Atomic And Molecular Clusters

---

Observation of highly dispersive bands in pure thin film C (1902.10827v1)

Drew W. Latzke, Claudia Ojeda-Aristizabal, Sinéad M. Griffin, Jonathan D. Denlinger, Jeffrey B. Neaton, Alex Zettl, Alessandra Lanzara

2019-02-27

While long-theorized, the direct observation of multiple highly dispersive C valence bands has eluded researchers for more than two decades due to a variety of intrinsic and extrinsic factors. Here we report a realization of multiple highly dispersive (330-520 meV) valence bands in pure thin film C on a novel substrate--the three-dimensional topological insulator BiSe--through the use of angle-resolved photoemission spectroscopy (ARPES) and first-principles calculations. The effects of this novel substrate reducing C rotational disorder are discussed. Our results provide important considerations for past and future band structure studies as well as the increasingly popular C electronic device applications, especially those making use of heterostructures.

Time-resolved observation of interatomic Coulombic decay induced by two-photon double excitation of Ne (1902.09882v1)

T. Takanashi, N. V. Golubev, C. Callegari, H. Fukuzawa, K. Motomura, D. Iablonskyi, Y. Kumagai, S. Mondal, T. Tachibana, K. Nagaya, T. Nishiyama, K. Matsunami, P. Johnsson, P. Piseri, G. Sansone, A. Dubrouil, M. Reduzzi, P. Carpeggiani, C. Vozzi, M. Devetta, M. Negro, D. Faccialà, F. Calegari, A. Trabattoni, M. C. Castrovilli, Y. Ovcharenko, M. Mudrich, F. Stienkemeier, M. Coreno, M. Alagia, B. Schütte, N. Berrah, O. Plekan, P. Finetti, C. Spezzani, E. Ferrari, E. Allaria, G. Penco, C. Serpico, G. De Ninno, B. Diviacco, S. Di Mitri, L. Giannessi, G. Jabbari, K. C. Prince, L. S. Cederbaum, Ph. V. Demekhin, A. I. Kuleff, K. Ueda

2019-02-26

The hitherto unexplored two-photon doubly-excited states [Ne()] were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD) which predominantly produces singly-ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne ions was recorded as a function of delay between the XUV pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly-excited states, 390 (-130 / +450} fs, and of the short-lived ones, less than 150~fs, are in good agreement with \emph{ab initio} quantum mechanical calculations.

Simulating the structural diversity of carbon clusters across the planar to fullerene transition (1902.09593v1)

Maëlle A. Bonnin, Cyril Falvo, Florent Calvo, Thomas Pino, Pascal Parneix

2019-02-25

Together with the second generation REBO reactive potential, replica-exchange molecular dynamics simulations coupled with systematic quenching were used to generate a broad set of isomers for neutral C clusters with , 42, and 60. All the minima were sorted in energy and analyzed using order parameters to monitor the evolution of their structural and chemical properties. The structural diversity measured by the fluctuations in these various indicators is found to increase significantly with energy, the number of carbon rings, especially 6-membered, exhibiting a monotonic decrease in favor of low-coordinated chains and branched structures. A systematic statistical analysis between the various parameters indicates that energetic stability is mainly driven by the amount of sp hybridization, more than any geometrical parameter. The astrophysical relevance of these results is discussed in the light of the recent detection of C and C fullerenes in the interstellar medium.

Magic Numbers for the Photoelectron Anisotropy in Li-Doped Dimethyl Ether Clusters (1902.09575v1)

Jonathan V. Barnes, Bruce L. Yoder, Ruth Signorell

2019-02-25

Photoelectron velocity map imaging of Li(CHOCH) clusters (1 n 175) is used to search for magic numbers related to the photoelectron anisotropy. Comparison with density functional calculations reveals magic numbers at n=4, 5, and 6, resulting from the symmetric charge distribution with high s-character of the highest occupied molecular orbital. Since each of these three cluster sizes correspond to the completion of a first coordination shell, they can be considered as 'isomeric motifs of the first coordination shell'. Differences in the photoelectron anisotropy, the vertical ionization energies and the enthalpies of vaporization between Li(CHOCH) and Na(CHOCH) can be rationalized in terms of differences in their solvation shells, atomic ionization energies, polarizabilities, metal-oxygen bonds, ligand-ligand interactions, and by cooperative effects.

Propensity rules in photoelectron circular dichroism in chiral molecules I: Chiral hydrogen (1806.09049v2)

Andres F. Ordonez, Olga Smirnova

2018-06-23

Photoelectron circular dichroism results from one-photon ionization of chiral molecules by circularly polarized light and manifests itself in forward-backward asymmetry of electron emission in the direction orthogonal to the light polarization plane. What is the physical mechanism underlying asymmetric electron ejection? How "which way" information builds up in a chiral molecule and maps into forward-backward asymmetry? We introduce instances of bound chiral wave functions resulting from stationary superpositions of states in a hydrogen atom and use them to show that the chiral response in one-photon ionization of aligned molecular ensembles originates from two propensity rules: (i) Sensitivity of ionization to the sense of electron rotation in the polarization plane. (ii) Sensitivity of ionization to the direction of charge displacement or stationary current orthogonal to the polarization plane. In the companion paper we show how the ideas presented here are part of a broader picture valid for all chiral molecules and arbitrary degrees of molecular alignment.

