Ionization potentials and charge localization in small charged group 12 clusters

International audienceFor small size, the cluster X n with X= Hg, Cd or Zn displays a van der Waals bond. A model Hamiltonian has been proposed for the singly (doubly) ionized van der Waals clusters X + n (X ++ n). The first and second ionization potentials of X n have been calculated. A good agreement is obtained with the available experiment values of the first ionization potential of mercury and cadmium. The stability and the metastability of X ++ n are discussed. The energy curves versus the bond length for Hg ++ 2 , Cd ++ 2 and Zn ++ 2 have been determined. The hole(s) in charged clusters X + n and X ++ n for 2≤n≤7 is (are) mainly delocalized, except for Hg + n with n≥4 where the hole is localized on 2 or 3 sites

Ionization potentials and charge localization in small charged group 12 clusters

International audienceFor small size, the cluster X n with X= Hg, Cd or Zn displays a van der Waals bond. A model Hamiltonian has been proposed for the singly (doubly) ionized van der Waals clusters X + n (X ++ n). The first and second ionization potentials of X n have been calculated. A good agreement is obtained with the available experiment values of the first ionization potential of mercury and cadmium. The stability and the metastability of X ++ n are discussed. The energy curves versus the bond length for Hg ++ 2 , Cd ++ 2 and Zn ++ 2 have been determined. The hole(s) in charged clusters X + n and X ++ n for 2≤n≤7 is (are) mainly delocalized, except for Hg + n with n≥4 where the hole is localized on 2 or 3 sites

Influence of the electron-scattering mechanism on the critical current given by flux pinning at grain or twin boundary in high Tc\mathit{T}_\mathrm{c} superconductors

The scattering caused by presence of grain or twin boundaries changes the local electronic properties of the material both in the normal state (mean free path, ...) and in the superconducting state (coherence length, ...). The consequences of electron-scattering mechanism on the critical current density given by ideal planar defects have been investigated for zero and large magnetic fields. For the case of large magnetic field, the critical current density dependences versus the magnetic field and the defect distance have been carried out. For the case of a 10 T applied magnetic field strength and for defect spacing largely smaller than 2 000 Å collective pinning is occurring, but for spacing largely larger than 2 000 Å it is the strong pinning. Finally the temperature behavior of the critical current density has been treated within the thermal flux creep model modified by the collective effects : i.e. the thermal activation energy is due to the contributions of the pinning well depth and of the bundle hopping energies

Etude théorique des pics satellites dans l'émission 1s de C60_{60}

We make a theoretical study of the shakeup structure of the C$_{60}$ 1s photoemission spectrum. The method takes into account the reactions of all the $\pi$ electrons which accompany the photoelectron emission. The calculation is a priori very long since there are $10^{34}$ possible final states to be considered. We show that the calculation can be restricted to the states which differ from the final ground state by creating electron-hole pairs in a set of 15 particular molecular wavefunctions. The comparison to experiment allows one to fix the parameters of the calculation. We find for the hopping integral : $\beta\sim -1,15-1,05$ eV and for the local attractive term which appears in the final state : $\Delta=2\beta$. Once these parameters are fixed, several features of the experimental spectrum are well explained by the model. We also calculate the shakeup structure of the 1s C$_{60}$ X-ray emission spectrum.Nous calculons la structure de basse énergie du pic de photoémission 1s de C$_{60}$ (pics satellites). La méthode consiste à prendre en compte la réaction de tous les électrons $\pi$ du système lors de l'éjection du photoélectron. Le calcul a priori très long peut être simplifié si l'on considère que, parmi les $10^{34}$ états finaux possibles, seuls interviennent ceux qui diffèrent de l'état final fondamental par création de paires électron-trou dans un ensemble de 15 fonctions d'onde moléculaires particulières. La comparaison à l'expérience permet d'ajuster les paramètres du calcul. On trouve pour l'intégrale de saut $\beta\sim -1,15-1,05$ eV et pour le terme local attractif apparaissant dans l'état final $\Delta=2\beta$. Ces paramètres fixés, plusieurs caractéristiques du spectre expérimental s'expliquent bien dans le cadre de ce modèle. Nous donnons aussi la structure de basse énergie du spectre d'émission X 1s de C$_{60}$

Spin diffusion on a linear spin polarised chain

We investigate the diffusion of a spin polarised projectile (silver atom) on a ferromagnetic spin polarised chain. The interaction between the projectile and the chain is described with a Heisenberg Hamiltonian which implied that we exclude the head-on collision which provokes strong electron exchange and fragmentation. We make a real time description of the spin of two interacting systems. We deduce from it the polarisation change of the projectile and the excitation in the chain versus the chain length, the impact parameter and the kinetic energy of the projectile. We analyse 3 different behaviors of the system according to whether the intra-chain excitation propagates slower or faster than the projectile

A theoretical study on electron transfer in low-energy collisions of a collinear meta-stable Na

Electron transfer in the collisions of a Na${_{3}^{+}}$ with a Na is theoretically studied. It is assumed that the target Na${_3^+}$ is collinear ($D_{\infty h}$) and that its electronic state is meta-stable triplet ($a ^3 \Sigma _u^+$) state. Adiabatic potential energy surfaces and non-adiabatic couplings of the Na${_{4}^{+}}$ system are calculated by using a semi-empirical diatomics-in-molecules (DIM) method. The positions of (avoided)-crossings of potential surfaces are investigated and the non-adiabatic couplings between two different electronic states are calculated. An avoided crossing is found in the region where the separation between the target and projectile is relatively large (10–15 bohr). A dynamical calculation demonstrates that this crossing causes charge transfer between the target and projectile. Another intersection at a smaller separation changes the target's spin state (from triplet state to singlet state or vice versa). The cross-sections for charge and spin transfer reaction are estimated at the collision energy of 6.8 keV. It is found that the charge transfer cross-section is extremely enhanced when the target cluster ion is in its meta-stable triplet state comared to the case where the cluster is the ground singlet state

Nanowires with well defined radii formed in operating liquid metal ion sources (LMIS)

For some elements such as germanium or tin, the mass spectra of ions emitted by liquid metal ion sources (LMIS) exhibit periodicities, i.e. series of equidistant peaks with an increase, ν, in the number of atoms between two peaks. We attribute it to the existence of jets in operating LMIS, the upper part of them being cylinders with $\nu{-}atom$ sections. The Ge$_{6m+1}^{3+}$ and Sn$_{6m+1}^{3+}$, m=3 to 8, and Ge$_{6m+4}^{3+}$, m= 9 to 14, observed ions can be explained by this mechanism, here $\nu=6$. We extend this mechanism to bismuth and gold and, in this last case, it allows the interpretation of a yet unexplained Au$_{8}^{3+}$ ion

Magnetic and vibration excitations induced by collision in ferromagnetic cluster

We investigate the excitations induced by the diffusion of a spin polarized projectile on a ferromagnetic spin polarized cluster. The interaction between the projectile and the target is described with a Heisenberg Hamiltonian which excludes the charge degree of freedom during the process. The repulsion between the nucleus has a Born-Meyer form. Our calculation includes both a real time description of the spins of the two interacting systems and also the atomic motion of the cluster atoms. The spin excitations induced are studied versus the cluster size and the trajectory conditions. The effects of the phonons on the spin excitations will be discussed. The collision transfer energy between the target and the projectile presents some resonances versus the velocity of the projectile. The nonadiabatic behaviour during the collision has been characterized by the spin temperature at the end of the the collision