A comparative study of carbon plasma emission in methane and argon atmospheres

Accepted Manuscript A comparative study of carbon plasma emission in methane and argon atmospheres

H. Yousfi, S. Abdelli-Messaci, O. Ouamerali, A. Dekhira PII: DOI: Reference:

S0584-8547(17)30327-0 doi:10.1016/j.sab.2018.02.006 SAB 5375

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date: Revised date: Accepted date:

24 July 2017 31 December 2017 11 February 2018

Please cite this article as: H. Yousfi, S. Abdelli-Messaci, O. Ouamerali, A. Dekhira , A comparative study of carbon plasma emission in methane and argon atmospheres. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sab(2017), doi:10.1016/j.sab.2018.02.006

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ACCEPTED MANUSCRIPT

A Comparative Study of Carbon Plasma Emission in Methane and Argon Atmospheres

H. YOUSFI a,b*, S. ABDELLI-MESSACI b, O. OUAMERALI a, A. DEKHIRA a Laboratoire de chimie théorique computationnelle et photonique, faculté de chimie, Université

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a

des Sciences et de la Technologie Houari Boumediene USTHB, BP 32 El-Alia, Bab Ezzouar,

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Alger 16111, Algeria b

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Centre de Développement des Technologies Avancées, cité 20 août 1956, BP 17, Baba Hassen, Alger, Algérie

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*[email protected]

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[email protected]

[email protected]

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[email protected]

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Corresponding author Name: Houssyen Yousfi Affiliation:

Laboratoire de chimie théorique computationnelle et photonique, faculté de chimie,

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a

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Université des Sciences et de la Technologie Houari Boumediene USTHB, BP 32 El-Alia, Bab Ezzouar, Alger 16111, Algeria b

Centre de Développement des Technologies Avancées, cité 20 août 1956, BP 17, Baba

Hassen, Algiers, Algeria Port.: +213 795589930 Tel.: +213 21 35 10 18 Fax: +213 21 35 10 39 E-mail address: [email protected] 1

ACCEPTED MANUSCRIPT Abstract The interaction between laser produced plasma (LPP) and an ambient gas is largely investigated by Optical Emission Spectroscopy (OES). The analysis of carbon plasma produced by an excimer KrF laser was performed under controlled atmospheres of methane and argon. For each ambient gas, the features of produced species have been highlighted. Using the time of flight

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(TOF) analysis, we have observed that the C and C2 exhibit a triple and a double peaks

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respectively in argon atmosphere in contrast to the methane atmosphere. The evolution of the

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first peaks of C and C2 follows the plasma expansion, whereas the second peaks move backward, undergoing reflected shocks. It was found that the translational temperature,

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obtained by Shifted Maxwell Boltzmann distribution function is strongly affected by the nature of ambient gas. The dissociation of CH4 by electronic impact presents the principal approach

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for explaining the emission of CH radical in reactive plasma. Some chemical reactions have

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been proposed in order to explain the formation process of molecular species.

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Key words:

Optical Emission Spectroscopy, Laser Ablation, Laser-induced carbon plasma, Shifted

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Maxwell Boltzmann Distribution Function.

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ACCEPTED MANUSCRIPT 1. Introduction Optical emission spectroscopy (OES) is one of the most powerful non-intrusive plasma diagnostic techniques that can be used to study the interaction between laser-produced plasma (LPP) and the ambient gas. The laser ablation (LA) technique, inducing the formation of plasma in gas environment, is very appealing for several technological applications, such as inductively

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coupled plasma atomic emission spectrometry (ICP-AES) [1], inductively coupled plasma mass

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spectrometry (ICP-MS) [2] and laser induced breakdown spectroscopy (LIBS) [3]. The latter

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presents many advantages such as rapid multi-elemental analysis of any kind of matter and no sample preparation. It is non-intrusive tool for analyzing plasma, and for gathering qualitative

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and quantitative information about emitting species [4]. The laser induced plasma spectroscopy (LIPS) diagnostics are generally performed in air at atmospheric pressure or in controlled

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environment. The LIBS technique helps in detecting transition species in the plasma plume as well as the particles from the background gas. The laser ablation is a short time-scale

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phenomena and plasma dynamics is dependent on the incident laser parameters (wavelength,

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pulse duration, fluence) and irradiated material properties (optical and thermodynamic properties) as well as on the nature and pressure of the ambient gas [5].

