Ion-matter interactions by MD simulations making use of reactive force fields

In the field of SIMS, ion-matter interactions have been largely investigated by numerical simulations like TRIM (or other programs using the binary-collision approximation) or molecular dynamics (MD) simulations. For MD simulations related to inorganic samples, mostly classical force fields assuming stable bonding structure have been used. In materials science, level-three force fields capable of simulating the breaking and formation of chemical bonds have recently been conceived. One such force field has been developed by Kieffer et al. 1–4 This potential includes directional covalent bonds, Coulomb and dipolar interaction terms, dispersion terms, etc. Important features of this force field for simulating systems that undergo significant structural reorganization are: (i) the ability to account for the redistribution of electron density upon ionization, formation, or breaking of bonds, through a charge transfer term; and (ii) the fact that the angular constraints dynamically adjust when a change in the coordination number of an atom occurs. In this work, we will present preliminary results of this potential, parameterized for silicon, for the simulation of atomic trajectories in samples subject to ion bombardment. Compared to normal force fields, ion-matter interactions as well as the sputtering of matter are expected to be described more accurately, especially when using reactive primary ions (oxygen or cesium) at low-impact energies. Copyright © 2010 John Wiley & Sons, Ltd.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/79412/1/3427_ftp.pd

Gold nanorods on the cathode electrode for enhancing the efficiency of polymer solar cells

Different densities of gold nanorods (GNRs) were incorporated on the back electrode of bulk heterojunction organic solar cell (OSC). GNRs layers (1, 3, and 5) were deposited on top of the poly(3-hexylthiophene) (P3HT) and phenyl- C61-butyric acid methyl ester (PCBM) layer using spin-casting technique. According to the optical and structural characterizations, the solar cell devices incorporated with one layer of gold nanorods showed an enhancement in both power conversion efficiency and short-circuit current by up to 14% and 22% respectively as compared to the devices without gold nanorods. This result suggests that GNRs in the back electrode of polymer solar cells act as backscattering elements. They not only increase the optical path length in the active layer but also store energy in localized surface plasmon resonance mode. Both mechanisms lead to enhancement of light absorption and in turn contribute to photocurrent generation and the overall power conversion efficiency. On the other hand, the solar cells with high density GNRs on the back electrode showed inferior performance compared to that of low density integrated ones. The decrease in PCE would stem from enhanced charge recombination induced by high density GNRs. Furthermore, generation of intense local electric fields named hotspots, would reduce the charge transportation and exciton dissociation probability. In such cases, the power conversion efficiency of the device is observed to be less than that for one layer GNRs or even the control device

Line shape diagnostics for solid density plasmas produced by ultra intense subpicosecond laser

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87325/2/158_1.pd

Dependence of hard x-ray yield on laser pulse parameters in the wavelength-cubed regime

Conversion efficiency and electron temperature scaling laws are experimentally studied in the wavelength-cubed (λ3)(λ3) regime, where a single-wavelength focus allows low energy pulses incident on a Mo target to produce x rays with excellent efficiency and improved spatial coherence. Focused intensity is varied from 2×10162×1016 to 2×1018 W/cm2.2×1018W/cm2. Conversion efficiency and electron temperature are best described by a power law for energy scaling while an exponential law best describes the scaling of these parameters with pulse duration. © 2004 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69364/2/APPLAB-84-13-2259-1.pd

High resolution hard x-ray spectroscopy of femtosecond laser-produced plasmas with a CZT detector

We present measurement of characteristic KαKα emission from Mo, Ag, and La targets irradiated by a 60 fs, 600 mJ, 10 Hz Ti:sapphire laser pulse at 1017–1019 W/cm2.1017–1019W/cm2. These x-ray emissions can potentially be used in applications from laser-based hard x-ray sources to x-ray mammography so detailed knowledge of the spectra is required to assess imaging of the figure of merit. We show here that high resolving hard x-ray spectroscopy can be achieved, with resolving powers (E/ΔE)(E/ΔE) of 60 at 18 keV, with cadmium–zinc–telluride detection system. The KαKα conversion efficiency from the laser light to the KαKα photon was optimized thanks to this diagnostic and values as high as 2×10−52×10−5 were obtained. © 2003 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70312/2/RSINAK-74-12-5035-1.pd

Study of hard x-ray emission from intense femtosecond TiTi:sapphire laser–solid target interactions

