Spectroscopic investigation of colliding plasma plumes

    Spectroscopic investigation of colliding plasma plumes Ravi Pratap Singh, Shyam L. Gupta, R.K. Thareja PII: DOI: Reference:

S0584-8547(13)00229-2 doi: 10.1016/j.sab.2013.08.006 SAB 4616

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date: Accepted date:

1 December 2012 14 August 2013

Please cite this article as: Ravi Pratap Singh, Shyam L. Gupta, R.K. Thareja, Spectroscopic investigation of colliding plasma plumes, Spectrochimica Acta Part B: Atomic Spectroscopy (2013), doi: 10.1016/j.sab.2013.08.006

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Spectroscopic investigation of colliding plasma plumes

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Ravi Pratap Singh, Shyam L. Gupta, and R. K. Thareja1

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Department of Physics, Indian Institute of Technology Kanpur- 208016 (Uttar Pradesh) India

Abstract:

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Time-resolved and space-resolved spectroscopic and imaging studies of colliding carbon

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plumes are reported, with the aim of understanding the dynamics of the ablated plume in comparison to single-plume carbon plasma. Laser produced colliding plumes and single

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plume were studied under vacuum (of the order 5x10-5 mbar) in a nitrogen environment using

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a flat graphite target. Due to the interaction of energetic particles of two colliding plumes, a new particle layer is formed that stagnate for a longer time than the seed plasma. Variation of

Key words:

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the plume front and dimension of the stagnation layer with time are also reported.

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Laser produced plasma, Colliding plumes, 2D imaging, Optical emission spectroscopy Author to whom the correspondence be addressed. Electronic mail: [email protected]

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ACCEPTED MANUSCRIPT 1. Introduction Laser-induced plasma spectroscopy is a very active area of research due to its widespread

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acceptance and use for widely diverse applications ranging from elemental analysis, ion beam

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source, space exploration, environmental analysis, surface processing, analysis of hazardous

deposition and bio-medical applications [1-6].

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materials, archaeometric applications, steel analysis, high harmonic generation, pulsed laser

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When a laser pulse is focused on to the target surface with fluence larger than the ablation threshold fluence of target material, a large fraction of electromagnetic energy stored in the

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pulse is transferred to the target, leading to vaporization and eventually to formation of plasma comprising of atoms, ions, molecules, nanoparticles, cluster and agglomerates.

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Various ionization and recombination processes taking place inside the plasma plume leads to

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the thermalization of plasma. The plasma is characterized by its physical parameters: among

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others, expansion velocity, electron temperature and number density, and persistence of emission. The properties of the plasma, in addition to the physico-chemical properties of the target, depend on laser parameters like irradiance, pulse width, wavelength, and ambient

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environment [2,3,6-8]. Various experimental techniques like Thomson scattering, X-ray scattering, Langmuir and microwave probes, interferometry, optical emission spectroscopy, and mass spectrometry have been developed for the measurement of plasma parameters [2, 9, 10]. In recent years, the influence of a plasma plume on the formation and dynamics of the second plasma plume [11] along with the interaction of two simultaneously produced plasma plumes in various geometries viz. orthogonal, cross beam and collinear geometries have attracted the interests of the scientific community essentially to understand the plume dynamics and properties in the interaction zone [12-16]. Colliding plumes have been used as accelerating medium, laboratory models of astrophysical plasma and in inertial confinement 2

ACCEPTED MANUSCRIPT fusion (ICF) [17-19]. The increased luminosity of the interaction zone in the colliding plasma [12] can improve limit of detection in LIBS diagnostics. The properties and type of

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stagnation attained in the interaction zone of the colliding plasma mainly depend on the inter

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seed plasma foci distance and the ion-ion mean free path which in turn depends on the

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relative velocity of plasma species [15, 20].

