Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The discovery of the photoelectric effect by Heinrich Hertz in 1887 set the foundation for over 125 years of hot carrier science and technology. In the early 1900s it played a critical role in the development of quantum mechanics, but even today the unique properties of these energetic, hot carriers offer new and exciting opportunities for fundamental research and applications. Measurement of the kinetic energy and momentum of photoejected hot electrons can provide valuable information on the electronic structure of materials. The heat generated by hot carriers can be harvested to drive a wide range of physical and chemical processes. Their kinetic energy can be used to harvest solar energy or create sensitive photodetectors and spectrometers. Photoejected charges can also be used to electrically dope two-dimensional materials. Plasmon excitations in metallic nanostructures can be engineered to enhance and provide valuable control over the emission of hot carriers. This Review discusses recent advances in the understanding and application of plasmon-induced hot carrier generation and highlights some of the exciting new directions for the field.

Plasmon-induced hot carrier science and technology

Many-particle charged-particle plasma simulations using spatial meshes for the electromagnetic field solutions, particle-in-cell (PIC) merged with Monte Carlo collision (MCC) calculations, are coming into wide use for application to partially ionized gases. The author emphasizes the development of PIC computer experiments since the 1950s starting with one-dimensional (1-D) charged-sheet models, the addition of the mesh, and fast direct Poisson equation solvers for 2-D and 3-D. Details are provided for adding the collisions between the charged particles and neutral atoms. The result is many-particle simulations with many of the features met in low-temperature collision plasmas; for example, with applications to plasma-assisted materials processing, but also related to warmer plasmas at the edges of magnetized fusion plasmas. >

Particle-in-Cell Charged Particle Simulations, plus Monte Carlo Collisions with Neutral Atoms, PIC-MCC

Imaging mass spectrometry combines the chemical specificity and parallel detection of mass spectrometry with microscopic imaging capabilities. The ability to simultaneously obtain images from all analytes detected, from atomic to macromolecular ions, allows the analyst to probe the chemical organization of a sample and to correlate this with physical features. The sensitivity of the ionization step, sample preparation, the spatial resolution, and the speed of the technique are all important parameters that affect the type of information obtained. Recently, significant progress has been made in each of these steps for both secondary ion mass spectrometry (SIMS) and matrix-assisted laser desorption/ionization (MALDI) imaging of biological samples. Examples demonstrating localization of proteins in tumors, a reduction of lamellar phospholipids in the region binding two single celled organisms, and sub-cellular distributions of several biomolecules have all contributed to an increasing upsurge in interest in imaging mass spectrometry. Here we review many of the instrumental developments and methodological approaches responsible for this increased interest, compare and contrast the information provided by SIMS and MALDI imaging, and discuss future possibilities.

Imaging mass spectrometry

A common misconception is that the irradiation of solids with energetic electrons and ions has exclusively detrimental effects on the properties of target materials. In addition to the well-known cases of doping of bulk semiconductors and ion beam nitriding of steels, recent experiments show that irradiation can also have beneficial effects on nanostructured systems. Electron or ion beams may serve as tools to synthesize nanoclusters and nanowires, change their morphology in a controllable manner, and tailor their mechanical, electronic, and even magnetic properties. Harnessing irradiation as a tool for modifying material properties at the nanoscale requires having the full microscopic picture of defect production and annealing in nanotargets. In this article, we review recent progress in the understanding of effects of irradiation on various zero-dimensional and one-dimensional nanoscale systems, such as semiconductor and metal nanoclusters and nanowires, nanotubes, and fullerenes. We also consider the t...

Ion and electron irradiation-induced effects in nanostructured materials

Molecular secondary ion formation under cluster bombardment: A fundamental review

A protocol for three-dimensional molecular thin-film analysis is described that utilizes imaging time-of-flight secondary ion mass spectrometry and large-area atomic force microscopy. As a test study, a 300-nm trehalose film deposited on a Si substrate was structured by bombardment with a focused 15-keV Ga+ ion beam and analyzed using a 40-keV C60+ cluster ion beam. A three-dimensional sputter depth profile was acquired as a series of high-resolution lateral SIMS images with intermittent erosion cycles. As the most important result of this study, we find that the structured film exhibits a highly nonuniform erosion rate, thus preventing a simple conversion of primary ion fluence into eroded depth. Instead, the depth scale calibration must be performed individually on each pixel of the imaged area. The resulting laterally resolved depth profiles are discussed in terms of the chemical damage induced by the Ga+ bombardment along with the physics of the C60+ induced erosion process.