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In reactive gas environment, the LA processes are governed by a variety of non-linear mechanisms. Indeed, electrons, ions, molecules and clusters containing plasma are separated in

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time and space in vacuum, which is not always the case in presence of gas. It is not easy to understand when and where these particles and molecules are formed [6]. Their formation can be explained by the atomic collision and the recombination process [7]. The interaction between plasma plume and gas particles leads to complex dynamics including different physicochemical mechanisms, inducing shockwaves, hydrodynamics instabilities and chemical reactions [8]. Haider et al. have reported the effect of ambient gas of air, helium and argon on graphite plasma in which they determined the temperature and electron density in each background

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ACCEPTED MANUSCRIPT environment. In the study performed by Krstulovic et al. using a combination of spectroscopic techniques concerning the laser ablation of Mn target in vacuum and in the presence of CH 4, the C2 and MnH bands have been observed [9]. Harilal et al. investigated the plasma gas dynamics role in the formation of AlO in laser-produced Al plasmas generated in air at atmospheric pressure. They found that plume hydrodynamics plays a significant role in

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redefining plasma thermodynamics and molecular formation [6]. Using the optical emission

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spectroscopy and the spectrally resolved and integrated fast imaging, Al-Shboul et al. have

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studied the dynamics of C2 emission in vacuum and in helium ambience. They observed linear and non-linear evolution in vacuum and helium gas respectively [10].

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In this work, we have studied the behavior of carbon plasma created by a KrF laser under methane and argon environments using optical emission spectroscopy (OES). We performed

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the experiments using a laser fluence of 9.6 J/cm2 and a pressure of 0.3 mbar for methane, whereas for argon, the two values of pressure applied were 0.3 and 1 mbar. A comparison

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between the time-of-flight profiles (TOF) of each emission species into the two gases has been made. Using Shifted Maxwell Boltzmann distribution function, an estimation of translational temperature and stream velocity of certain species have been carried out as function of distance

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from the target. Spatiotemporal evolution of species provided from plasma, gas, plasma-gas

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interaction has been studied in vacuum, methane and argon environments. We discussed the different chemical reactions leading to atomic and molecular formation in laser-induced carbon plasma.

2. Experimental setup Fig.1 shows the experimental setup for laser ablation and optical emission spectroscopy. We used in this experiment a graphite target of high purity (99.5%), which is slowly rotated to avoid drilling and cratering. The laser induced plasma (LIP) of graphite samples was generated by a 4

ACCEPTED MANUSCRIPT KrF excimer laser (Lambda Physik Compex 102, λ = 248 nm, 25 ns pulse duration) with a maximum pulse energy of 280 mJ. The graphite target was irradiated by laser beam at an incidence angle of 45° through a normal of the target surface for a perpendicular ejection of matter and to keep the transmission of the hublot. The laser beam was focused on impact surface by two cylindrical lenses in order to obtain the spot size of 0.02 cm2, providing a laser fluence

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of 9.6 J/cm2. The target was mounted inside a stainless steel chamber evacuated to a pressure

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of 10-6 mbar and then filled with methane at a pressure of 0.3 mbar and then with argon at a

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pressure of 0.3 and 1 mbar. An Acton 750 spectrometer was calibrated using a mercury lamp. The light emitting plasma was captured on an entrance slit of spectrometer of dimensions of

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100 µm width x 2mm length which is leading to a spectral resolution of 0.06 nm. The spectrometer was coupled to a fast Hamamatsu R294 photomultiplier (PM) with 2 ns rise time.

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The PM was connected to an oscilloscope (Tektronix TDS3032, 5GS/s, 300MHz) to record the signal of time of flight (TOF) of the emitting species. The TOF signal was finally presented on

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a computer using the Wavestar software package. An ICCD detector (Princeton instruments PI-

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MAX, 1024 x 256 pixels, pixel size= 24 x 24 µm) was used for recording plasma emission

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spectra at different times after onset plasma expansion.

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3. Plasma emission and species evolution 3.1 Emission spectra The laser-target, plasma-laser and plasma-gas interactions lead to complex physicochemical short duration processes that complicate the identification of the plasma emission composition. Description of plasma starts by trying to characterize the emitting species (atom, ion, and molecule) during the expansion stage [11]. In the first instant, the plasma emission is masked by the contribution of continuum emission as a result of free-free (inverse Bremsstrahlung) and free-bound (recombination) transitions near the target [3]. For these reasons, our results have 5

ACCEPTED MANUSCRIPT been recorded beyond 2 mm from the target surface. Carbon plasma has been created under three different ambiences: vacuum, methane and argon. The choice of methane gas referred to the deposition condition of hydrogenated amorphous carbon films. The argon was used to control the composition of plasma species under reactive gas as well as to compare the reactivity of formed molecular species in different environments. High intense plasma emission has been

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observed in argon ambience due to its higher atomic density compared to that of methane. The

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light emitting plasma was characterized by different transition wavelengths that correspond to

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atomic and molecular emissions. OES serves to identify different excited species (atoms, ions, molecules) in the LA and to determine the fundamental properties of the plasma such as

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temperature and electron density [12]. Fig.2 shows emissions of ionic and atomic carbon lines and molecular bands of C2 swan system and CH radicals. Typical emission spectra has been

