Convoluted effect of laser fluence and pulse duration on the property of a nanosecond laser-induced plasma into an argon ambient gas at the atmospheric pressure

Abstract

We studied the behavior of the plasma induced by a nanosecond infrared (1064 nm) laser pulse on a metallic target (Al) during its propagation into argon ambient gas at the atmospheric pressure and especially over the delay interval ranging from several hundred nanoseconds to several microseconds. In such interval, the plasma is particularly interesting as a spectroscopic emission source for laser-induced plasma spectroscopy (LIBS). We show a convoluted effect between laser fluence and pulse duration on the structure and the emission property of the plasma. With a relatively high fluence of about 160 J/cm2 where a strong plasma shielding effect is observed, a short pulse of about 4 ns duration is shown to be significantly more efficient to excite the optical emission from the ablation vapor than a long pulse of about 25 ns duration. While with a lower fluence of about 65 J/cm2, a significantly more efficient excitation is observed with the long pulse. We interpret our observations by considering the post-ablation interaction between the generated plume and the tailing part of the laser pulse. We demonstrate that the ionization of the layer of ambient gas surrounding the ablation vapor plays an important role in plasma shielding. Such ionization is the consequence of laser-supported absorption wave and directly dependent on the laser fluence and the pulse duration. Further observations of the structure of the generated plume in its early stage of expansion support our explanations.

Characteristics of indirect laser-induced plasma from a thin film of oil on a metallic substrate

“…In our experiment, according to the criteria of line selection, the Al I (309.3 nm) evaporated from the aluminum target, the Ar I (696.5 nm) and Ar II (480.6 nm) evaporated from the ambient gas were determined which were the same as the selection in Ref. [27]. The Na I (588.9 nm) and Cl I (837.6 nm) were evaporated from the oil film.…”

“…In this work, the electron density is calculated by the Stark broadening of the Ar I 696.5 nm line with aluminum substrate only in the Ar ambient gas, and the H α 656.27 nm line with aluminum substrate coated the oil film in the Ar ambient gas. As for Ar ambient gas, the Ar I 696.5 nm line has been verified to be a reliable reference line to calculated in our previous work [27]. And for the H α line, it is used widely to calculate the electron density when H α line can be detected in the plasma, which has a higher precision as it is well isolated, it is strongly affected by the linear Stark effect and does not exhibit self-absorption [31,32].…”

“…Multi-Saha-Boltzmann plots are used to deduce the temperature. A detailed description of the used procedures together with a list of the chosen lines can be found in our previous publication [27]. The relative standard deviations of the electron density and the temperature are calculated to be 20% for Ar I line, 15% for H α line and 10%, respectively.…”

Influence of pulse width on the laser ablation of zinc in nitrogen ambient

“…Dynamics of the ablated species can be theoretically modeled using a shock wave model at higher pressures [18], a blast wave model [19] at moderate pressures, and molecular dynamics simulations at very low pressures [20]. However, in the case of fs laser-produced plasmas (LPPs), the laser pulse does not interact with the generated plasma since the electron-ion collision time and heat conduction time are in the order of picoseconds [11,21]. Thus, fs laser ablation is a simple phenomenon but is followed by a fairly complex expansion exhibiting a different expansion dynamics when compared to ns LPP, which can be described using the plasma-annealing model and the two-temperature model [22].…”

“…Ablation yield, material removal, ionization, and plume expansion to the surroundings depend on parameters such as fluence [7][8][9], pulse width [10,11], wavelength [12][13][14], spot size [15], and material properties, along with the nature and pressure of the ambient gas [16,17]. Unlike femtosecond (fs) laser ablation, nanosecond (ns) laser ablation is a fairly complex phenomenon, as the ablated species can interact with the trailing edge of the relatively longer laser pulse, leading to further excitation and ionization.…”