Leonid V. Zhigilei

Microscopic mechanisms of laser ablation of organic solids in the thermal and stress confinement irradiation regimes

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and ...

Molecular dynamics simulation study of the fluence dependence of particle yield and plume composition in laser desorption and ablation of organic solids

Two distinct regimes of molecular ejection separated by a well-defined threshold fluence are observed in molecular dynamics simulation of pulsed laser irradiation of an organic solid. At fluences above the threshold a collective multilayer ejection or ablation occurs where large liquid droplets are ejected and the total yield of the ablated material follows a critical volume density of the deposited energy. Below threshold thermal desorption from the surface is observed and the desorption yield has an Arrhenius-type dependence on the laser fluence. The yield of monomers does not have a step increase at the threshold and thus deceptively does not give a straightforward interpretation of the ejection mechanisms.

Velocity distributions of molecules ejected in laser ablation

Based on the results of molecular dynamics simulations, we propose an analytical expression for the velocity distributions of molecules ejected in laser ablation. The Maxwell-Boltzmann distribution on a stream velocity, commonly used to describe the measured velocity distributions, is modified to account for a range of stream velocities in the ejected plume. The proposed distribution function provides a consistent description of the axial and radial velocity distributions. The function has two parameters that are independent of the desorption angle and have clear physical meaning, namely, the temperature of the plume and the maximum stream velocity or velocity of the plume propagation.

Molecular dynamics simulations of MALDI: laser fluence and pulse width dependence of plume characteristics and consequences for matrix and analyte ionization

Molecular dynamics simulations of matrix-assisted laser desorption/ionization were carried out to investigate laser pulse width and fluence effects on primary and secondary ionization process. At the same fluence, short (35 or 350 ps) pulses lead to much higher initial pressures and ion concentrations than longer ones (3 ns), but these differences do not persist because the system relaxes toward local thermal equilibrium on a nanosecond timescale. Higher fluences accentuate the initial disparities, but downstream differences are not substantial. Axial velocities of ions and neutrals are found to span a wide range, and be fluence dependent. Total ion yield is only weakly dependent on pulse width, and consistent with experimental estimates. Secondary reactions of matrix cations with analyte neutrals are efficient even though analyte ions are ablated in clusters of matrix.

Molecular dynamics simulations of matrix-assisted laser desorption—connections to experiment

The molecular dynamics (MD) simulation technique has been applied to investigate fundamental aspects of matrix-assisted laser desorption. In this paper, we focus on direct comparisons of the results from the simulations with experimental data and on establishing links between the measured or calculated parameters and the basic mechanisms of molecular ejection. The results on the fluence dependence of the ablation/desorption yields and composition of the ejected plume are compared with mass spectrometry and trapping plate experiments. Implications of the prediction of a fluence threshold for ablation are discussed. The strongly forward-peaked velocity and angular distributions of matrix and analyte molecules, predicted in the simulations, are related to the experimental distributions. The shapes and amplitudes of the acoustic waves transmitted from the absorption region through the irradiated sample are compared to recent photoacoustic measurements and related to the ejection mechanisms. The conformational changes during plume evolution and the ejection velocities of analyte molecules are studied and the directions for future investigations are discussed. Finally, we demonstrate that the MD simulation technique can be used to model other processes relevant to mass spectrometry applications, such as laser disintegration of aerosol particles and laser ablation in the presence of photochemical reactions.

On the threshold behavior in laser ablation of organic solids

The microscopic mechanisms of the fluence threshold behavior in laser ablation of organic solids have been delineated using a breathing sphere model and molecular dynamics simulations. Below threshold, evaporation is identified to occur and primarily single molecules are desorbed. Above threshold, collective ejection or ablation occurs in which large molecular clusters constitute a significant portion of the ejected plume. The laser induced pressure buildup and the phase explosion due to overheating of the irradiated material are identified as the key processes that determine the dynamics of laser ablation. q 1997 Elsevier Science B.V.

Pressure-transmitting boundary conditions for molecular-dynamics simulations

A scheme for establishing boundary conditions in molecular-dynamics simulations that prevent pressure wave reflection out of the simulation volume is formulated. The algorithm is easily implemented for a one-dimensional geometry. Its efficiency is tested for compressive waves in Cu.

A combined molecular dynamics and finite element method technique applied to laser induced pressure wave propagation

Analysis of a variety of dynamic phenomena requires simultaneous resolution at both atomistic and continuum length scales. A combined molecular dynamics and finite element method approach, which we discuss in this paper, allows us to find the balance between the necessary level of detail and computational cost. The combined method is applied to the propagation of a laser-induced pressure wave in a solid. We find good agreement of the wave profile in the molecular dynamics and finite element regions. This computational approach can be useful in cases where a detailed atomic-level analysis is necessary in localized spatially separated regions whereas continuum mechanics and thermodynamics is sufficient in the remainder of the system.

Structural Stability of Carbon Nanotube Films: The Role of Bending Buckling

In films, mats, buckypaper, and other materials composed of carbon nanotubes (CNTs), individual CNTs are bound together by van der Waals forces and form entangled networks of bundles. Mesoscopic dynamic simulations reproduce the spontaneous self-assembly of CNTs into continuous networks of bundles and reveal that the bending buckling and the length of CNTs are the two main factors responsible for the stability of the network structures formed by defect-free CNTs. Bending buckling of CNTs reduces the bending energy of interconnections between bundles and stabilizes the interconnections by creating effective barriers for CNT sliding. The length of the nanotubes is affecting the ability of van der Waals forces of intertube interactions to counterbalance the internal straightening forces acting on curved nanotubes present in the continuous networks. The critical length for the formation of stable network structures is found to be ∼120 nm for (10,10) single-walled CNTs. In the simulations where the bending buckling is artificially switched off, the network structures are found to be unstable against disintegration into individual bundles even for micrometer-long CNTs.

Velocity distributions of analyte molecules in matrix-assisted laser desorption from computer simulations

The mass dependence of the velocity distributions of analyte molecules in matrix-assisted laser desorption is analyzed based on the results of molecular dynamics simulations. The spread of the velocities along the direction of the flow is found to be independent of the mass of the analyte molecules and to reflect the entrainment of the analyte molecules in the expanding matrix. The radial velocity distributions for both matrix molecules and analyte molecules of different masses, on the other hand, fit well to a Maxwell–Boltzmann distribution with the same temperature, suggesting the association of the spread in the radial velocities with the thermal motion in the plume. A consistent analytical description of the complete velocity distribution for matrix molecules and analyte molecules of different masses is proposed based on the approximation of a range of stream velocities and a single temperature in the ejected plume.