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obtained at various times after onset plasma formation, in the wavelength range of 230-750 nm, at a pressure of 0.3 mbar of CH4 and at a distance of 4 mm from the target surface. For these

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spectra, the gate time used was 10 µs for a delay time of 100 ns. The spectrum in Fig.2a presents a molecular band dominated by the CH radical emissions of vibrational band sequence (Δv=0) and CH+, CII, CIII lines. Fig.2b and Fig.2c correspond to the

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C2 swan band emission of the sequence (Δv=1, 0). The mainly atomic lines and molecular bands

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observed in the plasma in vacuum and controlled atmosphere are summarized in Table. 1. We are interested in the following species: C at 247,8 nm, CII at 426,72 nm, ArII at 440,09 nm, the (0,0) band head of C2 at 516,52 nm, the (0,0) band head of CH at 431,4 nm and at 314,49 nm. 3.2 Analysis of time of flight signals The Fig.3 illustrates the time of flight signals of C, CII, ArII, C2 and CH emission species of carbon plasma under 0.3 mbar of argon and methane gases at different distances from the target surface. For all emission species, it is clear that the maximum intensities are located in defined 6

ACCEPTED MANUSCRIPT regions, which depend on excited population species. However, the shape of temporal profile corresponding to same species change with the nature of ambient gas. The TOF of C is very broad in argon compared to that in methane environment. The emission intensities of CII and C2 are more important in argon than in methane, whereas for the neutral atom of C, the measures are relatively very close and its temporal profile presents a single peak in methane and double

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peaks in argon, at different distances from the target. These double structures could be explained

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by the emission of two components with different velocities, which originate from two different

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processes.

3.3 Study of TOF signals by Shifted Maxwell Boltzmann distributions function

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The time of flight measurement provide an important information about kinetic features of

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particle such as velocity distribution, kinetic energy and translational temperature of different population appeared during plasma expansion. The TOF could be affected by many factors following the laser irradiation such as laser fluence, laser wavelength and nature and pressure

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of ambient gas [15]. Moreover, collisions between expansion plasma species and background gas could be elastic for mono-atomic gases (argon) and inelastic for polyatomic gases (CH4). The inelastic collisions affected directly the velocity distribution of species and reallocate the

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total energy between the translational energy and internal degrees of freedom [19]. In order to

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determine the translational temperature and the stream velocity of neutral and ionized emission species, we used the shifted Maxwell-Boltzmann distribution function (SMB) which is written as follows [16]. f(t) = A z𝑡

−4

𝑒𝑥𝑝

[−

2 𝑚 𝑧 ( −𝑉) ] 2𝑘𝐵 𝑇 𝑡

(1)

where f(t) is the intensity of emitting species, A is the normalization constant, kB the Boltzmann constant, m is the mass of the species, z is the distance from the target, V is the stream velocity, and T is the translational temperature. Several authors have used the equation (1) to determine

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ACCEPTED MANUSCRIPT the temperature and the stream velocity of several materials [15-17]. In our case, we fitted the Shifted Maxwell Boltzmann distribution function to the TOF spectra of neutral, ionic and molecular species as a function of the distance from the target surface under both methane and argon ambiences. The Fig. 4 shows the TOF of C2 species fitting with equation (1). Fig. 5a and Fig. 5b show the evolution of temperature and stream velocity of C, CII, C2 and CH at 0.3 mbar

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of methane ambience as function of distance from the target. For all emission species, the

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temperature decreases with increasing distance and has maximum values of 1.51, 1.32, 1.03

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and 0.9 eV for C2 ; CII, CH and C respectively in the distance region between 2.5 and 6 mm from the target. The stream velocity in Fig. 5b presents the same profile for all species and

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increases with the increase of the observation distance. The CH radical has the largest velocity followed by CII, C2 and then CI. Fig. 5c and Fig. 5d show the temperatures of C2 and CII in

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methane and in argon gases. The impact of the ambient gas on the translation temperature of C2 molecule is significant. The Figure clearly shows higher temperature values for C2 molecules

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in methane as compared to those in argon environment. Concerning CII, the temperature values

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in methane are very high as compared to the ones recorded in argon gas. Such significant difference may be originated from the difference in density between the two ambient gases. For

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the case of C2 at 4 mm from the target, Al-Shboul et al have reported temperature values of 0.78, 0.61 and 0.52 eV at 0.4, 1 and 5 Torr of helium pressures respectively, as well as 0.78 and

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0.44 eV for the double structure of CI at 0.4 Torr of helium at a distance of 7 mm from the target surface using femtosecond laser (= 800 ns,  = 40 fs) [17]. The fit using equation (1) is not always perfect and may lead to certain values with no physical meaning. In our case, negative values of velocity for neutral carbon at 0.3 mbar of argon gas have been found and rejected. Some authors have modified the equation (1) in order to obtain meaningful parameters and enhance the agreement with experimental data. Zhigilei et al. have used the results of molecular dynamics simulation to propose a modified Maxwell Boltzmann

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ACCEPTED MANUSCRIPT distribution function, with two clear physical parameters (the temperature of the plume and the stream velocity) [18]. Morozov evaluated the temperature of niobium, copper, graphite and gold using modified Maxwell Boltzmann distribution. The latter has been defined using a database obtained by direct simulation Monte Carlo method (DSMC) [19].

