The Curious Case of Transient Plasma Generated by ns-laser Ablation of Ag in Ar Gas

The continuous effort for transitioning pulsed laser deposition technique from an experimental tool into an industrial one is sustained by the use of complex technologies for in situ real-time monitoring of the deposition process. Floating Langmuir Probe (LP) and optical emission spectroscopy (OES) measurements were used for plasma monitoring during the pulsed laser deposition of Ag lms under various Ar pressure conditions. The electrical measurements revealed a complex multi-structured distribution of the ions, which was strongly inuenced by the increase of Ar pressure. The generation of highly energetic ions with the addition of Ar and the degree of Ar gas ionization was also explored. OES measurements conrmed the presence of highly ionized Ar species in the plasma. A peculiar effect was seen under pressures higher than 10 Pa, where the presence of high electronic charges lead to a perturbative behavior of plasma in the vicinity of the LP. The effect of these complex dynamics on the structure and quality of the deposited thin lm was also investigated revealing a strong correlation between the electronic distribution in plasma and thin lm thickness prole. equilibrium, heterogenous composition – photons, electrons, ions, molecules, clusters, etc..


Introduction
Understanding the fundamentals of well-established techniques like the pulsed laser deposition (PLD), has been the core driving force behind transitioning them from laboratory to industry 1 . The road to achieve this task has been arduous so far as there is a strong multi-component dependence (laser-targetgas-substrate) 2 that requires tailoring through empirical trial and error approaches for each material [3][4][5] , every desired properties, or applications. The complexities of the fundamental processes during PLD are well known. Our group in a recent series of papers discussed the plasma plume splitting 6 , ionic oscillations 7 or gas effect on metallic plasma dynamics 8 , complementing other groups results on phenomena like plasma ip-over effect 9 , plasma rebound 10 and complex structures formation 11 . The main approach for highlighting these processes has been the implementation of a wide range of diagnostic tools such as optical emission spectroscopy (OES) 12 , mass spectrometry 13 , laser-induced uorescence 14 , or Langmuir Probe (LP) method 15 . All of these methods, previously established for low or high temperature plasmas generated using different sources, were successfully adapted for the particularities of ns-laser produced plasmas 16 : rapid space and time evolution, local partial thermodynamic equilibrium, heterogenous composition -photons, electrons, ions, molecules, clusters, nanostructures, etc..
A particular interest has been given to laser-produced Ag plasma as Ag has shown promising results in a wide range of applications spanning from plasmonic resonance 17 , exible conductive electrodes 18 or even medical and biological applications 19 . From a diagnostic perspective several papers have reported on the behavior of LP in transient plasma generated by short or ultrashort laser ablation of Ag in various con gurations in terms of laser properties 20,21 , gas nature [22][23][24][25][26] and LP geometries 8,23,27,28 . The general consensus is that during the ablation process there are generated high energy multiple charged ions 9,14,20 (up to 100 eV), and complex nanostructures 20,22 that can affect the properties of the thin lms. Several studies have been focused on the angular distribution of Ag ions and the deposited material showing the dependence on the laser uence and gas nature with a widening of the ionic distribution seen with the increase of the background pressure 29 . However, the majority of the reported results only took into account the ionic saturation current as a measure of the ablation process 30 and used speci c at squared geometries placed against the expansion direction of the plasma. The range of measurements spanned from a few mm to tens of mm and often gave con icting results. Baraldi et al. 31 reported Ag ions with a kinetic energy of 50 eV measured at 4 cm in low vacuum 10 − 3 Pa on a wide range of laser uences 2.5-15 J/cm 2 , Amoruso et al. 29 report of 50 eV Ag ions recorded at 8 cm from the target generated by ablation with a laser uence of 2.4 J/cm 2 and a pressure of 10 − 6 Pa and Kang et al. 32 found at 4 cm 50 eV Ag ions generated with a 1.9 J/cm 2 uence at 10 − 4 Pa. These can be seen as limitation of using the saturation current regime of the probe as the LP does not differentiate between multiple charged ions or even different ionic species as the theory assumes only single ionized species in homogenous plasmas, which is unquestionably not true for ns-laser produced plasmas. We recently reported on an alternative approach for plasma monitoring 8 using oating probe measurements in connection with complementary OES investigations as an alternative to the classical approach for a more accurate tool to describe the kinetics of the ejected particles.
