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.
Isotope effect in multiphoton ultraviolet photolysis of CO
Two‐photon dissociation of carbon monoxide near 193 nm with a tunable ArF* laser to yield excited C(2 1D) atoms is reported. The atoms are detected by subsequent absorption of a third laser photon to C(3 1P°) followed by emission of 248 nm fluorescence to C(2 1S). The process is found to be isotopically sensitive; the C(2 1D) yield from 13C 16O and 12C 18O is a factor of 6 greater than from 12C 16O. The isotope effect is attributable to an enhancement in the two‐photon matrix element due to the shift in the near resonant a 2Π, v=2 intermediate state. C2 Swan band emission is also observed, arising from association of the free carbon atoms, and exhibits a corresponding isotope effect. Finally, collisional processes are observed involving excited carbon atoms and electrons, using two‐photon ionization of a small quantity of added xenon atoms as a new technique for producing a controlled density of free electrons.