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4. Kinetic study of plasma species

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4.1 In vacuum

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The evolution of emission species of neutral carbon C at 247.8 nm, ionized carbon at 426.7 nm and molecular carbon C2 at 516.51 nm have been investigated in vacuum at 10-6 mbar of

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pressure. Fig.6 represents the distance-time plots of C, CII and C2. We have observed a linear evolution of atomic, ionized and molecular species which indicate a free expansion of plasma.

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The CII species is the faster, followed by the C and the molecular species of C 2, with the estimated expansion velocities of 1.54, 1.4 and 0.75 (106 cm/s) respectively. A slight change in

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neutral carbon behavior, namely the increase in velocities from t>0.6 µs, was observed. The

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emission of ionized carbon and C2 molecular species are observed near the target at short duration, compared to the neutral carbon. The works of Shboul et al. indicate that C2 emission

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zone in vacuum was just present near the target at early times (less than 3 mm from the target) [10, 20]. The nanosecond laser ablation of carbon target in vacuum revealed the excited neutral

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carbon emission. The initial electrons created by multi-photonic and thermo-ionic effects and heated by inverse bremsstrahlung process induced the first ionization of neutral carbon and produced the ionized carbon CII [21]. In vacuum, the C2 species could be formed directly from the target or by cluster dissociation. 4.2 In methane atmosphere The interaction of laser-produced plasma into reactive ambient gas is more complex compared to its expansion in vacuum [22]. It leads to several physicochemical mechanisms such as 9

ACCEPTED MANUSCRIPT deceleration, diffusion, thermalization of the ablated species, attenuation, recombination, chemical reactions, clustering and shock waves formation [23]. In this study, we investigated the behavior of the emitting species of carbon plasma at a pressure of 0.3 mbar of methane. That value of pressure is deduced from an optimization study performed, in our laboratory, on hydrogenated carbon thin films deposition in methane atmosphere [24].As reported by Budai et

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al. [25], very few works have investigated the deposition of a-C: H films using PLD method in

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CH4 atmospheres in contrary to plasma assisted deposition, where the papers are abundant [25].

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In the same way, a little works have investigated the laser induced carbon plasma under methane atmosphere by optical emission spectroscopy. The latter technique allows identifying and

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estimating the kinetic energy of plasma species impinge on the substrate. We represent in Fig.7a, the distance-time plots of C, CII, C2 and CH species at a pressure of 0.3 mbar in CH4

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atmosphere. In the first region (d<5 mm), a linear behavior has been observed showing a strong expansion of plasma and slight affects of methane gas. The velocities of CII and CH are higher

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than those of C and C2. In the second region (d>6 mm), the ambient gas effect on the emitting

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species starts to appear, leading to a decreasing velocity. The C2 molecules present two components: slow (d<6 mm) and fast (d>10 mm). The slow component follows the C species

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and could be originated from the target surface, whereas the fast one follows the CII and CH species. The formation of the fast C2 is due to the recombination processes between carbon

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atomic species in the front edge of plasma. However, at 0.3 mbar of CH4 and in the region between 6 and 10 mm, the two components of C2 may exist and merged into one component forming the combination slow-fast profile. The latter is represented by one component in the time of flight signal of C2 in fig 3. As reported by Abdelli-Messaci et al. [26] and Al-Shboul et al. [20] using filtered ICCD imaging diagnostic, the slow-fast component of C2 have been also

observed during the first stage of plasma expansion, till 1µs.

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ACCEPTED MANUSCRIPT Up to a distance of 8 mm from the target surface, we notice that the CH at 431.4 nm and CH at 314.49 nm have similar behaviors then they behave quite differently, for higher distances. The emission of CH could be due to the fast electron impact dissociation of ambient methane gas. The Fig.7b shows the maximum emission intensities of C, CII, C2 and CH as a function of the distance from the target surface. The maximum emission intensities decrease until 4 mm from

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the target, then increase due to the collisions between plasma species and gas molecules. They

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are located at 7 mm from the target surface for C and CH (314.49 nm), at 8.5 mm for CII and

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CH (431.4 nm) and at 13 mm for C2. The population of plasma-excited species depends strongly on environment gas, which causes chemical reactions between plasma particles and ambient

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gas. Both atomic collisions and the recombination process explained the formation of molecular species such as C2. The dissociation of CH4 by fast electron impact and/or by collision with

