Sy-Bor Wen

Laser ablation induced vapor plume expansion into a background gas. II. Experimental analysis

Laser ablation of copper with a 4ns laser pulse at 1064nm was studied with a series of synchronized shadowgraph (100fs laser pulses at 400nm) and emission images (spectral line at 515nm). Data were obtained at two laser pulse energies (10 and 30mJ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conversion ratio and the amount of sample vaporized for ablation in each condition were obtained by the theoretical analysis reported in paper I from trajectories of the external shock wave, internal shock wave, and contact surface between the Cu vapor and the background gas. All three quantities were measured from shadowgraph and emission images. The results showed that E, the amount of energy that is absorbed by the copper vapor, decreases as the atomic mass of the background gas increases; and M, the mass of the sample converted into vapor, is almost independent of the background gas [Horn et al., Appl. Surf. Sci. 182, 91 (2001)]. A physical interpretation is given based...

Expansion of the laser ablation vapor plume into a background gas. I. Analysis

A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, E, and the vaporized sample mass, M, are specified. The values for E and M were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus E and M, which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal sh...

Characterization of pulsed laser-deposited LiMn2O4 thin films for rechargeable lithium batteries

The structure and characteristics of thin films of LixMn2O4 spinel prepared by pulsed laser deposition are reported. X-Ray Diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS) and Nuclear Reaction Analysis (NRA) show that the lithium content of the film is very sensitive to the conditions of deposition. A large target–substrate distance (9 cm) and a stoichiometric LiMn2O4 target lead to lithium deficient films, whereas lithium excess films are obtained for a shorter target–substrate distance (5 cm) and a target prepared with excess lithium (10% excess). SEM and AFM measurements show that the films are dense and quite rough. The local structure of the films was studied by infrared spectroscopy. The Fourier Transform Infrared Spectroscopy spectrum of LiMn2O4 consists of 2 major bands, the positions of which depend on the thickness of the film. The evolution of the FT-IR spectrum of LixMn2O4 over the temperature range −150°C to 250°C clearly shows the previously reported temperature-induced Jahn–Teller distortion between O and 10°C.

Metal particles produced by laser ablation for ICP-MS measurements

Pulsed laser ablation (266nm) was used to generate glass particles from two sets of standard reference materials using femtosecond (150fs) and nanosecond (4ns) laser pulses with identical fluences of 50Jcm(-2). Scanning electron microscopy (SEM) images of the collected particles revealed that there are more and larger agglomerations of particles produced by nanosecond laser ablation. In contrast to the earlier findings for metal alloy samples, no correlation between the concentration of major elements and the median particle size was found. When the current data on glass were compared with the metal alloy data, there were clear differences in terms of particle size, crater depth, heat affected zone, and ICP-MS response. For example, glass particles were larger than metal alloy particles, the craters in glass were less deep than craters in metal alloys, and damage to the sample was less pronounced in glass compared to metal alloy samples. The femtosecond laser generated more intense ICP-MS signals compared to nanosecond laser ablation for both types of samples, although glass sample behavior was more similar between ns- and fs-laser ablation than for metal alloys.

Laser-induced shockwave propagation from ablation in a cavity

The propagation of laser-induced shockwaves from ablation inside of cavities was determined from time-resolved shadowgraph images. The temperature and electron number density of the laser-induced plasma was determined from spectroscopic measurements. These properties were compared to those for laser ablation on the flat surface under the same energy and background gas condition. A theoretical model was proposed to determine the amount of energy and vaporized mass stored in the vapor plume based on these measurements.

Experimental and theoretical studies of particle generation after laser ablation of copper with a background gas at atmospheric pressure

Laser ablation has proven to be an effective method for generating nanoparticles; particles are produced in the laser induced vapor plume during the cooling stage. To understand the in situ condensation process, a series of time resolved light scattering images was recorded and analyzed. Significant changes in the condensation rate and the shape of the condensed aerosol plume were observed in two background gases, helium and argon. The primary particle shape and size distributions were measured using a transmission electron microscope, a scanning electron microscope, and a differential mobility analyzer. The gas dynamics simulation included nucleation and coagulation within the vapor plume, heat and mass transfer from the vapor plume to the background gas, and heat transfer to the sample. The experimental data and the calculated evolution of the shape of the vapor plume showed the same trend for the spatial distribution of the condensed particles in both background gases. The simulated particle size distr...

Energy deposition and shock wave propagation during pulsed laser ablation in fused silica cavities

Propagation of the shock wave generated during pulsed laser ablation in cavities was measured using laser shadowgraph imaging and compared with laser ablation on a flat surface. The cavities were fabricated in fused silica with the same diameter and variable depths to provide aspect ratios (depth/diameter) of 3 and 6. It was found that outside the cavity, after ~30 ns the radius of the expanding shock wave was proportional to t2/5, corresponding to a spherical blast wave as predicted by the Sedov–Taylor solution. The calculated pressures and temperatures of the shocked air outside of the cavities were higher than on the flat surface. The incident energy driving the shock wave outside the cavity was smaller than that on the flat surface; the greater the cavity aspect ratio, the smaller the energy supporting the shock wave. Plasma–wall interactions are discussed to explain the sharp decrease of the energy driving the shock wave outside the cavity.

Radiative cooling of laser ablated vapor plumes: experimental and theoretical analyses

A study was made of the cooling of the laser induced vapor plume in background air. The temperature and size variations of the vapor plume were determined from spectroscopic measurements during the first few tens of microseconds after the laser pulse. Experiments were carried out over a range of laser spot sizes and energies. The energy transport by thermal radiation from the vapor plume to the background air and to the test sample was formulated. Spectral line by line calculations were made by (a) calculating the detailed line emission profiles (valid for all optical depths), as well as by (b) dividing the lines into being either optically thin or optically thick. The calculations agreed with one another and with the experimental results for the decreasing vapor plume temperature. It was also shown that for optically thin conditions, which are often valid for small vapor plumes, the variation of the surface reflectivity of the test sample had very little effect on the cooling process. For optically thin ...

Analysis of laser ablation: Contribution of ionization energy to the plasma and shock wave properties

By fitting simulation results with experimentally measured trajectories of the shock wave and the vapor∕background gas contact surface, we found that inclusion of ionization energy in the analysis leads to a change in the evolution of the pressure, mass density, electron number density, and temperature of the vapor plume. The contribution of ionization energy to both the plasma and shock wave has been neglected in most studies of laser ablation. Compared to previous simulations, the densities, pressures, and temperatures are lower shortly after the laser pulse ( 50ns). The predicted laser energy conversion ratio also showed about a 20% increase (from 35% to 45%) when the ionization energy is considered. The changes in the evolution of the physical quantities result from the retention of the ionization energy in the vapor plume, which is then gradually transformed to kinetic and thermal energies. When ionization energy...

Expansion and radiative cooling of the laser induced plasma

To study the expansion and cooling process of the laser induced plasma generated by nanosecond pulsed laser ablation, experiments have been conducted which measure the position of the external shockwaves and the temperature of the vapor plumes. The positions of external shockwaves were determined by a femtosecond laser time-resolved imaging system. Vapor plume temperature was determined from spectroscopic measurements of the plasma emission lines. A model which considers the mass, momentum, and energy conservation of the region affected by the laser energy was developed. It shows good agreement to the experimental data.