Energy transfer mechanisms in laser-induced plasmas: Variation of physical traits mediated by the presence of single optically-trapped nanoparticulate material

Abstract

In the present work, authors provide new insight into the dual role played by air plasmas as the atomization and excitation source in single-particle LIBS (SP-LIBS). Moreover, the dependence of the dissociation and excitation mechanisms during laser-particle interaction and the sample's state, i.e., bulk, solid aerosol, and, single-particle, is discussed. For this purpose, copper nanoparticles were individually trapped in air at atmospheric pressure and then analyzed by laser-induced breakdown spectroscopy (LIBS). Plasma imaging and, for the first time, time-resolved LIBS spectroscopy on isolated nanoparticles were successfully performed to prove the proposed mechanism. Both methodologies yielded results coherent with an exchange of energy from the surrounding air plasma to the particle. Electronic temperature (Te) for plasmas formed in the presence and absence of nanoparticles were calculated using ionic nitrogen lines from the air plasma. Upon comparison of temperatures, slightly cooler plasmas were recorded when occluding trapped copper particles (30,880 ± 1150 K) than for ‘blank’ air plasmas (32,480 ± 930 K), which indicated heat transference into the nanoparticle. Finally, the emission sensitivity, which exponentially depends on the particle size, was calculated to quantify (using Hess-like cycles) the energy absorbed by the nanoparticle and its influence on the LIBS signal.