The optical absorption spectra of cyclo[n]carbons (n=10, 14, 18) are investigated in the framework of time-dependent density functional theory. The collective plasmon excitations well develop as the increases of the ring size and the symmetry group of cyclo[n]carbons. An increase in intensity for the main peaks with the growing number of atoms in cyclo[n]carbons is observed. With the increase of the radius of the monocyclic ring, as more electrons participate in the dipole oscillation the main excitation peaks are red-shifted to the lower energy. The highly symmetrical structures of cyclo[n]carbons (D_{nh}) possess degenerate levels, leading to simpler spectra with fewer peaks. The Fourier transform of the induced electron density of the cyclo[n]carbons (n=10, 14, 18) is investigated at the excitation frequencies.
We demonstrate that the model of zero-range potentials can be successfully employed for the description of attached electrons in atomic and molecular anions, for example, negatively charged carbon clusters. To illustrate the capability of the model we calculate the energies of the attached electron for the family of carbon cluster anions consisting of two-, three- (equilateral triangle), and four (tetrahedron) carbon atoms equidistant from each other as well as for a C3 molecule having a chain structure. The considered approach can be easily extended to carbon clusters containing an arbitrary number of atoms arranged in an arbitrary configuration.
Brian J. Anderson, Cameron N. Olson, Haje Korth
et al.
AbstractThe development of large‐scale Birkeland currents is examined using the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), which measures global Birkeland currents continuously on 10‐min time scales. The integrated current was used to identify onsets of at least 1 MA preceded by periods of quiescence lasting at least 3 hr. The Region 1 currents do not fully form without Region 2. Rather, they develop together, first on the dayside, then on the nightside, and lastly, they fill in and intensify at all local times to form the nominal statistical pattern. The onsets are closely correlated with enhancements of magnetospheric forcing as indicated by the solar wind electric field and the orientation of the interplanetary magnetic field. Nightside onsets correspond to intensifications of the auroral electrojet as reflected in the AE index; they are delayed by ~40 min relative to the increase of the dayside current and are 2.8 times more rapid than the dayside current increase. After nightside onset, Birkeland currents expand toward dawn and dusk and merge with the dayside currents while also intensifying at all local times. The dayside current pattern depends on the sign of the interplanetary magnetic field BY. The nightside current distributions are the same for positive and negative BY and display a Harang discontinuity independent of the sign of BY. The predominant development and intensification of Birkeland currents occur after nightside onset at all local times with roughly 75% of the total current, both Regions 1 and 2, appearing after nightside onset.
Carbon clusters have been generated by a novel technique of energetic heavy ion bombardment of amorphous graphite. The evolution of clusters and their subsequent fragmentation under continuing ion bombardment is revealed by detecting various clusters in the energy spectra of the direct recoils emitted as a result of collision between ions and the surface constituents.
We simulated the growth of 2D islands with 2 kinds of diffusion atoms using the kinetic Monte- Carlo (kMC) method. As a result, we found that the slow atoms tend to create nuclei and determine the island volume distribution, along with additional properties such as island density. We also conducted a theoretical analysis using the rate equation of the point-island model to confirm these results.
Molecular dynamics simulations and free energy calculations were employed to investigate the evolution, formation probability, detailed balance and isomerization rate of small C cluster isomers at 2500 K. For C10, the isomer formation probability predicted by free energy is in good agreement with molecular dynamics simulation. However, for C20, C30 and C36, the formation probability predicted by free energy is not in agreement with molecular dynamics and the detailed balance does not hold, indicating that the molecule system is in non-equilibrium. Such result may be attributed to the transformation barriers between cage, bowl and sheet isomers.
