O. Barthélemy

Temporal characterization of femtosecond laser pulses induced plasma for spectrochemical analysis of aluminum alloys

Abstract This paper reports studies on time-resolved space-integrated laser induced breakdown spectroscopy (LIBS) of plasmas produced by ultrashort laser pulses at atmospheric pressure, on aluminum alloy targets. The temporal behavior of specific ion and neutral emission lines of Al, Mg and Fe has been characterized. The results show a faster decay of continuum and spectral lines, and a shorter plasma lifetime than in the case of longer laser pulses. Spectroscopic diagnostics were used to determine the time-resolved electron density, as well as the excitation and ionization temperatures. In comparison with plasmas produced by ns laser pulses, the plasma generated by ultrashort pulses exhibits a faster thermalization. Analytical performances of fs-LIBS were also evaluated. Linear calibration curves for minor elements (Mg, Fe, Si, Mn, Cu) presented in aluminum alloys were obtained. The limits of detection are in the parts per million (ppm) range and are element-dependent.

Investigation of the State of Local Thermodynamic Equilibrium of a Laser-Produced Aluminum Plasma

In this work, the assumption of local thermodynamic equilibrium (LTE) for a laser-induced plasma in ambient air is examined experimentally using two different laser systems, namely an infrared short-pulse Ti:Sapphire laser and an ultraviolet long-pulse XeCl excimer laser. The LTE assumption is investigated by examining the plasma produced at a laser fluence of 10 J/cm2 from aluminum targets containing iron and magnesium impurities. The excitation temperature is deduced from Boltzmann diagrams built from a large number of spatially integrated neutral iron lines distributed from 3.21 to 6.56 eV. It is shown that at any time after the end of the laser pulse, the neutral excited states are in excellent Boltzmann equilibrium. Detailed investigation of Boltzmann equilibrium further validates previous temperature measurements using less accurate diagrams. However, observations of ion lines provide some evidence that the ionized species do not obey Saha equilibrium, thereby indicating departure from LTE. This could be explained by the fact that the plasma cannot be considered as stationary for these species.

Influence of the laser pulse duration on laser-produced plasma properties

In the framework of laser-induced plasma spectroscopy (LIPS) applications, time-resolved characteristics of laser-produced aluminium plasmas in air at atmospheric pressure are investigated for laser pulse durations ranging from 100 fs to 270 ps. Measurements show that for delays after the laser pulse longer than ~100 ns, the plasma temperature increases slightly with the laser pulse duration, while the electron density is independent of it. In addition, as the pulse duration increases, the plasma radiation emission lasts longer and the spectral lines arise later from the continuum emission. The time dependence of the continuum emission appears to be similar whatever the duration of the laser pulse is, while the temporal evolution of the line emission seems to be affected mainly by the plasma temperature. Finally, as far as spectrochemical applications (such as LIPS) of laser-produced plasmas are concerned, this study highlights the importance of the choice of appropriate temporal gating parameters for each laser pulse duration.

Influence of the laser pulse duration on spectrochemical analysis of solids by laser-induced plasma spectroscopy.

Quantitative analysis of aluminum and copper alloys by means of laser-induced plasma spectroscopy (LIPS) has been investigated for three representative laser pulse durations (80 fs, 2 ps, and 270 ps). The experiments were carried out in air at atmospheric pressure with a constant energy density of 20 J/cm2. Because the decay rate of the spectral emission depends on the laser pulse duration, the optimum detection requires an optimization of the temporal gating acquisition parameters. LIPS calibration (sensitivity and nonlinearity) and the limit of detection (LOD) are discussed in detail. While the LOD of minor elements embedded in alloy samples obtained by sub-picosecond or sub-nanosecond laser pulses are both time and element dependent, provided an appropriate temporal window is chosen, the optimum LODs (several parts per million (ppm)) prove to be independent of the laser pulse duration. Finally, it is found that for elements such as those detected here, gated LIPS spectra using picosecond or sub-picosecond laser pulses provide much better LOD values than non-gated spectra.

Numerical simulations of ultrashort laser pulse ablation and plasma expansion in ambient air

Abstract Using a self-consistent one-dimensional Cartesian Lagrangian fluid code, we modeled the ultrashort laser pulse ablation of solid aluminum and the subsequent plasma expansion in ambient air. A laser fluence of approximately 10 J/cm 2 is considered. The code axial plasma temperature and density are strongly inhomogeneous and the maximum radiation emission generally occurs in the front of the plasma. The code average plasma temperature is in good agreement with the experiments for all times, while larger discrepancies with respect to the experiments are observed at late times for the plasma density. Experimental results are in reasonable agreement with the condition of thermodynamic equilibrium, which is an important assumption in the model.

Laser-ablated volume and depth as a function of pulse duration in aluminum targets.

The ablated depth and volume per laser pulse from an aluminum target were measured for pulse durations that ranged from 80 fs to 270 ps at an average fluence of approximately 100 J/cm2 and a wavelength of 0.8 microm. The ablated volume showed a flat maximum for subpicosecond pulses and a minimum for approximately 6 ps. The crater diameters were rather constant up to pulse durations of approximately 6 ps and increased for larger pulse durations. As a result, the ablated depth also showed a plateau for subpicosecond pulses but decreased monotonically with pulse duration. A physical interpretation of these results and their consequences for laser applications are discussed.

Characterization of the expansion of an aluminum laser-induced plasma in ambient air by fast photography

Laser-induced aluminum plasma in ambient air at a fluence of 10 J/cm/sup 2/ is observed by means of fast photography. Plasma expansion is found to be nearly hemispherical. The existence of an air plasma at the edge of the Al plasma is suggested by comparing spectrally resolved and unresolved pictures.

Temporal evolution of the state of local thermodynamic equilibrium of a laser produced plasma at low fluence

Summary form only given, as follows. Of the few studies that have been devoted to examining departure from LTE conditions, several suggest that laser produced plasmas at low fluences could be far from LTE at early times (before typically 1 /spl mu/s). The present work addresses this issue by investigating the departure from LTE conditions in Al plasmas created by a 10 ns excimer laser pulse at a wavelength of 308 nm with fluences of about 10 J/cm/sup 2/. The Boltzmann diagram method is applied to a few lines emitted by iron impurities contained in an Al matrix. Time evolution of the LTE departure is investigated by probing the plume at various delays after the laser pulse interaction with the target.