Yudi Rosandi

Short-pulse Laser Induced Transient Structure Formation and Ablation Studied with Time-resolved Coherent XUV-scattering

The structural dynamics of short‐pulse laser irradiated surfaces and nano‐structures has been studied with nm spatial and ultrafast temporal resolution by means of single‐shot coherent XUV‐scattering techniques. The experiments allowed us to time‐resolve the formation of laser‐induced periodic surface structures, and to follow the expansion and disintegration of nano‐objects during laser ablation.

Making channeling visible: keV noble gas ion trails on Pt(111)

The impact of argon and xenon noble gas ions on Pt(111) in grazing incidence geometry are studied through direct comparison of scanning tunneling microscopy images and molecular dynamics simulations. The energy range investigated is 1–15 keV and the angles of incidence with respect to the surface normal are between 78.5° and 88°. The focus of the paper is on events where ions gently enter the crystal at steps and are guided in channels between the top most layers of the crystal. The trajectories of the subsurface channeled ions are visible as trails of surface damage. The mechanism of trail formation is analyzed using simulations and analytical theory. Significant differences between Xe+ and Ar+ projectiles in damage, in the onset energy of subsurface channeling as well as in ion energy dependence of trail length and appearance are traced back to the projectile and ion energy dependence of the stopping force. The asymmetry of damage production with respect to the ion trajectory direction is explained through the details of the channel shape and subchannel structure as calculated from the continuum approximation of the channel potential. Measured and simulated channel switching in directions normal and parallel to the surface as well as an increase of ions entering into channels from the perfect surface with increasing angles of incidence are discussed.

Grazing incidence ion erosion in the presence of adsorbates

The effect of a background pressure of adsorbable species on sputtering and surface damage in grazing incidence ion erosion of Pt(111) is investigated by scanning tunneling microscopy and molecular dynamics simulations. The background pressure implies a partial surface coverage with adsorbates, which in turn causes an enhancement of the erosion rate by a factor of up to 40 compared to the clean case. Partial pressures of molecular oxygen and carbon monoxide were maintained during ion erosion with 5keV Ar + for various grazing angles between 81 and 87 and temperatures ranging from 400 to 550K.

The Laser ablation of a metal foam: The role of electron–phonon coupling and electronic heat diffusivity

The incidence of energetic laser pulses on a metal foam may lead to foam ablation. The processes occurring in the foam may differ strongly from those in a bulk metal: The absorption of laser light, energy transfer to the atomic system, heat conduction, and finally, the atomistic processes—such as melting or evaporation—may be different. In addition, novel phenomena take place, such as a reorganization of the ligament network in the foam. We study all these processes in an Au foam of average porosity 79% and an average ligament diameter of 2.5 nm, using molecular dynamics simulation. The coupling of the electronic system to the atomic system is modeled by using the electron–phonon coupling, g, and the electronic heat diffusivity, κe, as model parameters, since their actual values for foams are unknown. We show that the foam coarsens under laser irradiation. While κe governs the homogeneity of the processes, g mainly determines their time scale. The final porosity reached is independent of the value of g.

Collision‐Induced Melting in Collisions of Water Ice Nanograins: Strong Deformations and Prevention of Bouncing

Collisions between ice grains are ubiquitous in the outer solar system. The mechanics of such collisions is traditionally described by the elastic contact theory of adhesive spheres. Here we use molecular dynamics simulations to study collisions between nanometer-sized amorphous water-ice grains. We demonstrate that the collision-induced heating leads to grain melting in the interface of the colliding grain. The large lateral deformations and grain sticking induced considerably modify available macroscopic collision models. We report on systematic increases of the contact radius, strong grain deformations, and the prevention of grain bouncing.

Melting of Al Induced by Laser Excitation of 2p Holes

Novel photon sources—such as XUV- or X-ray lasers—allow to selectively excite core excitations in materials. We study the response of a simple metal, Al, to the excitation of 2p holes using molecular dynamics simulations. During the lifetime of the holes, the interatomic interactions in the slab are changed; we calculate these using WIEN2k. We find that the melting dynamics after core-hole excitation is dominated by classical electron–phonon dynamics. The effects of the changed potential surface for excited Al atoms occur on the time scale of 100 fs, corresponding to the Debye time of the lattice.

Insight from molecular dynamics simulation into ultrashort-pulse laser ablation

Ultrashort-pulse laser irradiation may melt the target and, at higher intensities, lead to ablation. The state of the material shortly after irradiation is characterized by high temperatures and pressures (both tensile and compressive). In addition, the material may not yet be in thermal equilibrium. Molecular dynamics simulation is well suited to model the state of matter under these conditions. We give several examples of how molecular dynamics simulations have contributed to understanding the ablation phenomena after ultrashort-pulse laser irradiation.

Ultrashort-pulse laser ablation: insights from molecular-dynamics simulation

Laser ablation is a common technique for removing surface material from a target with pulsed laser light. The most familiar example is LASIK surgery, in which ophthalmologists use laser pulses to selectively remove material from a patient’s cornea to correct impaired vision. The technique has many industrial applications, including surface cleaning, etching, precision holedrilling, surface hardening, and more. Our research investigated the atomic-level thermomechanics and revealed mechanisms of picosecond (ps)-pulsed laser ablation that may lead to new applications for the technique. Computer simulation is an ideal tool to explore these atomic-level processes: for our research, we used 3D molecular-dynamics software developed in-house. In a metal, laser energy is absorbed by conduction electrons close to the target surface, in the ‘skin’ layer, with a depth on the order of 10nm. The excited electrons diffuse quickly into the target, while simultaneously imparting their energy to surrounding atoms. The electron-atom coupling time, , characterizes the time after which the electron-and-atom systems have reached thermal equilibrium. For aluminum, Š 3ps. By that time, the electrons have diffused to a heated depth of dT D 50nm.1 In this region, a zone of hot, thermally equilibrated material is created. The heated zone produces localized high-compressive stress within the material and attempts to expand. If ultrashort pulses are delivered to the material in less than the material’s characteristic electron-coupling time, there will be insufficient time for that stress to spread and the material to relax. For example, given a velocity of sound in a material vsound D 5km/s, the time needed to dissipate that stress is equivalent to the acoustic response time, dT =vsound D 10ps. This state of stress confinement Figure 1. Ablation of a 12.8nm-wide, 12.8nm-thick, aluminum film energized with 1.2eV/atom laser irradiation.2