Laser ablation of bicomponent systems: A probe of molecular ejection mechanisms

Abstract

A combined experimental and molecular dynamics simulation study of laser ablation of a model bicomponent system with solutes of different volatility provides a consistent picture of the mechanisms of material ejection. The comparison of the ejection yields shows that there are two distinct regimes of molecular ejection, desorption at low laser fluences, and a collective ejection of a volume of material or ablation at higher fluences. Ejection of volatile solutes dominates in the desorption regime, whereas nonvolatile solutes are ejected only in the ablation regime.

The role of the photochemical fragmentation in laser ablation: a molecular dynamics study

“…The cohesive energy of the original system is 0.6 eV [18], 0.57 eV for the system with lower cohesive energy, and 0.66 eV for the system with higher cohesive energy [22]. Even though for the two systems with modified cohesive energies the penetration depth is slightly higher (55 nm) than in the original system (50 nm), the assumption that the cohesive energy equal to the critical energy density is still valid.…”

“…The results of our recent laser ablation study of systems with different cohesive energies suggest that the cohesive energy of the sample is related to the value of the ablation threshold [22]. The ablation threshold is defined by the critical energy density E v * required for the collective ejection of a volume of material or ablation [23,24].…”

Computer Simulations of Laser Ablation of Molecular Substrates

“…This choice of equilibrium distance is based on the physical concept that the sublimation or cohesive energy of an organic solid is governed primarily by the interaction among atoms on the outside of the molecule and allows an easy means of simulating multicomponent molecular systems. 22,61,70,73,75,76,101 To simulate molecular excitation by photon absorption and vibrational relaxation of the excited molecules, an additional internal degree of freedom is attributed to each molecule. This internal degree of freedom, or breathing mode, is realized by allowing the particles to change their sizes.…”

“…Although a twice smaller, as compared to the simulations shown in Figure 3a,b, computational cell is used in the simulation performed with the nonreflecting boundary condition, no artifacts due to the pressure wave reflection are observed. The nonreflecting boundary conditions have been successfully used in simulations of laser ablation and damage of organic materials in which both planar 22,[67][68][69][70][71][72][73][74][75][76][77][78]101 and spherical 119 pressure waves were generated. Recently, the boundary conditions have been also implemented and tested for metals.…”

“…There is, however, a growing number of experimental studies that are specifically aimed at investigating the fundamental processes in laser ablation. In particular, systematic studies of the role of the laser pulse duration, [11][12][13][14] fluence and wavelength, [15][16][17] size of the laser spot, 15 number of successive laser pulses, 18 laser beam incidence angle, 19,20 initial temperature of the molecular substrate, 21 and molecular volatility 22 have been performed. In addition to the yields 11,15,16,21 and velocities 17,[23][24][25][26][27] of the ejected molecules and ions, that are commonly measured in time-of-flight mass spectrometry experiments, other parameters, such as cluster ejection [28][29][30] and profiles of the acoustic signals propagating from the ablation region [31][32][33][34][35][36][37][38][39] have been investigated, providing a more complete picture of the ablation process.…”