Laser ablated plasmas are frequently used for the growth of thin films viz. metal thin films, ZnO, carbon nitride and diamond like carbon coatings (DLC) on various substrates

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[14, 21]. DLC, a thin film of carbon having very high density, hardness, electrical resistivity

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and chemical inertness is used on polymers to increase their mechanical and chemical strength [14, 22]. The properties of the DLC films produced by pulsed laser deposition

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depend on various plasma properties and ambient atmosphere. Camps et al. [14] have used

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orthogonal geometry for the colliding plumes and reported DLC film with less splashing. In their configuration, two plumes expand and collide leading to stagnation and thermalization

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depending on separation and delay between two plumes [23]. The interaction region has a much larger density, and hence a much lower mean free path, leading to variable stagnation

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behaviour that depends on the delay between the two seed plasmas. In the configuration used in the present work, two seed plasmas are created by using two laser beams derived from a single beam. The properties of the interaction zone depend on the collisional processes of the lateral (radial) evolution of seed plasmas and the forward velocity component contributing to the physical movement of the interaction zone. Moreover, the distance between the two plumes, and hence the nature of collisions and interpenetration, can easily be controlled in our configuration. The aim of this work is to report the results of a comparative study of collinearly colliding carbon plasma and single carbon plasma using optical emission spectroscopy and

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ACCEPTED MANUSCRIPT time resolved imaging in an attempt to understand the modifications of plasma properties in

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the colliding region.

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2. Experimental

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A schematic diagram of experimental setup used is shown in Fig. 1. A flat circular disk of pure graphite, placed on a rotating mount, was used as a target in a vacuum chamber.

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The chamber was evacuated to a base pressure of 10-7 mbar and then filled with nitrogen gas at 5x10-5 mbar for providing a controlled environment in the vacuum chamber. A laser beam

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of wavelength 1064 nm from Nd:YAG pulsed laser with pulse width of 8 ns and a repetition rate of 10 Hz (LAB-190-10 series from Spectra Physics) was split into two equal halves using

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a 50:50 beam splitter and these two beams were focused at angle of 45degree on the target

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surface at a distance of 2.5 mm using focusing lenses of focal length 40 cm. An irradiance of

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4.6 x 109 W/cm2 on each focus was maintained throughout the experiment. The stepper motor was used to rotate the target in order to provide a fresh surface each time to the incident beam. The plasma plumes expand collinearly normal to the target surface and interact with

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each other along the direction of their lateral expansion. To collect the optical emissions of plasma, a quartz lens makes a 1:1 image of plasma at the input of an optical fiber coupled to the entrance slit of a spectrograph (Shamrock SR 303i , Andor Technology, USA) attached to a gated intensified charged coupled device (ICCD, DH-720, ANDOR Technology, USA) interfaced with a computer. The emission from colliding region was recorded by masking the radiation falling on the fiber input. To record the two dimensional image of plasma expansion, the spectrograph was replaced by a camera lens attached with the gated ICCD. This gated ICCD was set to capture a series of time resolved images with gate width of 10 ns. The two ablating pulses are made to reach the

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ACCEPTED MANUSCRIPT target surface without any temporal delay with respect to each other. The optical delay was controlled by monitoring with an oscilloscope the signals from two identical photodiodes

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inserted in the path of the beams. However, to produce the single plasma plume, only one of

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the beams coming from the beam splitter was focussed on the target.

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3. Results and discussion

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3.1 Time resolved imaging

Time-resolved imaging analysis of both colliding and single plumes of carbon plasma

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under similar experimental conditions was performed. These images were captured in interval of 10 ns up to 1 µs, keeping the gate width fixed. To understand the interaction processes

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between two collinear colliding plumes it was first necessary to explore the dynamics of a

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single plume. Plasma plume dynamics depends not only on the plasma parameters but also

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upon ambient gas and its pressure. Figure 2a shows the images of the single plume at different temporal delays with respect to the ablating pulse along with corresponding intensity plots. Single plume shows the splitting into two regions at later time delays. These