Protocols for three-dimensional molecular imaging using mass spectrometry.

Neutral silver clusters Agn with n ≤ 18 sputtered from a polycrystalline silver surface under bombardment with 5 keV Ar+ ions were detected by means of energy resolved time-of-flight mass spectrometry. Postionization of the ejected neutral particles was performed by an excimer laser operated with ArF (hv = 6.4 eV). The kinetic energy of the detected particles was determined from their flight time interval between the sputter pulse and the ionizing laser pulse. By saturating the ionization of the sputtered neutrals, relative cluster sputtering yields Y(Agn) were determined as a function of the cluster size. For n ≤ 6, the value of Y(Agn) is found to drop by approximately one order of magnitude if n is increased by one atom, whereas for cluster sizes larger than six atoms the decrease becomes significantly less pronounced and an odd-even intensity alternation similar to that observed for sputtered ion clusters becomes visible. The kinetic energy distributions of silver atoms, dimers, trimers, tetramers and pentamers determined from the time-of-flight method all look very similar and, in particular, show essentially the same high-energy dependence for sputtered atoms and molecules. In comparison, corresponding data taken by electron impact ionization with subsequent electrostatic energy analysis exhibit a significantly different asymptotic behaviour, the high energy slope of the energy spectra becoming steeper as the cluster size increases. From a detailed analysis of the measured signals, it is concluded that due to the concurrence of single- and multiphoton absorption processes the laser postionization results may be corrupted by photo-frag- mentation.

Sputtered neutral silver clusters up to Ag18

We report on a study of neutral Ag atoms and Agn clusters sputtered from a polycrystalline silver sample by keV rare gas ion bombardment. The ejected neutral species were photoionized by a pulsed laser and detected by time-of-flight mass spectrometry. In contrast to previous studies of sputtered neutral silver clusters, the ionization was performed by a VUV laser (λ = 157 nm) which, due to its high photon energy of 7.9 eV, permits nonresonant single photon ionization (SPI) of all investigated species. It is shown that the corresponding SPI ionization cross sections do not vary dramatically between silver atoms and the different clusters. As a consequence, fragmentation influences encountered in previous studies with longer wavelength lasers are practically eliminated from the determination of yields and kinetic energy distributions of the sputtered clusters. The resulting relative cluster sputtering yields (normalized to the yield of silver atoms) exhibit a power law dependence on the cluster size n according to n−δ with exponents δ ranging from 4.3 to 7.4 depending on the nature and the bombarding energy of the primary ions. The kinetic energy distributions of the sputtered neutral atoms and clusters are evaluated up to clusters containing seven atoms. It is found that the asymptotic decay of the energy distribution towards high emission energies becomes steeper from Ag to Ag3 and remains practically constant for larger clusters.

VUV photoionization of sputtered neutral silver clusters

The formation of neutral clusters during sputtering of polycrystalline Ag, Al, Nb and Ta surfaces was studied experimentally by non-resonant single photon post-ionization using a UV or VUV laser beam and time-of-flight mass spectrometry. From the laser intensity dependence of the resulting mass spectra, photoionization cross-sections as well as the yields of sputtered clusters are determined as a function of the cluster size. It is shown that in all cases investigated so far the cluster yields roughly exhibit a power law dependence on the cluster size, the exponent of which is found to be inversely correlated with the sputtering yield of the sample. This finding is of particular importance, since it rules out simple statistical combination models to describe the formation of large sputtered clusters. From the yield distributions it is inferred that — depending on the sputtering conditions — up to 46% of the sputtered atoms may be emitted in a bound state. The experimental results are briefly compared with theoretical model descriptions of the cluster emission/formation process available in the literature. It is found that only Molecular Dynamics computer simulations using modern many body potentials are able to reproduce the experimental findings.

Andreas Wucher

Papers

Molecular secondary ion formation under cluster bombardment: A fundamental review

Protocols for three-dimensional molecular imaging using mass spectrometry.

Sputtered neutral silver clusters up to Ag18

VUV photoionization of sputtered neutral silver clusters

The formation of clusters during ion induced sputtering of metals