Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

Abstract

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gasphase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N) ∼ N −τ with exponents different for small, up to ∼ 15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution.

Computer Simulations of Laser Ablation of Molecular Substrates

“…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.…”

Amorphous Solid Water (ASW): Pulsed Laser Ablation of ASW/CO2 Thin Films

“…In the latter, material is entrained such that different species assume the same forward (plume) velocity. 14,[30][31][32] We observe no such entrainment. Referring to Fig.…”

“…Most theoretical studies have focused on strongly bound solids, though a number have addressed molecular solids, 14,30-32 including ASW and doped ASW films.…”

“…10 Recent research with ASW and ASW/CO 2 has addressed the roles of sample morphology and its temperature variation, species segregation, and explosive boiling (phase explosion). [11][12][13][14][15][16][17][18][19][20] Pulsed infrared radiation (10 ns duration, 3424 cm -1 ) creates OH stretch excitation that converts to heat on a ps time scale. 4 Broad absorption spectra of ASW, crystalline ice, and liquid water ensure efficient excitation in samples whose character changes throughout the pulse.…”

“…14,[22][23][24][25][26][27][28][29][30][31][32] The groups of Garrison and Zhigilei have examined organic solids, 30-32 making seminal experimental and theoretical contributions. Likewise, theoretical studies of Lewis and coworkers 22,22,26,27,29 have provided a conceptual basis for judging experimental results.…”