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atomic carbon species lead to CH radical emission. 4.3 Argon atmosphere

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Argon as an inert gas, has been used for comparison purpose with the behavior of emitting species appeared in methane atmosphere. Fig.8 shows (a) the distance-time plots and (b) the maximum emission intensities of C, CII, ArII and C2 in argon ambience at a pressure of 0.3

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mbar. The CII, ArII and the first peak of C are the fast species followed by the second peak of

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C then the C2 species. The effect of argon starts to appear from 6 mm, all fast emission species undergoes collisions with argon particles, which is leading to decrease velocity. The first and the second peaks of C move forward the direction of plasma expansion. The first peak followed the evolution of CII species. Two physical processes are responsible to the emission of components of C: the first peak originating from the recombination process of CII, whereas the second ejected directly from the target surface by cluster dissociation. The C2 emission exhibits only one component, which is provided from the target or by cluster dissociation in the Knudsen

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ACCEPTED MANUSCRIPT layer in contrast to the case of methane gas, where a fast component of C2 appears due to the recombination process. Fig.8b shows the maximum emission intensities for C, CII, ArII and C2 as function of the observation distance. The neutral carbon is the most dominant species of plasma; the figure

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clearly shows that in the range 4-6 mm, the first and the second peaks have similar intensities. The CII, ArII and C2 have maximum intensities near the target at the same region. They

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decrease continuously with respect to the distance, in contrary to the intensity of emitting

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species in the methane ambience, which undergoes an enhancement at 4 mm from the target surface. The CII, ArII and C2 have maximum intensities near the target into the same region.

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The maximum emission intensities in argon ambience are completely different from those in

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methane. In order to highlight the origin and the evolution of the double structure of neutral carbon observed at 0.3 mbar, we have used a pressure of 1 mbar. Indeed, the double structure of CI appeared at 0.3 mbar becomes a multiple structure at 1 mbar (Fig.9). Furthermore, the C2

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species presents a double structure at the pressure of 1 mbar. The multiple structures of CI and C2 have not been observed in methane ambience.

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4.3.1 Double and triple structures Fig. 9 shows the TOF, distance-time plots and the maximum emission intensities of C2 and CI

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respectively at a pressure of 1 mbar in argon ambience. The temporal profile of the C 2 species presents two peaks (fig.9b). The first peak appears at an early time of the expansion, where the second peak appears from 2.5 µs. The origins of the first and the second peaks are different. Indeed, the evolution of the first peak of C2 follows the plasma expansion, whereas the second peak moves backward undergoing reflected shocks [27]. The first peak originates from the target surface or by cluster dissociation and the second one could undergo a reflection and an oscillation in the plasma core. Abdelli-Messaci et al. have reported the same behavior for the emission of CN species, in laser graphite ablation at pressures of 0.5 and 1 mbar of nitrogen 12

ACCEPTED MANUSCRIPT ambience, for fluence higher than of 12 J/cm2 [27]. They ascribed that to the reflected shocks. Depending in the laser fluence, Harilal et al. have observed a double and triple structures of C2 in laser produced graphite plasma, using an infrared wavelength of 1.06 µm under helium ambience. They explained the occurrence of multiple peaks of C2 species by the delays caused from different formation mechanisms (cluster dissociation and recombination) [28]. Al-Shboul

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et al. have observed a double structure for CN and C2 species in carbon plasma under nitrogen

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ambience. This multiple structures in the temporal evolution show that both C2 and CN have

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faster and slower components [20].

In our study, the TOF of neutral carbon revealed multiple structures (fig 9a) that have not

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observed at 0.3 mbar of argon and methane gases. The first and the second peaks appear at

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different instants. The third peak starts to appear at 6 mm from the target at time delay of 4 µs. Each one has a different time delay; it is shorter for the first compared to the second and the third, which is characterized by a relatively longer time delay. These peaks correspond to the

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different components of CI with different velocities. The first component is originated from the

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target surface and/or by recombination process of C+. The second component correspond to the returning CI species to the target surface as its velocity becomes negative with increasing

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distance, whereas the third component undergoes a second forward propagation. The distance-

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time plot of the first and second peaks of C at 0.3 mbar argon does not have the same behavior as the first, second and third peaks of C at 1 mbar argon. The second peak at 0.3 mbar argon moves in the same direction as that of the first peak, in contrast to the second peak at 1 mbar argon, which moves in the opposite direction of the first one. For the case of 1 mbar condition, the contribution of combination of CII to the C generation necessarily occurs the same way as for the case of 0.3 mbar, but they are not distinguished due to the confinement of plasma. The two components of C observed at 0.3 mbar, in figure 8a are merged into one component at 1 mbar and are represented by the first peak of figure 9c, due to the confinement of plasma at 13

