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Register a new userExcitation and relaxation in atom-cluster collisions
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Electronic and vibrational degrees of freedom in atom-cluster collisions are treated simultaneously and self-consistently by combining time-dependent density functional theory with classical molecular dynamics. The gradual change of the excitation mechanisms (electronic and vibrational) as well as the related relaxation phenomena (phase transitions and fragmentation) are studied in a common framework as a function of the impact energy (eV...MeV). Cluster transparency characterized by practically undisturbed atom-cluster penetration is predicted to be an important reaction mechanism within a particular window of impact energies.
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Charge transfer in collisions of Na_n^+ cluster ions with Cs atoms is investigated theoretically in the microscopic framework of non-adiabatic quantum molecular dynamics. The competing reaction channels and related processes affecting the charge transfer (electronic excitations, fragmentation, temperature) are described. Absolute charge transfer cross sections for Na_n^+(2.7 keV) + Cs --> Na_n + Cs^+ have been calculated in the size range 4 <= n <= 11 reproducing the size dependence of the experimental cross sections. The energy dependence of the cross section is predicted for n=4,7,9. An exotic charge transfer channel producing Cs^- is found to have a finite probability.
Based on a combined quantum-classical treatment, a complete study of the strong field dynamics of H2+, i.e. including all nuclear and electronic DOF as well as dissociation and ionization, is presented. We find that the ro-vibrational nuclear dynamics enhances dissociation and, at the same time, suppresses ionization, confirming experimental observations by I. Ben-Itzhak et al. [Phys. Rev. Lett. 95, 073002 (2005)]. In addition and counter-intuitively, it is shown that for large initial vibrational excitation ionization takes place favorably at large angles between the laser polarization and molecular axis. A local ionization model delivers a transparent explanation of these findings.
Molecular absorption and photo-electron spectra can be efficiently predicted with real-time time-dependent density-functional theory (TDDFT). We show here how these techniques can be easily extended to study time-resolved pump-probe experiments in which a system response (absorption or electron emission) to a probe pulse, is measured in an excited state. This simulation tool helps to interpret the fast evolving attosecond time-resolved spectroscopic experiments, where the electronic motion must be followed at its natural time-scale. We show how the extra degrees of freedom (pump pulse duration, intensity, frequency, and time-delay), which are absent in a conventional steady state experiment, provide additional information about electronic structure and dynamics that improve a system characterization. As an extension of this approach, time-dependent 2D spectroscopies can also be simulated, in principle, for large-scale structures and extended systems.
We investigate the modeling of positronium (Ps) states and their pick-off annihilation trapped at open volumes pockets in condensed molecular matter. Our starting point is the interacting many-body system of Ps and a He atom because it is the smallest entity that can mimic the energy gap between the highest occupied and lowest unoccupied molecular orbitals of molecules and yet the the many-body structure of the HePs system can be calculated accurately enough. The exact-diagonalization solution of the HePs system enables us to construct a pair-wise full-correlation single-particle potential for the Ps-He interaction and the total potential in solids is obtained as a superposition of the pair-wise potentials. We study in detail Ps states and their pick-off annihilation rates in voids inside solid He and analyse experimental results for Ps-induced voids in liquid He obtaining the radii of the voids. More importantly, we generalize our conclusions by testing the validity of the Tao-Eldrup model, widely used to analyse ortho-Ps annihilation measurements for voids in molecular matter, against our theoretical results for the solid He. Moreover, we discuss the influence of the partial charges of polar molecules and the strength of the van der Waals interaction on the pick-off annihilation rate.
Inter-Coulombic decay (ICD) resonances in the photoionization of Cl@C60 endofullerene molecule are calculated using a perturbative density functional theory (DFT) method. This is the first ICD study of an open shell atom in a fullerene cage. Three classes of resonances are probed: (i) Cl inner vacancies decaying through C60 outer continua, (ii) C60 inner vacancies decaying through Cl outer continua, and (iii) inner vacancies of either system decaying through the continua of Cl-C60 hybrid levels, the hybrid Auger-ICD resonances. Comparisons with Ar@C60 results reveal that the properties of hybrid Auger-ICD resonances are affected by the extent of level hybridization.
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