Physics - Atomic and Molecular Clusters Publications (50)


Physics - Atomic and Molecular Clusters Publications

When matter is exposed to a high-intensity x-ray free-electron-laser pulse, the x rays excite inner-shell electrons leading to the ionization of the electrons through various atomic processes and creating high-energy-density plasma, i.e., warm dense matter. Read More

The influence of the surface curvature on the surface tension of small droplets in equilibrium with a surrounding vapour, or small bubbles in equilibrium with a surrounding liquid, can be expanded as $\gamma(R) = \gamma_0 + c_1\gamma_0/R + O(1/R^2)$, where $R = R_\gamma$ is the radius of the surface of tension and $\gamma_0$ is the surface tension of the planar interface, corresponding to zero curvature. According to Tolman's law, the first-order coefficient in this expansion is assumed to be related to the planar limit $\delta_0$ of the Tolman length, i.e. Read More

In several settings of physics and chemistry one has to deal with molecules interacting with some kind of an external environment, be it a gas, a solution, or a crystal surface. Understanding molecular processes in the presence of such a many-particle bath is inherently challenging, and usually requires large-scale numerical computations. Here, we present an alternative approach to the problem - that based on the notion of the angulon quasiparticle. Read More

The degree of thermalization of electronically excited state manifolds of an absorber can be tested via optical spectroscopy. In the thermalized-manifold case, the ratio of absorption and emission is expected to follow a universal Boltzmann-type frequency scaling, known as the Kennard-Stepanov relation. Here, we investigate absorption and emission spectral profiles of rubidium, cesium, and potassium molecular dimers in a high-pressure argon buffer-gas environment and study the effect of collisionally induced redistribution. Read More

High-order harmonic generation (HHG) from aligned acetylene molecules interacting with mid infra-red (IR), linearly polarized laser pulses is studied theoretically using a mixed quantum-classical approach in which the electrons are described using time-dependent density functional theory while the ions are treated classically. We find that for molecules aligned perpendicular to the laser polarization axis, HHG arises from the HOMO orbital while for molecules aligned along the laser polarization axis, HHG is dominated by the HOMO-1 orbital. In the parallel orientation the harmonic spectrum comprises a double plateau structure with the inner plateau arising from excitation of an autoionizing state. Read More

We find that negative charges on an armchair single-walled carbon nanotube (SWCNT) can significantly enhance the migration of a carbon adatom on the external surfaces of SWCNTs, along the direction of the tube axis. Nanotube charging results in stronger binding of adatoms to SWCNTs and consequent longer lifetimes of adatoms before desorption, which in turn increases their migration distance several orders of magnitude. These results support the hypothesis of diffusion enhanced SWCNT growth in the volume of arc plasma. Read More

We study plasmonic excitations in the limit of few electrons, in one-atom thick sodium chains, and characterize them based on collectivity. We also compare the excitations to classical localised plasmon modes and find for the longitudinal mode a quantum-classical transition around 10 atoms. The transverse mode appears at much higher energies than predicted classically for all chain lengths. Read More

The molecular absorption of photons with angular momentum can result in twisted excitons with a well-defined quasi-angular momentum. Although they represent different physical properties, photonic and excitonic quanta can both be described in terms of topological charge, a conserved quantity. Multiple absorption events can be used to create a wide range of excitonic topological charges. Read More

This work takes inspiration from chemistry where the spectral characteristics of the molecules are determined by hybridization of electronic states evolving from the individual atomic orbitals. Based on analogy between quantum mechanics and the classical electrodynamics, we sorted dielectric microspheres with almost identical positions of their whispering gallery mode (WGM) resonances. Using these microspheres as classical photonic atoms, we assembled them in a wide range of structures including linear chains and planar photonic molecules. Read More

High harmonic generation in the interaction of femtosecond lasers with atoms and molecules opens the path to molecular orbital tomography and to probe the electronic dynamics with attosecond-{\AA}ngstr\"{o}m resolutions. Molecular orbital tomography requires both the amplitude and phase of the high harmonics. Yet the measurement of phases requires sophisticated techniques and represents formidable challenges at present. Read More