ACCEPTED MANUSCRIPT high pressure. Indeed, the two components of C at 0.3 mbar are adjacent and moving forward. Increasing the pressure to 1 mbar, makes them difficult to separate. The distance-time plot of the first peak agrees well with spherical shock wave model (R α t 0.4) (eq.3) in the region of 6-10 mm from the target. At 10 mm from the target, the plasma and

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ambient gas pressure are equilibrated, when the shock wave is detached from the shock front plasma-gas and continues to propagate and decelerate inside argon atmosphere. Some authors

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have highlighted the propagation of the shock wave, at high pressure by shadowgraphy

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technique [29, 30]. The distance-time plots of the three components of CI clearly showed different behaviors of each component. At 9.5 mm from the target surface, the intensities of the

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first and the second peaks present symmetric curves which indicating an emission of two

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populations of neutral carbon that move in two opposite directions. The third peak of CI undergoes an oscillation and moves with plasma expansion. The first and second peaks of CI and C2 correspond to two different populations, the first moves with the plasma expansion and

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the second moves backward. The reflection and oscillation behaviors of the plasma were reported by Bulkakov and Bulkakova [31]. They have used two-fluid gas-dynamic model and time-of-flight mass spectrometry diagnostic to study the expansion of laser ablation plume of

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YBa2Cu3O7−x under different ambient gases. In their theoretical study, they predicted a vortex

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phenomena formation due to the viscous effects of interaction between the plasma plume and ambient gas at the plume periphery. Using fast imaging diagnostic by ICCD camera, Geohegan et al. have observed several reflected shocks and oscillation within carbon plasma under argon atmosphere at high pressure of about 300 mbar [32]. Using combined continuous-microscopic models, Itina et al. have evidenced the oscillation of the plasma core [33]. In order to understand the origin of different components of C and C2, we represent in Fig. 10 the distance-time and distance-intensity plots of each one with emission species of CII and ArII at 1 mbar of argon gas. In order to investigate the recombination process, the distance-time plot of CII has been 14

ACCEPTED MANUSCRIPT added. The evolution of CII follows the first component of CI with similar velocities. Indeed, the CII could be recombined with an electron of plasma to form the neutral carbon CI. Compared to the CII, the ArII is faster. The second peaks of CI and of C2 have similar behaviors, particularly the motion in the opposite direction of plasma expansion. The maximum emission intensity of the first peak of CI decreases with increasing distance reaching a maximum at 4

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mm and at 14 mm from the target surface. Each maximum indicates a strong emission of CI

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caused by the collision of the plume with heavy argon gas or plasma species. The intensity of

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the first peak of C2 decreases with increasing distance and presents a maximum at about 5 mm from the target surface. The intensities of the second peaks of CI and C2 increase with increasing

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distance until 9 mm and 4 mm respectively. The ionized CII and ArII intensities present a maximum at early distance, which decreases continuously as function of distance from the

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target. The effect of argon pressure is evident and deduced by the confinement of the plasma plume. However, the plasma species is more confined at 1 mbar of argon pressure compared to

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the case of 0.3 mbar. The effect of ambient gas manifests in decreasing velocity by collisions.

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The evolution of plasma species undergoes exchange of energy and mass with the ambient gas. The kinetic energies of CI, CII, C2 and ArII are highly affected by the increasing of the argon

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pressure.

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5. Dynamic study: Drag and Shock wave models. A non linear expansion of plasma emission in reactive gas of methane was observed. This is caused by complex physico-chemical processes such as chemical reactions, shock waves formation and so on. A shock wave can be observed when the mass of the ambient gas, in motion is greater than the mass of ablated species, and the pressure of the plasma plume is higher than that of the ambient gas. However, the ambient gas in motion will be compressed by the high pressure of plasma plume, leading to the formation of a shock wave in the surrounding medium [6]. The shock wave can be observed in the limited spatial region when [34]: 15

ACCEPTED MANUSCRIPT 1

3Mp 3 (2πρ ) 0

1

𝐸

< 𝑅 < (P0 )3

(2)

0

where R is the propagation distance, Mp is the mass of the expanding plasma, ρ0 is the background gas density, 𝐸0 is the plume energy and P0 is the pressure of the surrounding gas. The drag and shock wave models have been applied on the spatiotemporal evolution of the

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emitting species in methane and argon atmospheres, in order to study the effect of pressure of

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ambient gas on the plasma expansion dynamics. The models are described by the following

Shock wave model equation [35]: 𝐸

𝑛

𝑅(𝑡) = ξ0 (ρ0 ) 𝑡 2𝑛

(3)

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0

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equations:

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where ξ0 ≈ 1 is the constant related to both geometric and thermodynamic quantities. Drag model equation [36]

(4)

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𝑑 = 𝑑𝑓 [1 − 𝑒𝑥𝑝(−𝛽𝑡) ]

where 𝑑𝑓 is the stopping distance of the plume and 𝛽 is the slowing coefficient.

