Dual-Pulse LIBS Using a Pre-Ablation Spark for Enhanced Ablation and Emission

Abstract

In this paper we report the first observations of dual-pulse laser-induced breakdown spectroscopy (LIBS) signal enhancements by using a pre-ablation spark. In this technique a laser pulse is brought in parallel to the sample surface and focused a few millimeters above it to form an air plasma or air spark. A few microseconds later a second laser pulse, which is focused on the sample, ablates sample material and forms the LIBS plasma from which analyte emission occurs. In this way, large LIBS signal enhancements, 11-to 33-fold, are observed for copper and lead, respectively, relative to the signal in the absence of the air spark. In all cases where enhanced LIBS signals are seen, greatly enhanced sample ablation also occurs.

Laser Induced Breakdown Spectroscopy

“…Since then, several papers dealing with double-pulse LIBS have reported, mainly, higher intensities of emission lines, typically up to 2 orders of magnitude, and lower LOD, down to parts per billion (ppb) or sub-ppb, in comparison with single-pulse LIBS. 107,[113][114][115][116][117][118][119][120][121][122][123][124] The first papers employing orthogonal configurations in re-heating and pre-ablation modes for double-pulse LIBS date from 1991 and 2000, respectively. 115,116 Although most papers dealing with multi-pulse excitation employ the double-pulse approach, a recent paper have demonstrated emission measurements after seven excitation pulses, using a microchip laser with 25 µs of interpulse separation, reporting up to 129 fold line emission enhancements in comparison with the singlepulse approach.…”

“…107,[113][114][115][116][117][118][119][120][121][122][123][124] The first papers employing orthogonal configurations in re-heating and pre-ablation modes for double-pulse LIBS date from 1991 and 2000, respectively. 115,116 Although most papers dealing with multi-pulse excitation employ the double-pulse approach, a recent paper have demonstrated emission measurements after seven excitation pulses, using a microchip laser with 25 µs of interpulse separation, reporting up to 129 fold line emission enhancements in comparison with the singlepulse approach. 107 It is important to point out that the analytical advantages of double or multi-pulse excitation cannot be explained just in terms of a higher total delivered energy due to the application of more than one pulse instead of just one, because some papers have also reported these improvements even when the total energy of a double or multi-pulse was the same of that of a single-pulse.…”

Laser-Induced Breakdown Spectroscopy: Fundamentals, Applications, and Challenges

“…These two pulses can excite the Figure 3: Dual pulse configurations (a) shows collinear configuration; the first pulse ablates the sample and the second one reheats the plasma [34], (b) is an orthogonal pre-ablative configuration; the first pulse creates a spark on the surrounding media and the second one ablates the sample [14,35], (c) shows the same idea as (a), but the plasma is reheated in an orthogonal way [36].…”

“…In the orthogonal configuration, the second pulse does not ablate more material, but reheats the plasma and reexcites the material ablated by the first pulse. Other authors have given reasons for the signal enhancement on preheating orthogonal configurations, such as a reduction in density or pressure of the surrounding media due to the first pulse [35,[37][38][39].…”

Tailored Ultrafast Pulses for Selective Energetic Residue Sampling

“…For the first method, a focused ns laser pulse was added to the plasma generated by the fs laser pulse. Double-pulse LIBS, i.e., adding two collinear pulses separated by hundreds of nanoseconds, has been shown to increase signals (6). The reasons for the enhancement are not entirely understood but are thought to be a combination of several factors, including a decrease of atmospheric gas density in the plasma (7).…”