Herein, the structure and stability of double icosahedron Ag$_{17}$M$_2$ (M = Ni, Cu, Zn) clusters are investigated using density functional theory (DFT) computations. The results indicate that the clusters favor endohedral configurations in the doublet state, as confirmed with four different functionals: BP86, PBE0, B3PW91, and TPSSh. Additionally, the doped clusters exhibit higher ionization energies and electronegativities compared those of the bare Ag$_{19}$ cluster. After doping, the ELF function increases at the Ag sites, which reveals important implications for catalysis.
Anirban Paul, Ian Carmichael, Dhananjay Nandi
et al.
Exploring the molecular fragmentation dynamics induced by low-energy electrons offers compelling insights into the complex interplay between the projectile and target. In this study, we investigate the phenomenon of dissociative electron attachment to ethyl acetate. The recorded yields of various fragment anions within an incident electron energy range of 1 to 13 eV reveal a diverse array of products with six different mass numbers. Examples include (M$-$H)$^-$, CH$_3^-$, C$_2$H$_5$O$^-$, CH$_3$CO$^-$, CH$_2$CHO$^-$, and CH$_3$COO$^-$, formed through the fracture of single bonds. Interestingly, the generation of other fragments, such as HCCO$^-$, suggests a more intricate structural rearrangement of the nuclei, adding a layer of complexity to the observed dissociation dynamics.
Complete dissociation dynamics of low energy electron attachment to nitrogen dioxide around 8.5 eV resonance has been studied using a velocity map imaging (VMI) spectrometer. Besides the three prominent resonant peaks at around 1.4 eV, 3.1 eV, and 8.5 eV, we have found an additional small resonance at the higher energy tail of the 8.5 eV resonance. We have collected the momentum distribution data of O$^-$ ions at different incident electron energies around the 8.5 eV resonance along with the smaller additional resonant peak. A theoretical analysis of these resonances with the momentum imaging experimental data on dissociative electron attachment to nitrogen dioxide in the gas phase is used to provide a detailed picture of the molecular dissociation process.
My thesis mostly focusses on the systems of porphyrin molecules adsorbed on single-crystalline metallic surfaces. Cyclic tetrapyrrole porphyrins play key roles in many important chemical and biological processes, such as oxygen transport in heme (iron porphyrin), electron transfer and oxidation reactions in photosynthetic chlorophyll (magnesium porphyrin). Comprehensive understanding of the magnetic and conformational properties of single porphyrin molecules adsorbed on metallic substrates attracts intensive research interest. In my thesis, I have studied the structural and electronics properties of porphyrin molecules by Low-temperature scanning tunneling spectroscopy (LT-STM) and scanning tunneling spectroscopy (STS) and theoretical methods. Owing to the high resolution of LT-STM, both geometric and electronic properties at the atomic level were probed. Moreover, the experimental results were understood by comprehensive theoretical methods, i.e. density functional theory, molecular dynamics, tight-binding and plane-wave expansion calculations.
In processes when particles such as nanodroplets, clusters, or molecules move through a dilute background gas and undergo capture collisions, it is often important to know how much translational kinetic energy is deposited into the particles by these pick-up events. For sticking collisions with a Maxwell-Boltzmann gas, an exact expression is derived which is valid for arbitrary relative magnitudes of the particle and thermal gas speeds.
We discuss the temporal picture of electron collisions with fullerene. Within the framework of a Dirac bubble potential model for the fullerene shell, we calculate the time delay in slow-electron elastic scattering by it. It appeared that the time of transmission of an electron wave packet through the Dirac bubble potential sphere that simulates a real potential of the C60 reaches up to 104 attoseconds. Resonances in the time delays are due to the temporary trapping of electron into quasi-bound states before it leaves the interaction region. As concrete targets we choose almost ideally spherical endohedrals C20, C60, C72, and C80. We present dependences of time-delay upon collision energy.
