fs/ns dual-pulse LIBS analytic survey for copper-based alloys

Abstract

The quantitative analytic capability of a fs/ns dual-pulse Laser-Induced Breakdown Spectroscopy technique, based on the orthogonal reheating of a fs-laser ablation plume by a ns-laser pulse, is presented. In this work, it is shown how the effect played by the delay times between the two laser beams can vary the analytical response of this dual-pulse LIBS configuration. In order to address this task, the Sn, Pb and Zn calibration curves of five certified copper-based samples have been investigated. These calibration curves have been obtained, in air at atmospheric pressure, by integrating the emission data collected in two different inter-pulse delay zones, one in the delay interval of 1–41 μs, the other within the range of 46–196 μs. For drawing the species calibration curves, the emission intensities of the considered Pb(I), Sn(I) and Zn(I) electronic transitions have been normalized with a non-resonant Cu(I) emission line. The experimental results have shown that, by varying the inter-pulse delay between the two laser beams, complementary analytical results can be induced. By considering at once all data acquired within the inter-pulse delay time of 1–196 μs, this hypothesis has been strengthened. The calibration curves obtained in this way are characterized by excellent linear regression coefficients (0.988–0.999) despite of the large Sn, Pb and Zn compositional variation of the targets employed. The results presented reveal, for the first time, that, by taking into account the role played by the inter-pulse delay time between the two laser beams, the fs/ns dual-pulse LIBS configuration here used can be improved and provide very good opportunities for performing quantitative analysis of copper-based alloys.

Introduction

The Laser-Induced Breakdown Spectroscopy (LIBS) analytical technique has been widely used for its versatility simplicity and for not needing any sample preparation. Nonetheless, several difficulties have been arisen as a consequence of the whole processes involved during the laser material interaction and the emitting plasma evolution. Thus, a large interest in characterizing the emission species parameters and relating them to the target material composition is continuously under study. In this context, a widespread attention about the advantages offered by ultra-short laser pulses, for these spectrometric technique applications, has largely developed [1], [2], [3], [4], [5], [6], [7], [8], [9]. The main benefit offered by ultra-short laser sources is related to their ability of preventing the ablated material from a diffuse thermal ablation mechanism ensuring, consequently, a high preservation of the sample stoichiometry [10], [11], [12], [13]. A peculiar characteristic of emitting plasma induced by these laser pulses is related to the absence of interactions between the laser beam and the formed expanding plasma. This effect provides background continuum and line intensity emissions lower than those induced by ns laser pulses. The former can be considered as an advantage in order to obtain good analytical results even without gated detectors [2], [14], whereas the latter can be seen as a drawback for the technique sensitivity [15]. With the aim of overtaking the low sensitivity supplied by ultra-short laser pulses, a dual-pulse LIBS (DP-LIBS) scheme, performed by combining an ultra-short laser pulse with a successive ns one, has been applied [14], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. The emission signal enhancements observed can represent a valid development for LIBS performed by ultra-short pulses in order to ensure the minimal thermal damage of the sample surface improving the technique sensitivity. In this work, our attention focused on the effects played by the inter-pulse delay between the ablating fs and the following ns laser beams on the analytical capabilities of this dual-pulse (DP) configuration. In this way, it has been shown how the observed fs laser-induced plasma components [26], [27], [28], [29], [30] might be related to the different DP-LIBS response obtained by changing the inter-pulse delay between the two laser beams. With this purpose, a survey on challenging materials such as copper-based alloys has been undertaken by using 1.0 J cm⁻² for the first ablating fs laser pulse and 130 J cm⁻² for the subsequent ns laser beam. The latter was focused at 0.5 mm from the target surface whereas the two beams inter-pulse delays were varied from 1 up to 196 μs. For highlighting the role played by the inter-pulse delay time on calibration linear responses of this set-up, five certified standard alloys have been employed. The elemental contents of these were used for drawing Pb, Sn and Zn calibration curves using, as internal standard, a non-resonant Cu(I) emission line belonging to the same spectral range of the investigated element.