Prediction of activation energy barrier of island diffusion processes using data-driven approaches (1902.10282v1)

Shree Ram Acharya, Talat S. Rahman

2019-02-22

We present models for prediction of activation energy barrier of diffusion process of adatom (1-4) islands obtained by using data-driven techniques. A set of easily accessible features, geometric and energetic, that are extracted by analyzing the variation of the energy barriers of a large number of processes on homo-epitaxial metallic systems of Cu, Ni, Pd, and Ag are used along with the activation energy barriers to train and test linear and non-linear statistical models. A multivariate linear regression model trained with energy barriers for Cu, Pd, and Ag systems explains 92% of the variation of energy barriers of the Ni system, whereas the non-linear model using artificial neural network slightly enhances the success to 93%. Next mode of calculation that uses barriers of all four systems in training, predicts barriers of randomly picked processes of those systems with significantly high correlation coefficient: 94.4% in linear regression model and 97.7% in artificial neural network model. Calculated kinetics parameters such as the type of frequently executed processes and effective energy barrier for Ni dimer and trimer diffusion on the Ni(111) surface obtained from KMC simulation using the predicted (data-enabled) energy barriers are in close agreement with those obtained by using energy barriers calculated from interatomic interaction potential.

Propensity rules in photoelectron circular dichroism in chiral molecules II: General picture (1806.09050v2)

Andres F. Ordonez, Olga Smirnova

2018-06-23

Photoelectron circular dichroism results from one-photon ionization of chiral molecules by circularly polarized light and manifests itself in forward-backward asymmetry of electron emission in the direction orthogonal to the light polarization plane. To expose the physical mechanism responsible for asymmetric electron ejection, we first establish a rigorous relation between the responses of unaligned and partially or perfectly aligned molecules. Next, we identify a propensity field, which is responsible for the chiral response in the electric-dipole approximation, i.e. a chiral response without magnetic interactions. We find that this propensity field, up to notations, is equivalent to the Berry curvature in a two-band solid. The propensity field directly encodes optical propensity rules, extending our conclusions regarding the role of propensity rules in defining the sign of forward-backward asymmetry from the specific case of chiral hydrogen to generic chiral systems. Optical propensity rules underlie the chiral response in photoelectron circular dichroism. The enantiosensitive flux of the propensity field through the sphere in momentum space determines the forward-backward asymmetry in unaligned molecules and suggests a geometrical origin of the chiral response. This flux has opposite sign for opposite enantiomers and vanishes for achiral molecules.

Linear increase of the 2+ ion concentration in the double photoionization of aromatic molecules (1805.00891v3)

D. L. Huber

2018-05-02

We investigate the linear behavior in the 2+ ion concentration observed in the double photoionization of a variety of aromatic molecules. We show it arises when the photoelectrons are emitted simultaneously. Neglecting the momentum of the incoming photon and the momentum transferred to the molecule, it follows that the momenta of the individual photoelectrons are oppositely directed and equal in magnitude. Under steady-state conditions, the ion concentration is proportional to the rate at which the ions are created which, in turn, varies as the product of the densities of states of the individual electrons. The latter vary as the square root of the kinetic energy, leading to overall linear behavior. The origin of the linear behavior in pyrrole and related molecules is attributed to the presence of atoms that destroy the periodicity of a hypothetical carbon loop. In contrast, the resonant behavior observed in pyridine and related molecules, where a fraction of the CH pairs is replaced by N atoms, is associated with electron transfer between the nitrogen atoms and carbon atoms that preserves the periodicity of the closed loop.

Autoionization dynamics of He nanodroplets resonantly excited by intense XUV laser pulses (1902.05332v1)

Y. Ovcharenko, A. LaForge, B. Langbehn, O. Plekan, R. Cucini, P. Finetti, P. O'Keeffe, D. Iablonskyi, T. Nishiyama, K. Ueda, P. Piseri, M. DiFraia, R. Richter, M. Coreno, C. Callegari, K. C. Prince, F. Stienkemeier, T. Moller, M. Mudrich

2019-02-14

The ionization dynamics of helium droplets in a wide size range from 220 to 10^6 He atoms irradiated with intense femtosecond extreme ultraviolet (XUV) pulses of 10^9 {\div} 10^{12} W/cm2 power density is investigated in detail by photoelectron spectroscopy. Helium droplets are resonantly excited in the photon energy range from ~ 21 eV (corresponding to the atomic 1s2s state) up to the atomic ionization potential (IP) at ~ 25 eV. A complex evolution of the electron spectra as a function of droplet size and XUV intensity is observed, ranging from atomic-like narrow peaks due to binary autoionization, to an unstructured feature characteristic of electron emission from a nanoplasma. The experimental results are analyzed and interpreted with the help of numerical simulations based on rate equations taking into account various processes such as multi-step ionization, interatomic Coulombic decay (ICD), secondary inelastic collisions, desorption of electronically excited atoms, collective autoionization (CAI) and further relaxation processes.

Dissociative electron attachment to pulsed supersonic O jet : Violation of selection rule and dependence on carrier gas proportion (1902.05279v1)

Irina Jana, Varun Ramaprasad, Dhananjay Nandi

2019-02-14

The formation of and ions via dissociative electron attachment to a pulsed supersonic jet of molecules containing weakly bound small van der Waals clusters seeded in a beam of argon is reported. The energy dependence of the and yield exhibits three peaks near 7, 11 and 16 eV incident electron energies. The 7 eV peak arises from the state of whereas, the 11 and 16 eV peaks are ascribed to two distinct resonance states: and states of , respectively, via a violation of the selection rule. The dependence of the cross-section of these two new peaks at 11 and 16 eV on the proportion of the carrier gas is also investigated and an optimum proportion has been observed experimentally which gives the lowest temperature of 14.86 K and highest Mach number of 72.31 for the pulsed supersonic jet.

---

Keeping updated in the latest research in atomic and molecular clusters!