We analyzed by means of LP and OES the dynamics of a laser produced Ag plasma in Ar gas at different pressures. The main focus was to accurately characterize the laser produced plasma plume from both ionic and electronic points of view and to understand the role of Ar gas pressure in the PLD process. The LP and OES measurements were correlated with the properties of the Ag thin lms resulted from the PLD process, determined by surface analysis techniques.

Results And Discussion
The classical approach in LP measurements of metallic plasmas consists of recording the ion saturation current evolution in time. Although its bene ts are well known, it has one major disadvantage: when using the saturation current, all the sub-structuring of the ionic cloud is mediated and integrated under the envelope of the current, offering little information on the inner structuring of the plasma. We have recently shown 8 the advantages of using the oating current approach for Ag expansion in N 2 . In Fig. 1 it is represented the oating probe measurements at 0° recorded for all the expansion conditions investigated.
For 0.05 Pa and 0.5 Pa, a three-peak structure of the ionic and electronic regions can be observed, similar to the distribution seen for Ag plasma expansion in N 2 8 . The structuring of the plasma has been investigated before and it is accepted that different groups of particles can be ejected through different ablation mechanisms 29,33 . Electrons and fast ions will be rstly ejected through Coulomb explosion mechanism 34 , which subsequently will induce a transient double layer at the edge of the plasma leading to a differential acceleration of single and multiply-charged ions forming the outer-edge of the plasma, while slow atoms clusters or nanoparticles will expand with a signi cant lower velocity forming the core of the plasma. With the increase of the Ar gas pressure (from 0.05 Pa to 10 Pa) we observed that the two peak structure seen for the early stage (t < 2µs time region) merged into a single one at 2 Pa and exhibited an overall increase in ion density by a factor of 3.3. A closer look into the evolution of the oating electronic current revealed an unexpected dynamic. The transition from 0.05 Pa to 0.5 Pa was followed by a decrease in oating electron current by 50% and by a steep increase of the same current with the pressure. However, it is worth discerning that although the electron density decreases, the average velocity, estimated through the arrival time of the maximum electronic density, increases for pressures below 2 Pa being followed by a decrease for subsequent higher pressures. This rather peculiar effect that showcases an interesting dynamic of electrons during expansion could strongly impact the overall deposition process especially with the increasing Ar pressure in the reaction chamber.
For a more in-depth analysis of this electron depletion of the plasma on the main expansion axis (0 o ), we performed angular measurements at each Ar pressure ( Fig. 2a-d). As the Ag angular density distribution is relatively known and has been thoroughly investigated for a wide range of background gasses and irradiation conditions [35][36][37] , the natural choice is to change the approach towards a kinetic one and thus we represented the expansion velocity angular distribution for each of the three ionic structures and the main electronic one. For an Ar pressure of 5 × 10 − 2 Pa three ionic structures ( Fig. 2-b) were seen expanding with different velocities (tens of km/s for the 1st and a few km/s for the latter two). This result is in good agreement with data recently reported 24,38 . The width of the angular distribution is also strongly dependent on the range of kinetic energies describing each of the three ionic structures. The 2nd and 3rd structures exhibited a wider angular distribution, which can be correlated with the Ag-Ar collisions, con rmed by data from Fig. 2 34,40 suggests that the electrical eld will be stronger closer to 0° axis and will be weaker towards the edges of the plasma.
Therefore, in front of the plume there will be a strong oscillatory movement (as seen in Fig. 1 inset) of the electron cloud leading to a loss in their kinetic energy. As the Ar pressure was increased, the thermalization of the electronic structure was observed ( Fig. 3-c), with the front of the plasma showing a wide angular distribution and a quasi-constant distribution in the ± 40° cone with velocities ranging from 140 to 120 km/s. Similar behavior was found for 10 Pa with a wide angular distribution of the velocity centered around 140 km/s.
