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Generation and detection of small particles

Contact : Jörg Hermann

In addition to the atomized vapor, laser-produced plasmas may contain small particles. The particles are generated during the ablation process or formed by condensation in the expanding vapor plume. The dimension of the particles is strongly variable. Typically, particles of nanometer size are ejected from the material during ablation by ultrashort (femtosecond) laser pulses, whereas micrometric particles are generated during nanosecond laser ablation. The characterization of the particles by their size distribution, and their kinetic and internal energies contribute to a better understanding of the mechanisms involved in laser ablation and particle generation. Thus, the size distribution of particles can be controlled via the regulation of the materials heat regime using two time-delayed laser pulses.

Fig. 6. Plume images recorded during ablation of copper using a single femtosecond laser pulse (upper) and two pulses delayed by 33 ps (lower). We observe an increase of the atomization degree for ablation with double pulses. The observation gate was delayed by 400 ns with respect to the laser pulses.

Optical breakdown or ablation plumes produced by pulsed lasers are recombining plasmas, characterized by large initial values of temperature and density, and followed by rapid cooling during fast expansion. In this scenario, the formation of particles by condensation prevails the invers process of particle vaporization. However, the balance between both processes can be inverted in favor to particle vaporization. This is in particular the case for laser-induced breakdown in a gas charged with particles. The particles within the volume of the breakdown plasma are vaporized by the interaction with the hot plasma. This process is explored in the development of a system for the measurements of size, density and chemical composition of particles.

Fig. 7. Emission spectrum of laser-induced breakdown in helium charged with alumina aerosols. The chemical composition of the aerosols is deduced from the simulation of the plasma emission spectrum on the base of a partial local thermodynamic equilibrium.

Publications :

Boudhib M., Hermann J., Dutouquet C., Compositional analysis of aerosols using calibration-free laser-induced breakdown spectroscopy, Anal. Chem. 88, 4029–4035 (2016)

Axente E., Mihailescu I.N., Hermann J., Itina T.E., Probing electron-phonon coupling in metals via observations of ablation plumes produced by two delayed short laser pulses, Appl. Phys. Lett. 99, 081502 1-3 (2011)

Noël S., Hermann J., Reducing nanoparticles in metal ablation plumes produced by two delayed short laser pulses, Appl. Phys. Lett. 94, 053120 (2009)