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The Fig. 11 shows the distance-time plots of C, CII, ArII, C2 and the CH fitting with shock wave and linear models, at 0.3 mbar of methane and argon atmospheres. The evolution of all

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emission species R depends on t0.4 (R α t0.4), which corresponds to a spherical shock wave appeared between 14 and 18 mm for the C, and beyond 5 mm for the ionized species of CII and ArII in argon gas. In methane environment, the CII undergoes a shock wave beyond 14 mm from the target surface. The CII behaves differently in the two environments and undergoes a spherical shock wave from t>1 µs in methane and from t>0.4 µs in argon. The shock wave appears rather in argon than in methane, due the greater mass of argon (39.95 g/mol) compared to the one of methane (16.011 g/mol). The distances 14 and 12 mm from the target of the C2 and the CH are in good agreement with (R α t0.4) respectively in methane ambience, whereas 16

ACCEPTED MANUSCRIPT the C2 undergoes a shock wave in argon beyond a distance of 6 mm. The effect of gas starts to appear at a defined region that depends on the nature and the pressure of gas. The atomic, ionic and molecular species have different kinetics properties, which allow them to apply a certain force on the ambient gas causing the formation of internal and external shock waves. The shock wave will be detached from the plasma edge and continuously propagate in the gas. Harilal et

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al. have observed in Al plasma generating under argon atmosphere, two shock fronts one behind

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the other using shadowgraphy technique [29]. All emission species presented in the Fig. 11

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exhibit a linear behavior near the target. The drag model (equation 4) has been also used and

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agrees well with long portions of distance-time plots of C, CII, C2 and CH species.

6. Chemical reactions within the plasma

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The neutral, ionic and molecular species are more reactive in CH4 than in argon. However, in argon ambience the C2 exhibits a double structure resulting from physical processes, in contrast

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to the triple structure of C, which is provided from physical processes and chemical reactions.

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The maximum emission intensity of C2 was observed far from the target surface in CH4, whereas it is near the target in argon ambience stating that C2 formation results from a chemical

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reaction in CH4. In vacuum (Fig.6), the neutral carbon originates directly from the target surface, but in methane ambience, two components of C have been observed with different

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velocities. The slow component is coming from the target surface and the fast one has a velocity close to that of C+. The fast component is produced from the three body recombination process of C+ following the reaction [37]. C+ + e + e

C+e

(1)

We distinguish two different behaviors of C2 in methane gas that are not observed in argon. For the first instant, the C2 is slow near the target, which means, the C2 is provided from the target

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ACCEPTED MANUSCRIPT surface or formed by the clusters dissociation. For the second instant, several formation options can be suggested, so that the most probable is the three-body reaction expressed as follows: C+ + C + M → C 2 + M

(2)

The particle M could be a species of plasma.

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The electron impact process is the most likely reaction [38] for explaining CH radical formation

CH4 + e → CH + H2 + H + e

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from the dissociation of methane gas with threshold energy of 11 eV:

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(3)

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The hydrogen formed in reaction (3), could be recombined with another species of plasma for giving CH radicals according to the following reaction: C + H + M → CH + M

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(4)

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An alternative reaction could also explain the CH emission as follows: (5)

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C + CHn→ CH + CHn−1, n= 1...4,

Radicals CHn (n=1, 2, 3) originating from the dissociation of CH4 by electron impact process

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and by the strong collisions of CH4 gas with the expanding plume species in laser-produced

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carbon plasma. These radicals could be observed using mass spectrometry, Fourier Transform IR spectroscopy (FTIR), Raman spectroscopy techniques, etc.

7. Conclusion A comparative study of carbon plasma emission in CH4 and Ar atmospheres has been carried out by optical emission spectroscopy. Each ambient gas plays a significant role in redefining the formation mechanisms of the emitting species. The spatiotemporal evolution of C2 in methane atmosphere, which is divided into slow and fast components, shows an evolution for

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ACCEPTED MANUSCRIPT both components and proves two different formation origins. The slow component is derived directly from the target surface, whereas the fast one is due to the recombination process of the atomic carbon at the plasma edge. The recombination of ionized carbon with an electron of plasma leads to fast neutral carbon. The dissociation of CH4 by electronic impact presents the principle approach for explaining the emission of CH radical in the plasma. The C and C2 exhibit

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triple and double peaks respectively at 1 mbar of argon atmosphere in contrast to the case of

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methane. The multiple structures highlight the reflecting shocks effect and oscillation behaviors

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of particles, in the plasma core and queue.

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ACCEPTED MANUSCRIPT Figure Captions Fig.1 : Experimental setup for laser ablation and optical emission spectroscopy Fig.2 : Typical emission spectra of carbon plasma recorded at 4 mm from the target surface under 0.3 mbar of CH4.The gate time used was 10 µs for a delay time of 100 ns.