Using the extensive set of stations in the SuperMAG collaboration, we introduce partial ring current indices, which provide new insights into ring current development. The indices are labeled SMR‐00, SMR‐06, SMR‐12 and SMR‐18 for their center local time range. These indices incorporate data from 98 mid and low latitude stations. The behavior of these local time indices during storms and substorms, on both an individual and superposed epoch basis, produces consistent patterns. The initial positive spike before a storm, which results from solar wind pressure enhancements, is seen simultaneously at all local times. Once the main phase of the storm begins, however, SMR‐18 nearly always drops fastest and furthest in magnitude, while SMR‐06 drops more slowly (i.e., has weaker ring current signatures), and never as far. Symmetry is, in fact, not reached until storm recovery is well underway, with a typical symmetry point of about 20–25 h after onset. If the main phase continues to new depths (larger magnitude negative SMR) over a longer time period, the SMR‐18 sector will continue to lead, and SMR‐06 to lag. There has been controversy over the extent to which substorm auroral and cross‐tail currents perturb Dst and SYM‐H signatures. The signature of substorms can be seen very clearly as a positive spike of roughly 10 nT magnitude in SMR‐00, and only to a much lesser extent elsewhere. The SMR global index, and thus also SYM‐H, experiences only a small immediate perturbation from a substorm onset, ending with a net drop of a few nT. Since there are typically only 1–2 substorms in a main phase, substorms are a minor factor in the development of the storm time ring current. Indeed, because even the peak perturbation of substorms currents in the most affected sector (which is midnight) is nearly an order of magnitude smaller than the storm perturbation (about 10 nT versus 80 nT), fluctuations in the cross‐tail and field‐aligned currents in general are not a major influence over SMR. The pattern of LT substorm responses, with a strong positive SMR‐00 effect, weak positive SMR‐06 effect, and negative SMR‐18 effect implies it is field‐aligned currents and not the cross‐tail currents which create modest perturbations in the putatively ring current indices.
Michael Renzler, Matthias Daxner, Nikolaus Weinberger
et al.
The mechanism of ionization of helium droplets has been investigated in numerous reports but one observation has not found a satisfactory explanation: How are $He^+$ ions formed and ejected from undoped droplets at electron energies below the ionization threshold of the free atom? Does this path exist at all? A measurement of the ion yields of $He^+$ and $He_2^+$ as a function of electron energy, electron emission current, and droplet size reveals that metastable $He^{*-}$ anions play a crucial role in the formation of free $He^+$ at subthreshold energies. The proposed model is testable.
A. Crawford-Uranga, D. J. Mowbray, D. M. Cardamone
We show that additional features can emerge in the linear absorption spectra of homonuclear diatomic molecules when the ions are described quantum mechanically. In particular, the widths and energies of the peaks in the optical spectra change with the initial configuration, mass, and charge of the molecule. We introduce a model that can describe these features and we provide a quantitative analysis of the resulting peak energy shifts and width broadenings as a function of the mass.
We study the dissociation of H$_2$$^+$ in uv laser pulses by solving the non-Born-Oppenheimer time-dependent Schrödinger equation as a function of the photon energy $ω$ of the pulse. Significant enhancements of the dissociation into highly excited electronic states are observed at critical $ω$. This is found to be attributed to a sequential resonant excitation mechanism where the population is firstly transferred to the first excited state by absorbing one photon and sequentially to higher states by absorbing another one or more photons at the same internuclear distance. We have substantiated the underlying dynamics by separately calculating the nuclear kinetic energy spectra for individual dissociation pathways through different electronic states.
J. Kissinger, F. D. Wilder, R. L. McPherron
et al.
AbstractThe Harang discontinuity is a longitudinally extended ionospheric signature near midnight of flow reversal from westward to eastward with decreasing latitude. Its occurrence indicates enhanced convection in the magnetotail that requires an upward field‐aligned current from the ionosphere due to diamagnetic ion drift. Previous reports using event studies have been conflicting as to the occurrence of the Harang discontinuity during a mode of enhanced magnetotail response called steady magnetospheric convection (SMC). With a comprehensive list of SMC events from 1997 through 2007, we utilize data from the Super Dual Auroral Radar Network and a novel spatial superposition technique to statistically examine the occurrence of the Harang discontinuity during SMC events. We find that the statistical signature of the Harang discontinuity begins before SMC starts and strengthens as the SMC events progress. We also detail the typical size and strength of the Harang discontinuity and find that it is more pronounced during interplanetary magnetic field +By conditions.
Magnesium atoms embedded in superfluid helium nanodroplets have been identified to arrange themselves in a metastable network, refered to as foam. In order to investigate the ionization dynamics of this unique structure with respect to a possible light-induced collapse the femtosecond dual-pulse spectroscopy technique is applied. Around zero optical delay a strong feature is obtained which represents a direct probe of the foam response. We found that upon collapse, ionization is reduced. A particlar intensity ratio of the pulses allows to address either direct ionization or photoactivation of the neutral complexes, thus affecting reaction pathways. A simplified excitation scheme visualizes possible scenarios in accordance with the experimental observations.
