Snowballs, quantum solvation and coordination: lead ions inside small helium droplets

International audienceAb initio calculations are used to construct an analytical many-body potential for Pb2+Hen and Pb+Hen clusters which accounts for non pairwise additive interactions. The potential surface reproduces the global minima for cluster sizes ranging from n = 1 to n = 16 obtained from explicit ab initio calculations and found in a previous search for ultrahigh coordination numbers. Ground state energies and structures obtained by accurate diffusion quantum Monte Carlo calculations are used to investigate if quantum effects qualitatively affect the formation of coordination shells. For Pb2+ doped clusters a first solvation shell is closed at n = 12 and gradually softened by additional helium atoms which start to form a distinct second shell only at n = 16. Spin-orbit coupling profoundly influences the structure of Pb+Hen clusters and causes a gradual structural evolution without pronounced solvation shells

Shell Structures in Pb2+^{2+}Hen_n and Pb+^+Hen_n Clusters

International audienc

Ab initio Quantum Monte Carlo Simulation of Charged Doped Helium Clusters

blablabl

Quantum Simulations of Helium Clusters with Ionic and Open Shell Dopants

PosterMany experiments on pure and doped helium clusters result in the production of a broad distribution of charged fragment ions after electron impact or laser ionisation. These ion yield distributions occasionally exhibit distinct stability patterns. We use the diffusion quantum Monte Carlo technique to study these stability patterns and structural features of helium clusters with open shell atomic and molecular dopants. These simulations require reliable many-body potentials which we construct from high level ab initio CCSD(T) calculations for several electronic states mixed by spin-orbit coupling and including non-additive interactions arising from induction. For small clusters numerically exact calculations of rovibrational properties are used to establish the quality of our potential surfaces. The approach is illustrated with applications to I$^{q}$@He$_n$, q=-1,0,+1,+2 clusters which appear in ionisation and photodissociation experiments. Spin-orbit coupling due to the iodine atom is found to have a profound effect on the structure of the mixed clusters. Radial and angular helium density profiles are computed and show that even in the presence of pronounced radial structuring exchange between 'shells' remains possible and angular disorder remains significant. The identification of a well defined ionic snowball core appears incorrect. We present first results for CO$^+$ ions in helium clusters. He-CO$^+$ is an interesting astrophysical collision system but its interest for helium cluster studies is the similarity of the CO$^+$ rotational constant with the one of neutral CO. Our present understanding of rotation inside helium clusters relies on studies of molecules where changing the molecule implies changing dynamical parameters and the interaction potential. The CO/CO$^+$ case allows to study the specific effect of changing only the interaction energy. CO$^+$@He$_n$ should be relatively easy to make by cluster ionisation or aggregation in ion drift tubes and its strong dipole and vibrational transition moment make it an easy target for high resolution spectroscopy. We have computed accurate ab initio surfaces for the two lowest electronic states of He-CO$^+$ to predict rovibrational spectroscopic and collisional properties. A many-body model using these surfaces is used to study larger CO$^+$He$_n$ clusters

Generation of Supersonic Plasma Flow by Means of Unipolar RF Discharges

Recently the unipolar discharges has been used as a plasma source which employ the flow of the working gas for the creation of plasma channels : At first the radio frequency unipolar one hollow cathode discharge the cathode of which acts simultaneously as a nozzle for working gas inlet to the reactor chamber has been used in the particular applications.[1,2]. On the base of this reactor the plasma-chemical reactor with a system of unipolar multi-hollow-cathode discharges has been developed. This reactor is able to deposit the composite thin films and multi-layer structures onto internal walls of cavities, tubes and on the components with complicated shapes. Secondly the RF unipolar torch discharge has been used as a source of the plasma Channel. The advantage of this system is that it is possible to use it up to atmospheric pressure of the working gas and even in the liquid environment[3],[4]. In the present report the generation of the plasma channels by means of mentioned RF unipolar plasma discharges is discussed. At first the devices with RF unipolar hollow cathode discharge are presented and after that the device with the torch discharge is mentioned

Helquats: a facile, modular, scalable route to novel helical dications

The synthesis and properties of helical extended diquat (helquat), and derivatives that bear resemblance to diquat and azoniahelicene, was reported. Triyne with elongated tethers connecting the heterocyclic moiety with the pendant alkyne functionalities undergoing cycloisomerization give helquat featuring two seven-membered rings. The seven helquats reported are accessed uniformly in three steps from commercially available starting materials, entailing a Sonogashira coupling, bisquaternization, cycloisomerization, and 2+2+2 cycloisomerization. The evidence for the reversible electrochemical Weiz-type manifold and regular columnar stacks in crystal structures suggest the potential of helquats as electroactive functional elements

Emotional processing in Parkinson's disease and anxiety: an EEG study of visual affective word processing

A general problem in the design of an EEG-BCI system is the poor quality and low robustness of the extracted features, affecting overall performance. However, BCI systems that are applicable in real-time and outside clinical settings require high performance. Therefore, we have to improve the current methods for feature extraction. In this work, we investigated EEG source reconstruction techniques to enhance the extracted features based on a linearly constrained minimum variance (LCMV) beamformer. Beamformers allow for easy incorporation of anatomical data and are applicable in real-time. A 32-channel EEG-BCI system was designed for a two-class motor imagery (MI) paradigm. We optimized a synchronous system for two untrained subjects and investigated two aspects. First, we investigated the effect of using beamformers calculated on the basis of three different head models: a template 3-layered boundary element method (BEM) head model, a 3-layered personalized BEM head model and a personalized 5-layered finite difference method (FDM) head model including white and gray matter, CSF, scalp and skull tissue. Second, we investigated the influence of how the regions of interest, areas of expected MI activity, were constructed. On the one hand, they were chosen around electrodes C3 and C4, as hand MI activity theoretically is expected here. On the other hand, they were constructed based on the actual activated regions identified by an fMRI scan. Subsequently, an asynchronous system was derived for one of the subjects and an optimal balance between speed and accuracy was found. Lastly, a real-time application was made. These systems were evaluated by their accuracy, defined as the percentage of correct left and right classifications. From the real-time application, the information transfer rate (ITR) was also determined. An accuracy of 86.60 ± 4.40% was achieved for subject 1 and 78.71 ± 0.73% for subject 2. This gives an average accuracy of 82.66 ± 2.57%. We found that the use of a personalized FDM model improved the accuracy of the system, on average 24.22% with respect to the template BEM model and on average 5.15% with respect to the personalized BEM model. Including fMRI spatial priors did not improve accuracy. Personal fine- tuning largely resolved the robustness problems arising due to the differences in head geometry and neurophysiology between subjects. A real-time average accuracy of 64.26% was reached and the maximum ITR was 6.71 bits/min. We conclude that beamformers calculated with a personalized FDM model have great potential to ameliorate feature extraction and, as a consequence, to improve the performance of real-time BCI systems