There is growing interest in adapting time-resolved laser-induced incandescence (TiRe-LII) to size metal nanoparticles, owing to their emerging applications in materials science. Extending TiRe-LII to new aerosols requires a model for the heat transfer between the laser-energized nanoparticles and the surrounding gas. Unfortunately, the thermal accommodation coefficient, α, which defines the energy transferred when a gas molecule scatters from the particle surface, is rarely available. This parameter can sometimes be obtained from LII measurements made on a reference aerosol sized using electron micrography, but this process is notoriously time-consuming, and thermophoretic sampling of metal nanoparticles is often problematic. These challenges have precluded interpretation of data from several pioneering TiRe-LII studies on metal nanoparticles, including one by Murakami et al. [1] that intended to determine how the bath gas influences the growth of molybdenum nanoparticles formed through laser-induced photolysis of Mo(CO)6. Alternatively, it is sometimes possible to estimate α using molecular dynamics (MD). In this technique, a pairwise potential between the gas molecule and metal atoms is derived from ab initio (generalized gradient approximations of density functional theory, GGA-DFT) calculations of the gas/surface potential. The potentials then differentiated to obtain forces, and Newton's equations of motion are timeintegrated to obtain atomic trajectories during a gas/surface scattering event. Finally, α is found through Monte Carlo integration over all incident gas molecular This approach was initially used to characterize α between soot and various gases, and is presently being extended to metal nanoparticles. Preliminary results show that MD-derived Preliminary thermal accommodation coefficients for accommodation coefficients are highly metal nanoparticles sensitive to the potential well depth. αMD αexp Unfortunately, a well-known limitation of Ni/Ar 0.20±0.02 GGA-DFT is that they cannot describe the Fe/He 0.07±0.01 0.01 [2] long-range electron correlations Fe/Ar 0.04±0.01 0.1 [2], 0.13 [3] responsible for van der Waals (vdW) Mo/He 0.006±0.002 forces, which contribute to the potential Mo/Ar 0.04±0.01 well. While the Ni/Ar interaction is dominated by a strong Casimir force, vdW forces are thought to play a major role in other systems. Accordingly, true accommodation coefficients are probably larger compared to ones found using ab initio derived gassurface potentials with no vdW correction. Current research is focused on identifying an appropriate heuristic correction that can account for the dispersive forces.
MD simulation of an argon molecule scattering from a laser- trajectories. energized iron nanoparticle