Hollow Gold Nanoparticles Produced by Femtosecond Laser Irradiation.

Metallic hollow nanoparticles exhibit interesting optical properties that can be controlled by geometrical parameters. Irradiation with femtosecond laser pulses has emerged recently as a valuable tool for reshaping and size modification of plasmonic metal nanoparticles, thereby enabling the synthesis of nanostructures with unique morphologies. In this Letter, we use classical molecular dynamics simulations to investigate the solid-to-hollow conversion of gold nanoparticles upon femtosecond laser irradiation. Here, we suggest an efficient method for producing hollow nanoparticles under certain specific conditions, namely that the particles should be heated to a maximum temperature between 2500 and 3500 K, followed by a fast quenching to room temperature, with cooling rates lower than 120 ps. Therefore, we describe the experimental conditions for efficiently producing hollow nanoparticles, opening a broad range of possibilities for applications in key areas, such as energy storage and catalysis.

Formation of Hollow Gold Nanocrystals by Nanosecond Laser Irradiation.

The irradiation of spherical gold nanoparticles (AuNPs) with nanosecond laser pulses induces shape transformations yielding nanocrystals with an inner cavity. The concentration of the stabilizing surfactant, the use of moderate pulse fluences, and the size of the irradiated AuNPs determine the efficiency of the process and the nature of the void. Hollow nanocrystals are obtained when molecules from the surrounding medium (e.g., water and organic matter derived from the surfactant) are trapped during laser pulse irradiation. These experimental observations suggest the existence of a subtle balance between the heating and cooling processes experienced by the nanocrystals, which induce their expansion and subsequent recrystallization keeping exogenous matter inside. The described approach provides valuable insight into the mechanism of interaction of a pulsed nanosecond laser with AuNPs, along with interesting prospects for the development of hollow plasmonic nanoparticles with potential applications related to gas and liquid storage at the nanoscale.

Ab initio intermolecular potential energy surface for the CO2-N2 system and related thermophysical properties.

A four-dimensional potential energy surface (PES) for the interaction between a rigid carbon dioxide molecule and a rigid nitrogen molecule was constructed based on quantum-chemical ab initio calculations up to the coupled-cluster level with single, double, and perturbative triple excitations. Interaction energies for a total of 1893 points on the PES were calculated using the counterpoise-corrected supermolecular approach and basis sets of up to quintuple-zeta quality with bond functions. The interaction energies were extrapolated to the complete basis set limit, and an analytical site-site potential function with seven sites for carbon dioxide and five sites for nitrogen was fitted to the interaction energies. The CO2-N2 cross second virial coefficient as well as the dilute gas shear viscosity, thermal conductivity, and binary diffusion coefficient of CO2-N2 mixtures were calculated for temperatures up to 2000 K to validate the PES and to provide reliable reference values for these important properties. The calculated values are in very good agreement with the best experimental data.

Collision-induced rotational excitation in N2 (+)((2)Σg (+),v=0)-Ar: Comparison of computations and experiment.

The collisional dynamics of N2 (+)((2)Σg (+)) cations with Ar atoms is studied using quasi-classical simulations. N2 (+)-Ar is a proxy to study cooling of molecular ions and interesting in its own right for molecule-to-atom charge transfer reactions. An accurate potential energy surface (PES) is constructed from a reproducing kernel Hilbert space (RKHS) interpolation based on high-level ab initio data. The global PES including the asymptotics is fully treated within the realm of RKHS. From several ten thousand trajectories, the final state distribution of the rotational quantum number of N2 (+) after collision with Ar is determined. Contrary to the interpretation of previous experiments which indicate that up to 98% of collisions are elastic and conserve the quantum state, the present simulations find a considerably larger number of inelastic collisions which supports more recent findings.

Communication: Equilibrium rate coefficients from atomistic simulations: The O((3)P) + NO((2)Π) → O2(X(3)Σg(-)) + N((4)S) reaction at temperatures relevant to the hypersonic flight regime.