The absolute differential oscillator strengths (DOS's) for the photoabsorption of the Ne, Ar, and Xe atoms encapsulated in the C$_{60}$ have been evaluated using the time-dependent-density-functional-theory, which solves the quantum Liouvillian equation with the Lanczos chain method. The calculations are performed in the energy regions both inside and outside the C$_{60}$ giant resonance. The photoabsorption spectra of the atoms encaged in the C$_{60}$ demonstrate strong oscillations inside the energy range of the C$_{60}$ giant resonance. This type of oscillation cannot be explained by the confinement resonance, but is due to the energy transfer from the C$_{60}$ valence electrons to the photoelectron through the intershell coupling.
Massively parallel ionization of many atoms in a cluster or bio-molecule is identified as new phenomenon of light-matter interaction which becomes feasible through short and intense FEL pulses. Almost simultaneously emitted from the illuminated target the photo-electrons can have such a high density that they interact substantially even after photo-ionization. This interaction results in a characteristic electron spectrum which can be interpreted as convolution of a mean-field electron dynamics and binary electron-electron collisions. We demonstrate that this universal spectrum can be obtained analytically by summing synthetic two-body Coulomb collision events. Moreover, we propose an experiment with hydrogen clusters to observe massively parallel ionization.
In the present paper, we have studied atomic structure of nitrogenous austenite. High precision ab-initio calculation was utilized for the calculation of the pair potentials of interatomic interactions N-N in FCC Fe lattice. These potentials were used for the Monte Carlo modeling of the short range order in the Fe-N system. It was discovered that in FCC Fe lattice, nitrogen atoms might be partially ordered. In this case, atomic structure of nitrogenous austenite is characterized by availability of the Fe6N phase with the short range order over the N atoms located in the third coordination sphere.
The interaction of intense laser fields with silver and argon clusters is investigated theoretically using a modified nanoplasma model. Single pulse and double pulse excitations are considered. The influence of the dense cluster environment on the inner ionization processes is studied including the lowering of the ionization energies. There are considerable changes in the dynamics of the laser-cluster interaction. Especially, for silver clusters, the lowering of the ionization energies leads to increased yields of highly charged ions.
We study the elastic scattering of atomic argon by a electron in the presence of a bichromatic laser field. The numerical calculation is done in the first Born approximation (FBA) for a simple screening electric potential. With the help of numerical results we explore the dependence of the differential cross sections (DCS) on the relative phase between the two components of the radiation field and discuss the influence of the number of photons exchanged on the phase-dependence effect. Moreover, we also discuss the numerical results of the DCS for different scattering angles and impact energies.
The surface plasmon in simple metal clusters is red-shifted from the Mie frequency, the energy shift being significantly larger than the usual spill-out correction. Here we develop a variational approach to the RPA collective excitations. Using a simple trial form, we obtain analytic expressions for the energy shift beyond the spill-out contribution. We find that the additional red shift is proportional to the spill-out correction and can have the same order of magnitude.
We study the carbon-dope aluminum clusters by using time-of-flight mass spectrum experiments and {\em ab initio} calculations. Mass abundance distributions are obtained for anionic aluminum and aluminum-carbon mixed clusters. Besides the well-known magic aluminum clusters such as Al$_{13}^-$ and Al$_{23}^-$, Al$_7$C$^-$ cluster is found to be particularly stable among those Al$_n$C$^-$ clusters. Density functional calculations are performed to determine the ground state structures of Al$_n$C$^-$ clusters. Our results show that the Al$_7$C$^-$ is a magic cluster with extremely high stability, which might serve as building block of the cluster-assembled materials.
A flexible scheme for decomposing the vibrational density of states in terms of pair vibrational density of states is presented. This scheme provides the linkage between the site vibrational density of states and pair vibrational density of states so that vibrational modes, in particular localized modes, can be conveniently examined in terms of the correlation between the vibration at a given site and those at its neighboring sites. Furthermore, within the framework of a total energy vibrational spectrum study, this scheme allows the analysis of vibrational modes in terms of their electronic origin. A case study of the vibrational dynamics of the relaxed Si$_{87}$ cluster is carried out to demonstrate the flexibility of the scheme in analyzing the properties of vibrational modes, particularly for complex systems with reduced or no symmetry.