A robust potential wherein is embedded the crucial core polarization interaction is used in the Regge Pole methodology to calculate low energy electron elastic scattering total cross section (TCS) for the C60 fullerene in the electron impact energy range 0.02 through 10.0 eV. Read More

We report a theoretical analysis showing that Rayleigh scattering could be used to monitor the growth of nanoparticles under arc discharge conditions. We compute the Rayleigh scattering cross sections of the nanoparticles by combining light scattering theory for gas-particle mixtures with calculations of the dynamic electronic polarizability of the nanoparticles. We find that the resolution of the Rayleigh scattering probe is adequate to detect nanoparticles as small as C60 at the expected concentrations of synthesis conditions in the arc periphery. Read More

Morphology and its stability are essential features to address physicochemical properties of metallic nanoparticles. By means of Molecular Dynamics based simulations we show a complex dependence on the size and material of common structural mechanisms taking place in mono-metallic nanoparticles at icosahedral magic sizes. We show that the well known Lipscomb s Diamond Square Diamond mechanisms, single step screw dislocation motions of the whole cluster, take place only below a given size which is material dependent. Read More

We present the results of classical molecular dynamics simulations of collision-induced fusion and fragmentation of C$_{60}$ fullerenes, performed by means of the MBN Explorer software package. The simulations provide information on the structural differences of the fused compound depending on kinematics of the collision process. The analysis of fragmentation dynamics at different initial conditions shows that the size distributions of molecular fragments produced are peaked for dimers, which is in agreement with a well-established mechanism of C$_{60}$ fragmentation via preferential C$_2$ emission. Read More

Tributyl-phosphate (TBP), a ligand used in the PUREX liquid-liquid separation process of spent nuclear fuel, can form explosive mixture in contact with nitric acid, that might lead to violent explosive thermal runaway. In the context of safety of a nuclear reprocessing plant facility, it is crucial to predict the stability of TBP at elevated temperatures. So far, only the enthalpies of formation of TBP is available in the literature with a rather large uncertainties, while those of its degradation products, di-(HDBP) and mono-(H$_2$MBP}) are unknown. Read More

The mechanisms of phase change of argon during picosecond laser internal ablation are studied using molecular dynamics simulations. It is found that propagation of stress wave and fluctuation of temperature are periodical. The phase change process from solid to liquid to supercritical fluid then back to solid occurs as combined results of heating and the propagation of tensile stress wave induced by the laser pulse and the limited internal space. Read More

We present a comprehensive analysis of photon emission and atomic collision processes in two-phase argon doped with xenon and nitrogen. The dopants are aimed to convert the VUV emission of pure Ar to the UV emission of the Xe dopant in the liquid phase and to the near UV emission of the N2 dopant in the gas phase. Such a mixture is relevant to two-phase dark matter and low energy neutrino detectors, with enhanced photon collection efficiency for primary and secondary scintillation signals. Read More

Using examples of several well-known, two-body interaction models, this work finds violations of the universality of the large scattering length, $a$, limit. Two classes of underlying interactions are identified. For hard interactions the density approximately scales as $1/k^4$ for momenta that are much less than the inverse of the effective range, $r_e$. Read More

Accurate prediction of the electronic and hydrogen storage properties of linear carbon chains (Cn) and Li-terminated linear carbon chains (Li2Cn), with n carbon atoms (n = 5 - 10), has been very challenging for traditional electronic structure methods, due to the presence of strong static correlation effects. To meet the challenge, we study these properties using our newly developed thermally-assisted-occupation density functional theory (TAO-DFT), a very efficient electronic structure method for the study of large systems with strong static correlation effects. Owing to the alteration of the reactivity of Cn and Li2Cn with n, odd-even oscillations in their electronic properties are found. Read More

The hydration of ions in nanoscale hydrated clusters is ubiquitous and essential in many physical and chemical processes. Here we show that the hydrolysis reaction is strongly affected by relative humidity. The hydrolysis of CO32- with n = 1-8 water molecules is investigated by ab initio method. Read More

