Gas dynamics of the pulsed emission of a perfect gas with applications to laser sputtering and to nozzle expansion

Laser-Solid Interaction and Dynamics of the Laser-Ablated Materials

Rapid transformations through the liquid and vapor phases induced by laser-solid interactions are described by our thermal model with the Clausius-Clapeyron equation to determine the vaporization temperature under different surface pressure condition. Hydrodynamic behavior of the vapor during and after ablation is described by gas dynamic equations. these two models are coupled. Modeling results show that lower background pressure results lower laser energy density threshold for vaporization. the ablation rate and the amount of materials removed are proportional to the laser energy density above its threshold. We also demonstrate a dynamic source effect that accelerates the unsteady expansion of laser-ablated material in the direction perpendicular to the solid. a dynamic partial ionization effect is studied as well. a self-similar theory shows that the maximum expansion velocity is proportional to cs/α, where 1 – α is the slope of the velocity profile. Numerical hydrodynamic modeling is in good agreement with the theory. With these effects, α. is reduced. therefore, the expansion front velocity is significantly higher than that from conventional models. the results are consistent with experiments. We further study the plume propagates in high background gas condition. Under appropriate conditions, the plume is slowed down, separates with the background, is backward moving, and hits the solid surface. then, it splits to be two parts when it rebounds from the surface. the results from the modeling will be compared with experimental observations where possible.

Measurement of mitochondrial DNA synthesis in vivo using a stable isotope-mass spectrometric technique

We describe here a new stable isotope-mass spectrometric technique for measuring mitochondrial DNA (mtDNA) synthesis. Growing (2-4 mo old) and weight-stable (8-10 mo old) Sprague-Dawley rats were primed with (2)H(2)O (deuterated water) to 2.0-2.5% body water enrichment, via intraperitoneal injection, and then given 4% (2)H(2)O in drinking water for 3-11 wk. Mitochondria were isolated from cardiac and hindlimb muscle, and mtDNA was isolated and enzymatically hydrolyzed to deoxyribonucleosides. PCR confirmed the absence of nuclear DNA contamination. The isotopic enrichment of the deoxyribose moiety of deoxyadenosine was determined by GC-MS analysis, and percent new mtDNA was calculated by comparison to genomic DNA enrichments in a tissue with nearly complete turnover (bone marrow). Initial label incorporation into deoxyadenosine of mtDNA was linear, and turnover of mtDNA was observed in nongrowing adult female rats (1.1-1.3% new mtDNA per day in cardiac and skeletal muscle). Die-away curves of mtDNA after discontinuing (2)H(2)O administration gave a similar turnover rate constant. Human subjects were also given (2)H(2)O for up to 6 wk, and mitochondria from platelets were isolated. Incubation with DNase removed any contaminating genomic DNA; platelet mtDNA exhibited linear incorporation from (2)H(2)O and reached plateau values identical to those in genomic DNA from fully turned over cells (circulating monocytes). In conclusion, replication of mtDNA can be directly measured in vivo in rodents and humans without the use of radioactivity. Use of this technique may allow improved understanding of the regulation of mitochondrial biogenesis in health and disease.

Temporal and spatial evolution of a laser-induced plasma from a steel target

The space and time evolution of a laser-induced plasma from a steel target has been studied using optical time-of-flight and shadowgraphic techniques. The results, obtained for two distinct laser energy regimes, allow us to individuate two different regions in the plume, one characterized by air and continuum emissions produced by the shock wave ionization and the other characterized by emissions from ablated material. Moreover, it was shown that a sufficiently high laser fluence and short delay time of acquisition are needed to avoid inhomogeneous effects in the plasma, as required in analytical applications such as laser-induced breakdown spectroscopy.

Direction-selective free expansion of laser-produced plasmas from planar targets

Direction-selective expansion of laser-produced plasmas from planar slab targets of Al, Ni, Mo, and Ta are reported. Angular distributions of the particles emitted from the targets, produced by a 130 mJ, 5 nsec, Nd:YAG laser, were obtained by means of a retarding potential analyzer and a quartz crystal. It was observed that the angular distributions of the particles had mainly three characteristics. For a given laser energy and a given target element, the angular distribution showed more preferential focusing toward the target normal as the value of the focal spot size B increased. Second, for a given laser energy and a given focal spot size, the focusing was more pronounced toward the target normal as the atomic mass number of the target materials increased. Third, for a given energy, a given focal spot size and a given element, the particles with higher ionization states were much more focused toward the target normal. Our experimental results confirm the Monte Carlo simulation results of the earlier works taking into account collision and recombination processes.

Reflectivity and energy balance in pulsed-laser deposition experiments from mono- and bi-atomic targets

Reflectivity and complete energy balance from the planar targets of aluminum, copper, nickel, and molybdenum have been measured using a Nd: glass laser (lambda=1060 nm,tau=5 ns) in the low intensity regime of laser plasma interaction as a function of focal spot size. The magnitude of partition of total incident laser energy into different channels is observed to decrease as the focal spot size increases. It is further observed that the magnitude of partition of the incident energy into these channels, in general, decreases as the atomic number increases for any given focal spot size, although, the reflectivity component of the partitioned energy increases with focal spot size for any given element. The reflectivities of copper and tungsten and their alloy were measured separately. The reflectivity from the alloy plasma was reduced by a factor of 6 compared to either element separately. This observation confirms the recent theory that in the multiion plasma the ion acoustic waves are additionally damped due to additional Joule, thermal diffusion, and viscous terms in the modified ion-fluid theory of the ion acoustic waves in a multiion species plasma.

Time of flight measurements on ion-velocity distribution and anisotropy of ion temperatures in laser plasmas

Using a 200-mJ, 5-ns, Nd:YAG laser (where YAG denotes yttrium aluminum garnet), time of flight spectra for ions produced from slab targets of carbon, aluminum, nickel, and tantalum were obtained at a laser intensity of approximately 5x10(10) W/cm(2). The velocity distribution function of ions of each ionization state, at several angles, in the plane of incidence, was investigated and was observed to depart significantly from the Maxwell-Boltzmann distribution function. Kinetic temperatures of ions of each ionization state were estimated separately at various angles in the plane of incidence and were observed to show a highly anisotropic property leading one to conclude that the equipartition of energy between the plasma electrons and ions has not taken place. The results are discussed and the relevant conclusions are presented.

Double layer effects in laser-ablation plasma plumes

Charge-collector probe measurements have been performed to elucidate ion acceleration in laser-induced plasma plumes over a range of laser fluences important for pulsed laser deposition. The fundamental (1064 nm) or second (532 nm) harmonics of a Nd:YAG laser were used for ablation. The evolution of the time-of-flight ion signal from single-peaked to double-peaked and again to single-peaked with increasing laser fluence in the range of 2-25 J/cm(2) has been followed. The analysis of the ion velocity distributions shows that increasing laser fluence results in the appearance of a portion of accelerated ions that can be recognized as an additional fast peak in the time-of-flight distribution. The dependencies of the ion signal on the target-to-collector distance, the background pressure, and the wavelength of laser radiation have been studied. The results are discussed from the viewpoint of the generation of a self-consistent ambipolar electric field (so-called double layer). The observed ion acceleration suggests that formation in the plume of a high-energetic electron tail due to absorption of laser radiation is responsible for the development of a double layer.