Further exploration into the Ar pressure effect on the ionic and electronic charge distribution involves the analysis of charge density distribution with the expansion velocity. This particular analysis consists of transforming the electronic or ionic time-of-ight signal into energy distribution. There are two approaches reported: the rst one reported by Krassa et al. 41 and Toftmann et al. 42 where the ionic saturation current is transformed into a charge yield distribution on their respective kinetic energy and the second one based on revising the LP classical approach for owing plasmas 15,16 . Both techniques involve the value of the ionic current in the saturation region of the I-V characteristic. It is worth noting that these techniques work well for pure, single-element plasmas with mostly single ionized species present, which is a well-known limitation of the technique. To overcome this, we used the oating current as a reference and implemented the approach from our previous report 8  Furthermore, the data can be analyzed in the framework of the two expansion regimes for laser-produced plasmas 26 : free expansion regime (collision-less) and the collisional regime. Given the wide range of Ar pressures used in this work we were able to capture dynamics without collisions (10 − 5 -10 − 2 Pa) and collisional regimes (for Ar pressure 10 − 1 -10 Pa). The transition collision-less/collisional occurs around 10 − 1 Pa where it appears a high energy ionic peak, therefore we can conclude that the interaction of high energetic plasma front with the Ar gas leads to the ionization of the gas and subsequent acceleration of the Ar ions. In the framework of our expansion scenario the ionization is an energetically favorable process as both electrons (127.3 eV @ 5 × 10 − 1 Pa) and ions (I -38 eV, II -20.5 eV @ 5 × 10 − 1 Pa) have enough kinetic energy to generate 1st and 2nd ionization states for Ar. The possibility of the background gas ionization scenario has been previously reported by Amoruso et al. 26  At higher pressures (5 Pa and 10 Pa) we observed a peculiar electron dynamics. In a time-span starting from 300 µs up to 3 ms, a modulated oscillatory regime was seen, with the effect being present for all measuring angles. This strange behavior has been previously reported for positively biased electrodes immersed in Ar plasma under similar pressures (> 2 Pa). It represents, according to the report of Dimitriu et al. 45 that a self-organized structure called plasma reball appeared on the surface of the electrode. The oscillations are induced by the disruption of the double layer formed around the reball and the ionic core being expelled in the plasma. The data presented here in Fig. 5 is of paramount importance for LP measurements and their limitation as real-time monitoring technique of PLD process, considering the relatively large measuring distance, 37 mm, the low voltage applied on the probe (4-20 V) and the PLD conditions (5-10 Pa -Ar gas). It means that in high Ar pressure conditions, for biases higher than 4 V a reball-like structure is formed on the probe surface, and thus LP is becoming perturbative for the deposition process. This is the rst report of a perturbative working regime for LP in PLD systems, offering a quantitative limitation in terms of pressure, gas nature and deposition geometries.
To con rm the Ar gas ionization scenario alternative, complementary techniques have to be implemented in order to reveal the presence of ionized Ar species in the collisional expansion regime. OES is one of the best techniques to complement the LP analysis as it can show the nature of the plasma species, with many reports in literature for its successful implementation for elemental analysis 12,46,47 and for background gas analysis in Ag expanding plasma 48 . The global emission (volume and time-integrated) from the plasma was acquired using an integration time of 100 µs. The results are presented in Fig. 5a where the overall emission spectra of an Ag plasma expanding in Ar at 10 Pa is represented. We identi ed using the specialized databases 49 the presence of atomic and single ionized species of Ag (Ag I -520.4 nm, Ag II -531.6 nm) and atomic, single and double ionized species of Ar (Ar I -455.5 nm, Ar II -655.8 nm, Ar III -350.2 nm). The increase in Ar pressure lead to an increase in all emission lines and thus an increase in thermal energy of the electrons consistent with our ndings in 8 where we investigated the effect of N 2 gas on Ag plasma. With the mention that Ar I optical signature in the 300-700 nm spectral range is scarce, we can safely assume that the Ar background gas was partially ionized throughout the plasma volume 50 . Spatially resolved optical emission spectroscopy was implemented to validate the partially ionization scenario. The results, presented in Fig. 5c revealed the excitation of new Ar II and Ar I species in the proximity of the substrate thus con rming that during expansion Ag plasma ionizes the Ar gas generated new species.