Rotation of molecules embedded in He nanodroplets is explored by a combination of fs laser-induced alignment experiments and angulon quasiparticle theory. We demonstrate that at low fluence of the fs alignment pulse, the molecule and its solvation shell can be set into coherent collective rotation lasting long enough to form revivals. With increasing fluence, however, the revivals disappear -- instead, rotational dynamics as rapid as for an isolated molecule is observed during the first few picoseconds. Read More

A novel system containing nanoporous materials and carbonate ions is proposed, which is capable to capture CO2 from ambient air simply by controlling the amount of water (humidity) in the system. The system absorbs CO2 from the air when the surrounding is dry, whereas desorbs CO2 when wet. A design of such a CO2 absorption/desorption system is investigated in this paper using molecular dynamics and quantum mechanics simulations, and also verified by experiments. Read More

We investigate the impact of dissipation on the energy balance in the electron dynamics of metal clusters excited by strong electro-magnetic pulses. The dynamics is described theoretically by Time-Dependent Density-Functional Theory (TDDFT) at the level of Local Density Approximation (LDA) augmented by a self interaction correction term and a quantum collision term in Relaxation-Time Approximation (RTA). We evaluate the separate contributions to the total excitation energy, namely energy exported by electron emission, potential energy due to changing charge state, intrinsic kinetic and potential energy, and collective flow energy. Read More

Structural characterization of metalloporphyrins in complex systems such as native hydrocarbons is in the focus of scientific and industrial interests since many years. We describe electron-nuclear double resonance (ENDOR) of crude oil from the well without any additional sample treatment (i.e. Read More

In this research, we employ time-dependent density functional calculations for photoelectron spectroscopy of nitrogen molecule in short laser pulses. First, the optical absorption spectrum of nitrogen is calculated by using full time propagation and linear response techniques. Then laser pulses with different frequencies, intensities, and lengths are applied to the molecule and the resulting photoelectron spectra are analyzed. Read More

We theoretically analyze angle-resolved photo-electron spectra (ARPES) generated by the interaction of C$_{60}$ with intense, short laser pulses. In particular, we focus on the impact of the carrier-envelope phase (CEP) onto the angular distribution. The electronic dynamics is described by time-dependent density functional theory, and the ionic background of $\csixty$ is approximated by a particularly designed jellium model. Read More

Hydrogen storage by physisorption in carbon based materials is hindered by low adsorption energies. In the last decade doping of carbon materials with alkali, earth alkali or other metal atoms was proposed as a means to enhance adsorption energies, and some experiments have shown promising results. We investigate the upper bounds of hydrogen storage capacities of $C_{60}Cs$ clusters grown in ultracold helium nanodroplets by analyzing anomalies in the ion abundance that indicate shell closure of hydrogen adsorption shells. Read More

The interaction of a helium atom with intense short 800 nm laser pulse is studied theoretically beyond the single-active-electron approximation. For this purpose, the time-dependent Schr\"odinger equation for the two-electron wave packet driven by a linearly-polarized infrared pulse is solved by the time-dependent restricted-active-space configuration-interaction method (TD-RASCI) in the dipole velocity gauge. By systematically extending the space of active configurations, we investigate the role of the collective two-electron dynamics in the strong field ionization and high-order harmonic generation (HHG) processes. Read More

In this contribution I will review some of the researches that are currently being pursued in Padova (mainly within the In:Theory and Strength projects), focusing on the interdisciplinary applications of nuclear theory to several other branches of physics, with the aim of contributing to show the centrality of nuclear theory in the Italian scientific scenario and the prominence of this fertile field in fostering new physics. In particular, I will talk about: i) the recent solution of the long-standing "electron screening puzzle" that settles a fundamental controversy in nuclear astrophysics between the outcome of lab experiments on earth and nuclear reactions happening in stars; the application of algebraic methods to very diverse systems such as: ii) the supramolecular complex H2@C60, i.e. Read More