Interaction of intense TiTi:sapphire laser with solid targets has been studied experimentally by measuring hard x-ray and hot electron generation. Hard x-ray (8–100 keV)(8–100keV) emission spectrum and KαKα x-ray conversion efficiency (ηK)(ηK) from plasma have been studied as a function of laser intensity (1017–1019  W/cm2)(1017–1019W∕cm2), pulse duration (70–400)fs(70–400)fs, and laser pulse fluence. For intensity I>1×1017 W/cm2I>1×1017W∕cm2, the Ag ηKAgηK increases to reach a maximum value of 2×10−52×10−5 at an intensity I = 4×1018 W/cm2I=4×1018W∕cm2. Hot electron temperature (KTh)(KTh) and ηKηK scaling laws have been studied as a function of the laser parameters. A stronger dependence of KThKTh and ηKηK as a function of the laser fluence than on pulse duration or laser intensity has been observed. The contribution of another nonlinear mechanism, besides resonance absorption, to hard x-ray enhancement has been demonstrated via hot electron angular distribution and particle-in-cell simulations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71221/2/PHPAEN-11-9-4439-1.pd

X‐ray spectroscopy of hot solid density plasmas produced by subpicosecond high contrast laser pulses at 1018–1019 W/cm2

Analysis is presented of K‐shell spectra obtained from solid density plasmas produced by a high contrast (1010:1) subpicosecond laser pulse (0.5 μm) at 1018–1019 W/cm2. Stark broadening measurements of He‐like and Li‐like lines are used to infer the mean electron density at which emission takes place. The measurements indicate that there is an optimum condition to produce x‐ray emission at solid density for a given isoelectronic sequence, and that the window of optimum conditions to obtain simultaneously the shortest and the brightest x‐ray pulse at a given wavelength is relatively narrow. Lower intensity produces a short x‐ray pulse but low brightness. The x‐ray yield (and also the energy fraction in hot electrons) increases with the laser intensity, but above some laser intensity (1018 W/cm2 for Al) the plasma is overdriven: during the expansion, the plasma is still hot enough to emit, so that emission occurs at lower density and lasts much longer. Energy transport measurements indicate that approximately 6% of the laser energy is coupled to the target at 1018 W/cm2 (1% in thermal electrons with Te≊0.6 keV and 5% in suprathermal electrons with Th≊25 keV). At Iλ2=1018 W μm2/cm2 (no prepulse) around 1010 photons are emitted per laser shot, in 2π srd in cold Kα radiation (2–9 Å, depending on the target material) and up to 2×1011 photons are obtained in 2π srd with the unresolved transition array (UTA) emission from the Ta target. © 1995 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69900/2/PHPAEN-2-5-1702-1.pd

Ultrafast x‐ray sources@f|

Time‐resolved spectroscopy (with a 2 psec temporal resolution) of plasmas produced by the interaction between solid targets and a high contrast subpicosecond table top terawatt (T3) laser at 1016 W/cm2, is used to study the basic processes which control the x‐ray pulse duration. Short x‐ray pulses have been obtained by spectral selection or by plasma gradient scalelength control. Time‐dependent calculations of the atomic physics [Phys. Fluids B 4, 2007, 1992] coupled to a Fokker–Planck code [Phys. Rev. Lett. 53, 1461, 1984] indicate that it is essential to take into account the non‐Maxwellian character of the electron distribution for a quantitative analysis of the experimental results.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70417/2/PFBPEI-5-7-2676-1.pd

Tabletop imaging of structural evolutions in chemical reactions

The introduction of femto-chemistry has made it a primary goal to follow the nuclear and electronic evolution of a molecule in time and space as it undergoes a chemical reaction. Using Coulomb Explosion Imaging we have shot the first high-resolution molecular movie of a to and fro isomerization process in the acetylene cation. So far, this kind of phenomenon could only be observed using VUV light from a Free Electron Laser [Phys. Rev. Lett. 105, 263002 (2010)]. Here we show that 266 nm ultrashort laser pulses are capable of initiating rich dynamics through multiphoton ionization. With our generally applicable tabletop approach that can be used for other small organic molecules, we have investigated two basic chemical reactions simultaneously: proton migration and C=C bond-breaking, triggered by multiphoton ionization. The experimental results are in excellent agreement with the timescales and relaxation pathways predicted by new and definitively quantitative ab initio trajectory simulations