Thin lm analysis
The holy grail of PLD technique is often considered the direct linking of plasma properties and those of the deposited lms. With such a complex and layered behavior of Ag plasma generated by ns-laser ablation in Ar gas, as seen through LP and OES, an important question arises, how do these properties of the plasma affect the deposition of Ag lms. To tackle this, we have produced nanometric Ag thin lms in four conditions: 1 × 10 − 4 Pa, 5 × 10 − 2 , 2, and at 10 Pa Ar pressure. The chosen pressures are in line with the phenomena observed from LP measurements where different behaviors were seen for low (10 − 5 Pa -10 − 2 Pa) and high Ar pressures (10 − 1 Pa -10 Pa), coinciding with the non-collisional/collisional expansion regime of the laser produced plasma. As reported by Scharf 51 the expected effect of the background pressure over the Ag lms is re ected by the deposition rate. We con rm the results reported in 51 , where the deposition rate increases with the addition of Ar and is followed by a decrease at pressures higher than 4 Pa. The evolution is explained based on the ion thermalization which reduces the thin lm sputtering during the deposition process. Results expected as for the conditions used there in terms of laser uence (6-7 J/cm 2 ) and pulse duration (20 ns). We would like to mention that although the thermalization of the plasma is better re ected by the electrons and Ar ions where we see a decrease in their kinetic energy, for Ar pressure higher than 10 − 1 Pa we see steep increases in the ionic energy of about 50% attributed to the Ar ions generated during expansion. Therefore, the plasma thermalization cannot be the only mechanism in uencing the deposition process as their distribution is less affected by the addition of Ar (Fig. 3). Our angular measurements presented unique distributions for both the ionic and electronic particle clouds which will no doubt affect the thickness distribution of the Ag lms. To better understand that we performed optical transmission spectroscopy for the estimation of the thin lm thickness. The angular distribution of the Ag lm growth was analyzed from an experimental deposition on a microscope slide glass substrate 25 × 75 mm 2 . The images of deposited lms are shown in Figure  S1 (Supporting Information). The thickness was estimated by tting an appropriate model to the obtained transmittance spectra. The model is based on Fresnel equations of a thin, semi-absorbing layer on a thick, non-interference, transparent layer (glass substrate) using the optical functions for Ag from 52 . We included Ag lm roughness in this model as an additional layer with optical functions derived from the effective medium approximation of two components: silver and voids (Bruggeman model). The thickness was estimated by tting an appropriate model to the obtained transmittance spectra. The example of t for four different Ag layer thicknesses of the Ag layer is shown in Fig. 6a, where the thickness estimated from the tting is displayed for each of the curves. Here are shown some examples of the spectrum measured for different thicknesses within the same deposited lm. The tted simulation is depicted as a dashed, red curve.
The thickness distribution of the thin lm (Fig. 6b) resemblance the angular distribution of the electron cloud: with energetic minima at low Ar pressure in the central region (0° ± 10°) and higher electron energies towards the edge of the plasma. Thus, we see that there is a more intimate connection between the distribution of electrons and the thickness of the lm, which infers that the deposition rate will be in uenced by the kinetics of the electrons in the substrate vicinity. When considering the dynamics at the front of the plume where the in the framework of ambipolar diffusion mechanism we can further correlate the deposition scenario with the charged particle dynamics at the front of the plasma plume. The drop in the center of the plume (0° ± 10°) will lead to reduced acceleration and lower oscillatory behavior which will mean that in the central part of the plume the ions are not signi cantly affected. While at the edge of the plasma ions will suffer a higher acceleration and thus leading to an enhancement of ion density accumulated in the outer regions of the plasma. Results are supported by the oating probe angular measurements where 2nd and 3rd ionic structures have a wider distribution with maxima close to 30°. With the increase of the pressure the scenario changes, the thermalized electronic cloud is now uniformly accelerating the plasma ions and thus the resulting lm thickness pro le will follow the shape of the ablated ionic cloud (Fig. 2d).