Partially-self-consistent gap-renormalization GW (grGW) is introduced to calculate quasiparticle (QP) energies within the many-body perturbation theory of Hedin. Self-consistency of the Green's function is obtained by renormalization of the band gap, removing the most significant approximation of the single-shot $\text{G}_{0}\text{W}_{0}$ approach. The formalism is performed as a post-processing step and thus, can be implemented within any GW algorithm which calculates the full frequency-dependent self-energies. Read More

We model an isothermal aggregation process of particles/atoms interacting according to the Lennard-Jones pair potential by mapping the energy landscapes of each cluster size $N$ onto stochastic networks, computing transition probabilities for the network for an $N$-particle cluster to the one for $N+1$, and connecting these networks into a single joint network. The attachment rate is a control parameter. The resulting network representing the aggregation of up to 14 particles contains 6417 vertices. Read More

Quasiparticle (QP) excitations are extremely important for understanding and predicting charge transfer and transport in molecules, nanostructures and extended systems. Since density functional theory (DFT) within Kohn-Sham (KS) formulation does not provide reliable QP energies, a many-body perturbation technique within the GW approximation are essential. The steep computational scaling of GW prohibits its use in extended, open boundary, systems with thousands of electrons and more. Read More

Chirality is ubiquitous in nature and fundamental in science, from particle physics to metamaterials.The most established technique of chiral discrimination - photoabsorption circular dichroism - relies on the magnetic properties of a chiral medium and yields an extremely weak chiral response. We propose and demonstrate a new, orders of magnitude more sensitive type of circular dichroism in neutral molecules: photoexitation circular dichroism. Read More

The ground bound states in the five-body muonic ions $a b \mu e_2$ (or $(a b \mu e_2)^{-}$), where $(a, b) = (p, d, t)$, are considered for the first time. As follows from accurate numerical computations of these five-body ions they are similar to the negatively charged hydrogen ion H$^{-}$ with a three-particle central quasi-nucleus $a b \mu$ (or $(a b \mu)^{+}$). These five-body ions play some role in the muon-catalyzed fusion of nuclear reaction in liquid deuterium and/or deuterium-tritium mixtures. Read More

We present an object-oriented Python library for computation of properties of highly-excited Rydberg states of alkali atoms. These include single-body effects such as dipole matrix elements, excited-state lifetimes (radiative and black-body limited) and Stark maps of atoms in external electric fields, as well as two-atom interaction potentials accounting for dipole and quadrupole coupling effects valid at both long and short range for arbitrary alignment of the atomic dipoles. The package is cross-referenced to precise measurements of atomic energy levels and features extensive documentation to facilitate rapid upgrade or expansion by users. Read More

We study the dynamics of transient charges formed in methane clusters following ionization by intense near-infrared laser pulses. Cluster ionization by 400 fs ($I=1 \times 10^{14}$ W/cm$^2$) pulses is highly efficient, resulting in the observation of a dominant C$^{3+}$ ion contribution. The C$^{4+}$ ion yield is very small, but is strongly enhanced by applying a time-delayed weak near-infrared pulse. Read More

We present a combined experimental and theoretical study of the charging dynamics of helium nanodroplets doped with atoms of different species and irradiated by intense near-infrared (NIR) laser pulses (<10^15 Wcm-2). In particular, we elucidate the interplay of dopant ionization inducing the ignition of a helium nanoplasma, and the charging of the dopant atoms driven by the ionized helium host. Most efficient nanoplasma ignition and charging is found when doping helium droplets with xenon atoms, in which case high charge states both of helium (He2+) and of xenon (Xe^21+) are detected. Read More

The results for binding energies of $^6$Li He$_2$ and $^7$Li He$_2$ systems are presented. They are obtained by solving Faddeev equations in configuration space. It is shown that the excited states in both systems are of the Efimov-type. Read More

It was recently shown that the exact potential driving the electron's dynamics in enhanced ionization of H$_2^+$ can have large contributions arising from dynamical electron-nuclear correlation, going beyond what any electrostatics-based model can provide[1]. This potential is defined via the exact factorization of the molecular wavefunction that allows the construction of a Schr\"odinger equation for the electronic system, in which the potential contains exactly the effect of coupling to the nuclear system and any external fields. Here we study enhanced ionization in isotopologues of H$_2^+$ in order to investigate nuclear-mass-dependence of these terms for this process. Read More

