 043405 (2003).
· "Time-resolved photo-imaging of image-potential states in carbon nanotube", M. Zamkov, N. Woody, B. Shan, H.S. Chakraborty, Z. Chang, U. Thumm, and P. Richard, Phys. Rev. Lett. 93 (2004).
· "Circular dichroism in laser-assisted proton - hydrogen collisions", T. Niederhausen, B. Feuerstein, and U. Thumm, Phys. Rev. A 70, 023408 (2004).
Interactions between ions and surfaces are not well understood at a microscopic level despite their importance for applications in surface chemistry (catalyses, corrosion prevention), accelerator design, and controlled fusion devices. Similarly, the detailed understanding of electron--transfer and electron emission in collisions between ions and fullerenes is of relevance for future applications. These collisions yield information on the physical and chemical properties of fullerenes that may enable the successful syntheses of new materials (fullerene chemistry).
In an attempt to better understand the interaction mechanisms in collisions of highly charged ions with clusters and surfaces, we developed (mostly) classical models, that simulates electron transfer in terms of classical currents across an effective potential barrier located between projectile and target. These simulations yield a variety of observables, such as charge transfer cross sections, projectile deflection angles, projectile kinetic energy gains, and charge--states distributions that probe the basic interaction mechanisms at a femto--second time scale. Some of these observables have been measured recently, compare favourably with our simulations, and allow us to carefully test our model assumptions in view of the fundamental interaction mechanisms and the basic static and dynamical properties of the collision partners.
We plan to continue to improve these models, both for cluster targets (C60) and for insulating surfaces. In particular, we plan to calculate projectile energy gains and projectile deflection angles in HCI-C60 interactions, in order to provide support for recent experiments [1--3] and to streamline our recent dynamical over--barrier simulations for HCI-surface interactions [4].
In addition to these simulations we are performing ab--intio calculations in which the time--dependent Schroedinger equation is solved by means of a two--center close--coupling expansion. These calculations allow us to investigate the hybridization characteristics, level shifts and widths, and neutralization probabilities in slow ion--surface collisions, with possible inclusion of external electric fields [5]. We are currently investigating the applicability of these calculations to the interaction of ions with thin metallic films [6].
1. S. Martin, J. Bernard, L. Chen, A. Denis, and J. Desesquelles, Eur. Phys. J.
D 4 (1998).
2. N. Selberg, A. Barany, C. Biedermann, C.J. Setterlind, H. Cederquist, A. Langereis, M.O. Larsson, A. Wannstrom, and P. Hvelplund, Phys. Rev. A 53, 874 (1996).
3. U. Thumm, Comments At. Mol. Phys. 34, 119 (1999); B. Walch, U. Thumm, M. Stockli, C.L. Cocke, and S. Klawikowski, Phys. Rev. A 58, 1261 (1998).
4. J.J. Ducree, F. Casali, and U. Thumm, Phys. Rev. A 57, 338 (1998); J.J. Ducree, H.J. Andra, and U. Thumm, Phys. Rev. A 60, 3029 (1999).
5. P. Kurpick and U. Thumm, Phys. Rev. A 58, 2174 (1998); B. Bahrim and U. Thumm, Surf. Sci. 451 (2000).
6. U. Thumm, P. Kurpick, and U. Wille, Phys. Rev. B 61, 3067 (2000).
In the field of electron--atom collisions, alkali--metal atoms and noble gases are frequently chosen targets for detailed experimental and theoretical study, owing both to their relative theoretical simplicity and to the relative ease with which they can be handled experimentally. The heavier targets, like Rubidium, Cesium, or Francium allow exploration of relativistic effects which are too small to be easily observed in lighter atoms.
We have performed a relativistic calculation based on the Dirac R--matrix method within a two--electron model potential approach [1]. Our results for electron--Cesium scattering include negative--ion binding energies, as well as elastic, inelastic, and superelastic cross sections. We calculated bound orbitals of neutral Cesium, starting from the Dirac equation based on an effective core potential that includes dipole and quadrupole polarization terms.
For the electron--Cesium system, two multiplets of narrow shape resonances (with J = 0 , 1 , 2 and widths of a few meV) are of particular interest. We have shown [1] that these resonances are influenced by both (two--electron) core polarization and relativistic effects. The former convert the 3PoJ negative ion states from bound states to resonances. The latter gives fine--structure splittings and finite autoionization widths to 3PeJ states that in LS coupling are strictly uncoupled to the adjacent continuum. Our calculations predict the same resonances to occur in Rb- and Fr-.
Photoelectron spectroscopy was recently used to experimentally confirm the dipole allowed J=1 term of the aforementioned 3PoJ resonances [2]. My further research interests in the field of electron--atom scattering include the extension of our relativistic calculations to photoprocesses [3], the calculation of atomic alignment and coherence parameters, the inclusion of more bound target states, and the application to others targets, such as, e.g., carbon nanotubes [4,5].
1. U. Thumm and D.W.Norcross, Phys.Rev.Lett. 67, 3495 (1991); Phys. Rev. A45, 6349 (1992); A47, 305 (1993); C. Bahrim and U. Thumm, Phys. Rev. A 61 (2000).
2. Scheer et al., Phys. Rev. Lett. 80, 684 (1998); C. Bahrim and U. Thumm, Phys. Rev. A 64, 022716 (2001).
3. "Boundary conditions for the Pauli equation: application to photodetachment of Cs-", C. Bahrim, I.I. Fabrikant, and U. Thumm, Phys. Rev. Lett. 87, 123003 (2001).
4. "Time-resolved photo-imaging of image-potential states in carbon nanotube", M. Zamkov, N. Woody, B. Shan, H.S. Chakraborty, Z. Chang, U. Thumm, and P. Richard, Phys. Rev. Lett. 93 (2004).
5. "Image-potential states of single- and multi-walled carbon nanotubes", 5. M. Zamkov, H.S. Chakraborty, A. Habib, N. Woody, U. Thumm, and P. Richard, Phys. Rev. B 70, 115419 (2004).
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