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Fig.3: TOF signals of CI, CII, ArII, C2 and CH at 0.3 mbar of methane and argon gases.

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Fig.4 : The TOF signals of C2 fitted by SMB function at 3.5 and 5.25 mm from the target.

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Fig.5: Translational temperatures and stream velocity of CI, CII, C2 and CH obtained by SMB

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fit

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Fig.6 : Distance-time plots of C, CII and C2 in vacuum

Fig.7: The distance-time plots and the normalized maximum emission intensities versus

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distance of C, CII, C2 and CH

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Fig.8: The distance-time plots and the maximum emission intensities versus distance of C, CII, C2 and ArII

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mbar of argon

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Fig.9: TOF, distance-time and maximum emission intensities-distance plots of CI and C2 at 1

Fig.10: Distance-time and intensity-distance plots of CI, C2, CII and ArII at 1 mbar of argon Fig.11: Distance-time plots with drag and shock wave models fits into CH4 and Ar gases. Table caption Table 1 : Wavelength and excitation energy of mainly atomic lines and molecular bands of the plasma emission recorded during laser ablation into methane atmosphere

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ACCEPTED MANUSCRIPT References [1] L. Moenke-Blankenburg, T. Schumann, Direct Solid Soil Analysis by Laser Ablation Inductively Coupled Plasma Atomic Emission Spectrometry, J. Anal. Atomic Spectrometry 9 (1994) 1059-1062. [2] R.E. Russo, X.L. Mao, H.C. Liu, J. Gonzalez, S.S. Mao, Laser ablation in analytical

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ACCEPTED MANUSCRIPT [10] K. F. Al-Shboul, S. S. Harilal, A. Hassanein, M. Polek, Dynamics of C2 formation in laserproduced carbon plasma in helium, Environment, J. Appl. Phys. 109(2011)053302. [11] D.A. Cremers, L. J. Radziemski, Handbook of Laser-Induced Breakdown Spectroscopy, Wiley, 2013. [12] H. J. Kunze, Introduction to Plasma Spectroscopy; Springer-Verlag: Heidelberg, Germany,

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ACCEPTED MANUSCRIPT [21] S. Abdelli-Messaci, T. Kerdja, A. Bendib, S.M. Aberkane, S. Lafane, S. Malek, Investigation of carbon plasma species emission at relatively high KrF laser fluences in nitrogen ambient, Applied Surface Science 252 (2005) 2012–2020. [22] A. Bogaerts, Z. Y.Chen, R.Gijbels, A.Vertes, Spectrochimica Acta Part B 58 (2003) 1867−1893.

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ACCEPTED MANUSCRIPT [30] F. Kokai, K. Takabachi, K. Shimizu, M. Yudasaka, S. Iijima, Shadowgraphic and emission imaging spectroscopic studies of the laser ablation of graphite in an Ar gas atmosphere, Appl. Phys. A 69 (1999) S223-S227] [31] A.V. Bulgakov, N.M. Bulgakova, Gas-dynamic effects of the interaction between a pulsed

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ACCEPTED MANUSCRIPT Table 1

431.4 314.49

CH+ (A1 Π→X1 Ʃ)

422.62 (R Head) 423.53 (Q(1) Head)

Transition 1 S- 1P0 2 S- 2P0 2 D- 2F0 3 S- 3P0 1 D- 1P0 1 P- 1P0 4 D- 4P0 3 0 3 F- D

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CH (A2Δ →X2Π) CH (C2Ʃ+→X2Π)

Vibrational band (0-0) (1-1) (2-2) (1-0) (2-1) (3-2) (4-3) (5-4) (6-5)

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C2 (d3Πg →a3Πμ) Swan system

Wavelength (nm) 516.52 512.93 509.77 473.71 471.52 469.76 468.48 467.8 468

Excitation energy (eV) 7.68 16.33 20.9 32.2 42.97 19.22 45.936 32.55

2.4

(0-0) (0-0)

2-3

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(0-0) DOUGLAS-HERZBERG system

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Excitation energy (eV)

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Molecular species

Wavelength (nm) 247,8 283.7 426.72 464.64 418.69 424.73 428.223 469.66

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Atomic species CI CII CII CIII CIII CIII CIII CIII

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ACCEPTED MANUSCRIPT Highlights 

The emitting spectra of carbon plasma under methane show the emission of atomic carbon, molecular bands of C2 and radicals of CH and CH+.



Time of flight signals of CI, CII, C2 present different behaviors in methane and argon atmospheres.

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The translational temperatures, estimated by Shifted Maxwell Boltzmann distribution



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function, have a significant meaning.

The multiple structures of the temporal profile of CI and C2 are observed only in argon

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The dynamic expansion of plasma undergoes reflected shocks.

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

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atmosphere.

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Graphics Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11