Further investigations on particle kinetic energy at the substrate surface were performed using XRD analysis and AFM measurements. All deposited silver lms are polycrystalline ( Figure S2 from Supporting Information) and have one phase (FCC structure) with a lattice constant a = 0.408 ± 0.001 nm. The lattice parameter didn't change for silver lms deposited at different argon pressure. We observed only that the intensity of re exes correlated to lm thickness. In Fig. 7 we can see the morphology of silver lms in the central part of lms in the vicinity of 0°. The crystal size for lms deposited at 1.1 × 10 − 4 , 5 × 10 − 2 , 2 Pa in the range 20-80 nm. The addition of Ar leads to the formation of smaller sized nanoparticles nanocrystals (20-30 nm) and an increase of layer thickness by a factor of 3, result correlated with a uniformization of the electron cloud and the overall plasma thermalization. The subsequent increase of Ar pressure (5-10 Pa) also leads to the generation of a higher density of Ar ionic species which could hinder the deposition process through sputtering processes. The decrease of the thin lm thickens previously believed to be driven by the scattering of the Ag species on Ar gas.
Finally, let us address the LP perturbative behavior effect on the deposition process. The thin lm analysis did not reveal any impurities nor were the Ag lm growth mechanisms affected, since the perturbative regime appears at a high Ar pressure. To understand these results two important aspects need to be addressed. The rst one is related to the perturbation propagation within the plasma volume. Any perturbation will be con ned within the plasma Debye sphere. This volume is typically described by a radius larger than the Debye Length by a factor of ten. Our investigations revealed the values for the Debye lengths at high Ar pressure of the order of µm which means that the maximum plasma volume that will be perturbed by the probe will be 4 × 10 3 µm 3 , a value that decreases with the addition of supplementary Ar gas in the system (con nement of the Ag plasma). The reduced volume and the relatively higher distance between probe and substrate (13 mm) ensure the fact that even when the probe enters a perturbative regime the pulsed laser deposition process is not affected.

Conclusions
Langmuir probe measurements and optical emission spectroscopy were implemented for in-situ measurements of pulsed laser deposition of Ag in various pressures of Ar, from 10 − 5 Pa to 10 Pa. Floating probe regime was used to investigate the Ar pressure effect on the plasma multi -structuring. Three ionic and electronic peaks were observed at low Ar pressures with the addition of Ar leading to the con nement of these multiple structures. Angular distribution of ions and electron revealed a complex transition of the electron kinetic energy. The increase in Ar pressure results in a widening of the ionic angular distribution with a pronounced effect on the low energy ions.
An energy depletion was observed for the free expansion conditions across the main expansion axis followed by thermalization of the electrons in the collisional regime. The increase of the Ar pressure resulted in the appearance of a new high energetic ion structure induced from the ionization of the Ar gas by the Ag plasma front. Optical emission spectroscopy measurements con rmed the complete ionization of the gas through spectral signatures of Ar single and multiple ionized ions during pulsed laser deposition. Thin lms deposited under identical conditions revealed that the thickness of the Ag lms mirrors the electron angular distribution revealing the in uence of an electrostatic mechanism over the thin lm growth. The thermalization of the Ag plasma also leads to enhancement of lm thickness up to an optimum of 2 Pa. The subsequent increase in Ar pressure leads to sputtering of the lm by the ionized Ar species. Positive biasing of the probe in Ag plasma expanding in 10 Pa of Ar lead to the formation of a reball like structure on the probe surface, which induced an oscillatory regime in the 300 µs -3 ms timespan. This is a novel perturbative regime of the Langmuir probe, which potentially can affect the deposition process. In our irradiation conditions and Ar gas pressures there are no signi cate in uences of this perturbative regime onto the PLD process.