The two primary methods responsible for solar wind magnetosphere coupling are magnetic reconnection and the viscous interaction. The viscous interaction is generated due to the antisunward dragging of plasma inside the magnetopause by the plasma flowing in the magnetosheath, creating a return flow deeper inside the magnetosphere and producing a circulation pattern. This viscous circulation pattern is mapped into the ionosphere via magnetic field lines, which results in ionospheric electric field in the nonrotating Earth's frame. We measure this interaction in terms of an electric potential, the viscous potential. In this paper, we use the results obtained from the Lyon‐Fedder‐Mobarry (LFM) simulation model during periods of purely northward interplanetary magnetic field (IMF) for different solar wind velocity and ionospheric conductivity, showing a reduction of the viscous potential with increasing magnitude of northward IMF. The viscous potential is found to settle around 5–10 kV for large +Bz values. The decrease in viscous potential was found to be associated with a weak or nonexistent sunward plasma flow in the nightside plasmasheet. Instead, the return flow to the dayside occurs at high latitudes and is associated with the reconnection topology and dynamics that occur during northward IMF periods. We also show that the magnetosphere remains closed during purely northward IMF, except for two small regions—one on each hemisphere, where the magnetic reconnection occur. We argue that the reduction of the viscous potential is due to a reduction of the velocity shear across the magnetopause and the lack of sunward convection in the equatorial tail.
The relationship between monoenergetic electron acceleration and broadband electron acceleration is uncertain, although some have speculated that the latter is a temporal transient, and may evolve into the former. Here we have taken advantage of DMSP satellite coincidences to investigate the issue. We consider 1668 cases where one DMSP satellite observed an electron acceleration event covering at least 2 s, hence as many discrete accelerated spectra, and a second satellite subsequently observed an electron acceleration at the same location. The spatial coincidence required was tight, with a maximum separation of 0.1° magnetic latitude and 0.15 h magnetic local time. Time separations of 0–10 min were considered in 1 min bins, with auroral acceleration flagged as either monoenergetic, broadband, or a mixture of both. Within the first temporal bin (0–1 min), the second satellite had a high probability of observing the same type of aurora as the first, establishing consistency. When the first satellite observed monoenergetic aurora, the second satellite also observed monoenergetic aurora (about 80% of the time), and this continued to up to about 10 min of UT separation. In most of the other 20% of the cases, the second satellite also recorded monoenergetic acceleration but with an additional mixture of broadband acceleration. Thus monoenergetic aurora does not seem to typically evolve on a time scale of minutes. However, when the first satellite encounter was with broadband acceleration, the second encounter was highly time‐dependent, with broadband dominating the second satellite encounter only in the 0–1 min bin. Between 1 and 5 min, the probability of observing a mixture of auroral types jumped, and after 6 min, the auroral acceleration was nearly as likely to be monoenergetic as broadband. Finally, if the first satellite encountered a mixture of acceleration, the second encounter was progressively more likely to be entirely monoenergetic aurora as time increased. These results are consistent with the idea that broadband aurora may be inherently a transient, and often progresses to monoenergetic aurora, while the latter is quasi‐static.
UHF/VHF beacon receivers deployed in Alaska measure the latitude profile of relative total electron content over Alaska and frequently across the auroral oval. On March 9, 2008, these receivers observed a plasma density enhancement at 147 km. These observations are supported by collocated scanning photometer measurements. A simple model current consistent with magnetic field perturbations observed by a chain of magnetometers across Alaska corresponds spatially to the enhanced density observed by the receivers. Hence, these measurements suggest that the auroral electrojet is flowing at 147 km, which is unusually high. Indeed, the ratio of the 630.0 nm and 427.8 nm emissions and the strength of the magnetic perturbation suggest the electrojet is flowing at a lower altitude. We attribute this difference to thermospheric compositional changes associated with particle and Joule heating that could result in a lower ratio of the 630.0 nm and 427.8 nm emissions and higher conductivity corresponding to observed density enhancement.
The linear optical absorption spectra of three isomers of planar boron cluster B$_{13}$ are calculated using time-dependent spin-polarized density functional approach. The geometries of these cluster are optimized at the B3LYP/6-311+G* level of theory. Even though the isomers are almost degenerate, the calculated spectra are quite different, indicating a strong structure-property relationship. Therefore, these computed spectra can be used in the photo-absorption experiments to distinguish between different isomers of a cluster.
