Laser ablation of nanoscale particles with 193 nm light
Laser interaction with nanoscale particles is distinct and different from laser-bulk material interaction, where a hot plasma is normally created. Here, we review our studies on 193 nm laser ablation of various nanoscale particles including NaCl, soot, polystyrene, and gold. The 20 ns laser beam with fluences up to 0.3 J/cm 2 irradiates nanoparticles in a gas stream at laser repetition rates from 10 to 100 Hz. The particle size distributions before and after irradiation are measured with a scanning mobility particle sizer (SMPS), and particle morphology is examined with electron microscopy. All the nanomaterials studied exhibit a similar disintegration pattern and similar particle formation characteristics. No broadband emission associated with particle heating or optical breakdown is observed. The nanoparticles formed after irradiation have a smaller mean diameter and an order of magnitude higher number concentration with a more spherical shape compared to the original particles. We use the photon-atom ratio (PAR) to interpret the laser-particle interaction energetics.
Nucleation and Cluster Formation
The formation of nuclei and clusters from single atomic or molecular species plays an important or even decisive role in laser-induced gas-phase synthesis of powders,Nanopowders laser-CVD, pulsed-laser deposition (PLD) etc. Of similar importance are (heterogeneous) nucleation processes in laser-annealing, recrystallization, thin-film formation, etc. Depending on the particular application, nucleation and cluster formation can be desirable or undesirable. An example is PLD, where clusters formed within the vapor plume may condense on the substrate and thereby deteriorate epitaxial film growth. However, by employing a moderate background pressure, the same technique can be used to fabricate thin nanocrystalline films with unique physical properties.
Analysis of Sulfuric Acid Aerosols by Laser-Induced Breakdown Spectroscopy and Laser-Induced Photofragmentation
Laser-induced breakdown spectroscopy and laser photofragmentation spectroscopy are investigated as potential approaches to the detection and quantification of the sulfuric acid aerosols resulting from the oxidation of dimethylsulfide in air. The former, direct technique, where aerosols are introduced in the plasma formed by focusing the fundamental frequency of a Nd:YAG laser in air, and atomic sulfur emission is measured in the region 180 nm, gives a limit of detection of 165 ppbv for a 15 min integration time, which is inadequate for our purposes. The second, indirect approach, based on the photofragmentation, with 193 nm photons, of the compound resulting from interaction between sodium chloride and sulfuric acid aerosols, gives a detection limit of 46.5 ppbv in 10 s measuring time. With this method, a complete “titration curve” for sulfuric acid aerosols can be obtained in “quasi” real-time.