Methods
A YAG Laser (λ = 266 nm, Energy = 81 mJ) at 10 Hz repetition rate was focused on a silver target (99.99% purity) at a uence of 3.8 J/cm 2 . The 6 mm thick silver target was continuously rotated to provide a fresh surface per each shot and avoid the local heating and crater formation. The target to substrate and to LP distances were 50 mm and 37 mm, respectively. The plasma investigations were performed under a residual vacuum of 5 × 10 − 5 Pa, and at 5 × 10 − 2 , 5 × 10 − 1 , 2, 5, and 10 Pa Ar pressure. Each experiment was preceded by a cleaning procedure during which the LP was shielded from the incoming transient plasma while the rotating silver target was laser irradiated with 1200 pulses (for other details on the setup geometry please see 8 ). The signal from tungsten LP (diameter -0.2 mm and exposed length -2 mm) was measured by collecting the voltage signal across a load resistor with a Tektronix DPO 4140 oscilloscope. All the performed investigations were time-synchronized by a fast silicon photodiode (Thorlabs FDS100) establishing the initial measuring moment as the moment when the laser was red.
The light emitted from plasma was transmitted through a linear bundle of 19 channel optical bers, which simultaneously record the optical emission from a 40 mm length strip oriented parallel to the plasma expansion axis. Optical spectra were collected by an iHR550 Imaging Spectrometer (Horiba) using a 2400 mm − 1 grating. Plasma image was recorded with a LN 2 cooled Symphony CCD camera (Horiba) having 2048 × 512 pixels in a wavelength range of 200-900 nm.
Ag thin lms were deposited on glass and Si substrates positioned at 50 mm from the target using 5000 consecutive pulses without substrate heating in identical irradiation conditions selected values of Ar pressures (in vacuum at1 × 10 − 4 Pa and in Ar at 5 × 10 − 2 , 2 and 10 Pa). The lateral distribution of the Ag lm growth was analyzed from an experimental deposition on a glass substrate 25 × 75 mm 2 . The measurement setup was arranged from a light source, light-collecting optics, and a minispectrophotometer. The transmittance spectra were measured by a spectrometer USB4000, Ocean Optics, in the spectral range from 300 to 890 nm. The light source, DT-mini-2-GS -Ocean Optics, was coupled with a focusing lens by an optical ber with a 200 µm core diameter. Light spot diameter on the sample was about 2 mm, which determined the lateral resolution of the transmittance measurement. Atomic force microscopy (AFM Dimension ICON, Bruker) was used to investigate the surface morphology, roughness and thin lm thickness. The measurements were performed under ambient conditions and images were obtained by the Peak Force Tapping mode using ScanAsystAir tips with scan areas of 1 × 1 µm 2 .
XRD measurements were carried out using the X-ray diffractometer (Empyrean, Malvern Panalytical) with a monochromated Cu K <α> radiation (photon energy E = 8.04 keV, wavelength λ = 1.54151 Å, U = 45 kV, I = 30 mA) in grazing incidence X-ray diffraction mode. The incident angle was 0.85° that corresponds to ~ 10 mm of sample irradiation. The XRD measurements were made from a central part of deposited lm MN was responsible for the OES measurements and data interpretation; LF was responsible for the AFM measurements and interpretation; JB was responsible for optical characterization of the lms and modeling of the data; VC and JL supervised the project and were responsible for the surface analysis. All authors contributed to writing the and reviewing the manuscript Figure 7 Transmittance spectra of Ag lm deposited at 10 Pa Ar pressure (blue, solid curves), tted simulation (red, dashed curves) (a) and angle distribution of thin lm thickness for selected Ar pressures 1.1×10-4, 5 ×10-2, 2, 10 Pa (b).

Supplementary Files
This is a list of supplementary les associated with this preprint. Click to download.