Anion-radical form of the oxygen centers O(-) is predicted at the DFT level for small silver oxide particles having the AgO stoichiometry. Model clusters (AgO)n appear to be ferromagnetic with appreciable spin density at the oxygen centers. In contrast to these clusters, the Ag2O model cluster have no unpaired electrons in the ground state. The increased O/Ag ratio in the oxide particles is proved to be responsible for the spin density at oxygen centers.
Barbara Grüner, Martin Schlesinger, Philipp Heister
et al.
The vibrational wave-packet dynamics of diatomic rubidium molecules (Rb2) in triplet states formed on the surface of superfluid helium nanodroplets is investigated both experimentally and theoretically. Detailed comparison of experimental femtosecond pump-probe spectra with dissipative quantum dynamics simulations reveals that vibrational relaxation is the main source of dephasing. The rate constant for vibrational relaxation in the first excited triplet state is found to be constant ~0.5ns^-1 for the lowest vibrational levels v< 15 and to increase sharply when exciting to higher energies.
Photoionization by attosecond (as) extreme ultraviolet (xuv) pulses into the laser-dressed continuum of the ionized atom is commonly described in strong-field approximation (SFA), neglecting the Coulomb interaction between the emitted photoelectron (PE) and residual ion. By solving the time-dependent Schödinger equation (TDSE), we identify a temporal shift $δτ$ in streaked PE spectra, which becomes significant at small PE energies. Within an eikonal approximation, we trace this shift to the combined action of Coulomb and laser forces on the released PE, suggesting the experimental and theoretical scrutiny of their coupling in streaked PE spectra. The initial state polarization effect by the laser pulse on the xuv streaked spectrum is also examined.
We present an illustration of using a quantum three-body code being prepared for public release. The code is based on iterative solving of the three-dimensional Faddeev equations. The code is easy to use and allows users to perform highly-accurate calculations of quantum three-body systems. The previously known results for He$_3$ ground state are well reproduced by the code.
It is well known that plasmons in bulk metals cannot be excited by direct photoabsorption, that is, by coupling of volume plasmons to light. Here we demonstrate that the situation in nanoclusters of the same metals is entirely different. We have carried out a photodepletion measurement for Na_20 and Na_92 and identified a broad volume plasmon absorption peak centered slightly above 4 eV, revealing the possibility of optical excitation of volume-type collective electronic modes in a metallic system. The observed phenomenon is related to different selection rules for finite systems.
Fabien Chirot, Pierre Labastie, Sébastien Zamith
et al.
We investigated the nucleation process at the molecular level. Controlled sticking of individual atoms onto mass selected clusters over a wide mass range has been carried out for the first time. We measured the absolute unimolecular nucleation cross sections of cationic sodium clusters Na_{n}^{+} in the range n=25-200 at several collision energies. The widely used hard sphere approximation clearly fails for small sizes: not only should vapor-to-liquid nucleation theories be modified, but also, through the microreversibility principle, cluster decay rate statistical models.
Patrick Claas, Georg Droppelmann, Claus-Peter Schulz
et al.
The dynamics of vibrational wave packets excited in Na2 dimers in the triplet ground and excited states is investigated by means of helium nanodroplet isolation (HENDI) combined with femtosecond pump-probe spectroscopy. Different pathways in the employed resonant multi-photon ionization scheme are identified. Within the precision of the method, the wave packet dynamics appears to be unperturbed by the helium droplet environment.
Standard density functional approximations greatly over-estimate the static polarizability of longchain polymers, but Hartree-Fock or exact exchange calculations do not. Simple self-interaction corrected (SIC) approximations can be even better than exact exchange, while their computational cost can scale only linearly with the number of occupied orbitals.
We report ground state energies and structural properties for small helium clusters (4He) containing an H- impurity computed by means of variational and diffusion Monte Carlo methods. Except for 4He_2H- that has a noticeable contribution from collinear geometries where the H- impurity lies between the two 4He atoms, our results show that our 4He_NH- clusters have a compact 4He_N subsystem that binds the H- impurity on its surface. The results for $N\geq 3$ can be interpreted invoking the different features of the minima of the He-He and He-H- interaction potentials.