In this communication, an effective set of the Hartree-Fock equations are derived only for electrons of the muonic systems, i.e., molecules containing a positively charged muon, conceiving the muon as a quantum oscillator. Read More

Free-electron laser hard X-ray light sources can provide high fluence, femtosecond pulses, enabling the time-resolved probing of structural dynamics and elementary relaxation processes in molecules. Traditional X-ray elastic scattering from crystals in the ground state consists of sharp Bragg diffraction peaks that arise from pairs of molecules and reveal the ground state charge density. Scattering of ultrashort X-ray pulses from gases, liquids, and even single molecules is more complex and involves both single- and two- molecule contributions, diffuse (non-Bragg) features, elastic and inelastic components, contributions of electronic coherences in nonstationary states, and interferences between scattering off different states (heterodyne detection). Read More

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. Read More

Recent experimental progress in creating and controlling singular electron beams that carry orbital angular momentum allows for new types of local spectroscopies. We theoretically investigate the twisted-electron energy loss spectroscopy (EELS) from the C60 fullerene. Of particular interest are the strong multipolar collective excitations and their selective response to the orbital angular momentum of the impinging electron beam. Read More

Hydrogen bond (H-bond) covalency has recently been observed in ice and liquid water, while the penetrating molecular orbitals (MOs) in the H-bond region of most typical water dimer system, (H2O)2, have also been discovered. However, obtaining the quantitative contribution of these MOs to the H-bond interaction is still problematic. In this work, we introduced the orbital-resolved electron density projected integral (EDPI) along the H-bond to approach this problem. Read More

We describe a beam splitter for polar neutral molecules. An electrostatic hexapole initially confines and guides a supersonic expansion of ammonia, and it then smoothly transforms into two bent quadrupole guides, thus splitting the molecular beam in two correlated fractions. This paves the way towards molecular beam experiments wherein one beam is modified through interactions with, e. Read More

We have recorded the coherent diffraction images of individual xenon clusters using intense extreme ultraviolet free-electron laser pulses tuned to atomic and ionic resonances in order to elucidate the influence of light induced electronic changes on the diffraction pattern. The data show the emergence of a transient core-shell structure within the otherwise homogeneous sample. Simulations indicate that ionization and nanoplasma formation result in a cluster shell with strongly altered refraction. Read More

Non Born-Oppenheimer quantum dynamics of H$_{2}^{+}$ excited by shaped one-cycle laser pulses linearly polarized along the molecular axis have been studied by the numerical solution of the time-dependent Schr\"odinger equation within a %three-body three-dimensional model, including the internuclear separation, $R$, and the electron coordinates $z$ and $\rho$. Laser carrier frequencies corresponding to the wavelengths $\lambda_{l}=25$~nm through $\lambda_{l}=400$~nm were used and the amplitudes of the pulses were chosen such that the energy of H$_{2}^{+}$ was close to its dissociation threshold at the end of any laser pulse applied. It is shown that there exists a characteristic oscillation frequency $\omega_{\rm osc} \simeq 0. Read More

Recent developments in attosecond spectroscopy yield access to the correlated motion of electrons on their intrinsic time scales. Spin-flip dynamics is usually considered in the context of valence electronic states, where spin-orbit coupling is weak and processes related to the electron spin are usually driven by nuclear motion. However, for core-excited states, where the core hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin-flips on a much shorter time scale. Read More

The reliable prediction of optical and fundamental gaps of finite size systems using density functional theory requires to account for the potential self-interaction error, which is notorious for degrading the description of charge transfer transitions. One solution is provided by parameterized long-range corrected functionals such as LC-BLYP, which can be tuned such as to describe certain properties of the particular system at hand. Here, bare and 3-mercaptoprotionic acid covered \ce{Cd33Se33} quantum dots are investigated using the optimally tuned LC-BLYP functional. Read More