The O((3)P) + NO((2)Π) → O2(X(3)Σg(-)) + N((4)S) reaction is among the N- and O- involving reactions that dominate the energetics of the reactive air flow around spacecraft during hypersonic atmospheric re-entry. In this regime, the temperature in the bow shock typically ranges from 1000 to 20,000 K. The forward and reverse rate coefficients for this reaction derived directly from trajectory calculations over this range of temperature are reported in this letter. Results compare well with the established equilibrium constants for the same reaction from thermodynamic quantities derived from spectroscopy in the gas phase which paves the way for large-scale in silico investigations of equilibrium rates under extreme conditions.

Computational study of collisions between O(3P) and NO(2Π) at temperatures relevant to the hypersonic flight regime.

Reactions involving N and O atoms dominate the energetics of the reactive air flow around spacecraft when reentering the atmosphere in the hypersonic flight regime. For this reason, the thermal rate coefficients for reactive processes involving O((3)P) and NO((2)Π) are relevant over a wide range of temperatures. For this purpose, a potential energy surface (PES) for the ground state of the NO2 molecule is constructed based on high-level ab initio calculations. These ab initio energies are represented using the reproducible kernel Hilbert space method and Legendre polynomials. The global PES of NO2 in the ground state is constructed by smoothly connecting the surfaces of the grids of various channels around the equilibrium NO2 geometry by a distance-dependent weighting function. The rate coefficients were calculated using Monte Carlo integration. The results indicate that at high temperatures only the lowest A-symmetry PES is relevant. At the highest temperatures investigated (20,000 K), the rate coefficient for the "O1O2+N" channel becomes comparable (to within a factor of around three) to the rate coefficient of the oxygen exchange reaction. A state resolved analysis shows that the smaller the vibrational quantum number of NO in the reactants, the higher the relative translational energy required to open it and conversely with higher vibrational quantum number, less translational energy is required. This is in accordance with Polanyi's rules. However, the oxygen exchange channel (NO2+O1) is accessible at any collision energy. Finally, this work introduces an efficient computational protocol for the investigation of three-atom collisions in general.

Argon solid response upon Rydberg photoexcitation of the NO chromosphore: case of using ab initio potential energy surfaces and comparison to similar studied systems.

Molecular dynamics simulations of NO-doped Ar solid upon Rydberg photoexcitation of the impurity have been carried out taking into account angular dependent potential energy surfaces (PESs) in the ground and excited states. To go beyond isotropic potentials simulations, the effects of anisotropy of potentials on the structure, dynamics, and energetics are investigated by taking into account two cases, namely, the whole PESs and the isotropic parts. Results have been compared to those obtained in previous works for similar NO-doped rare gas systems. Radial distribution functions (RDF) for the ground and excited state indicate that for both cases the shell structure of the lattice is kept ordered and is characterized by well-defined bands. No influence of the anisotropy of potentials has been detected in the RDFs since the anisotropy is rather manifested at short distances. The well part, which has been proven to be unimportant for the dynamics in previous works, arises here to be important for the right simulation of the spectrum. In general, our results show a reasonable agreement with respect to the experimental values for the dynamics and energetics when ab initio potentials are used, although better results can be obtained if higher level ab initio PESs are used.

An ab initio study of the Ar-NO(A 2Sigma+) intermolecular potential.

More complete molecular dynamics simulations of NO doped Ar solid upon photoexcitation of the impurity should include effects of angular variations of Ar-NO intermolecular potential. This is the main reason for presenting in this work an ab initio study of the Ar-NO(A (2)Sigma(+)) intermolecular potential. Ab initio calculations were carried out at the level of CASSCF-MRCI, with the aug-cc-pVTZ basis sets. In order to evaluate the influence of the quadruple excitations on the topology of the potential energy surface (PES), two cases were considered, that is, with and without taking into account Davidson's correction for quadruple excitations during the calculations. An analytical representation of the PES has been obtained as a function of the Jacobi coordinates of the system. In general, the PES is repulsive, except for linear directions, where two shallow wells appear. When quadruple excitations are considered, wells are located at 4.2 A (alpha=0 degrees) and 6.08 A (alpha=180 degrees) with energies of -20 and -15 cm(-1), respectively; and when are not considered, wells are located at 6.1 A (alpha=0 degrees) and 6.8 A (alpha=180 degrees) with energies of -15 and -10 cm(-1), respectively. For distances beyond 7 A, it is observed a very low energy decay and a rapid tendency to isotropic interactions.