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regions move with different velocities in forward direction (Fig. 2b) and expand linearly in the lateral direction (Fig. 2c). The observed splitting of plasma plume can be attributed to different thermal velocities of plume constituents, viz. electrons, ions, neutrals and molecular/dimer (C2) species due to their different masses and charge states [24, 25]. Figure 3 shows the captured images for colliding plumes along with corresponding intensity plots at different time delays with respect to the ablating pulse. These two plumes initially expand independently and start interacting with each other after ~70 ns. This observation matches with the time taken by individual plume to traverse nearly half of inter plume distance in the lateral direction (Fig.2c). These two plumes start interpenetrating at time >70 ns and the emission intensity between the seed plasma region increases as the interpenetration 5

ACCEPTED MANUSCRIPT increases as a result of various collisional processes. The interaction zone evolve with time and also splits at later times (~180 ns) owing to the different thermal velocities of

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constituents of plasma, as discussed above. As seen in the intensity plot, at about 200 ns the

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intensity of the stagnation layer becomes comparable to that of the seed plasma. The

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interaction region is constantly fed with energetic particles from the seed plasma. Therefore, the emission intensity in the interaction layer keeps increasing continuously while that of seed plumes decreases. Fast imaging observations mark the presence of single plume up to

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300 ns, while that of interaction zone of the colliding plumes can be seen up to 800 ns with

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respect to the ablating pulse.

Figure 4 shows the temporal variation of the plume front position of the slow and fast moving

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components of the interaction zone. The faster component expands almost freely with a

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velocity of (4.4 ± 0.1) x106 cm/sec. However, the species in the slower component are slowed down due to the drag force. The temporal variation of plume front position of the slower

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component is fitted with the drag model R(t )  Rm (1  et ) , where Rm is the maximum distance (vi/α) travelled by the plasma front having vi as initial velocity [7] before stopping.

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Our experimental observations yield [4.5(1-exp (-0.002t)] for the above relation. The length (Fig. 4b) and width (not shown here) of slow moving component are found to saturate for time delays > 350 ns, thus implying stagnation in the interaction zone. 3.2 Optical emission spectroscopy The optical emission spectra of single and colliding plumes were recorded at various times and at different spatial positions. The presence of various ionic, neutral and molecular transitions including relatively prominent transition, viz. C I at 247.7 nm, C II at 283.7 nm, 392.1 nm, 426.7 nm, 657.8 nm, along with some signatures of C2 band have been identified using available literature [26]. The most intense spectral transition in the observed carbon 6

ACCEPTED MANUSCRIPT spectra corresponds to C II at 426.7 nm, which persists for longer time compared to other emissions. At the beginning, spectroscopic transitions from the single plume dominate

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whereas at later times the relative intensity of various spectroscopic transitions from

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interaction zone dominates over those of the single plume.

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To study the dynamic behaviour of the plasma, optical time of flight measurements were performed using the intensity distribution of C II at 426.7 nm with time at 0.5, 2.5, 4.5, 6.5

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and 8.5 mm away and parallel to the target surface for both colliding and single plumes. The results are shown in Fig.5a and b. The full width half maximum (FWHM) of the intensity

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distribution of C II at 426.7 nm in the colliding plume is larger compared to that of the single plume: this finding agrees with imaging observations showing the persistence of the colliding

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region for longer times. The maximum peak intensity of C II at 426.7 nm decreases with

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distance from the target for both single and colliding plumes (Fig. 6(a)).

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The observed spectral intensity recorded in the optical emission spectra can be related to the abundance of the emitting species passing through an observation point at a given instance of

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time. Therefore, the time of observation of maximum spectral intensity as a function of distance of the observation point from the target surface can be used to measure the most probable velocity of a particular plasma species [23]. The most probable velocity of C II plasma species was determined by plotting the spatio-temporal variation of maximum spectral intensity at 426.7 nm (Fig. 6(b)). It is clearly observed that the C II is moving with a larger velocity in a single plume as compared to the colliding plumes. This difference can be attributed to the collisional processes occurring in the colliding plumes. 3.3 Electron temperature and number density Electron temperature of the plasma is calculated assuming the plasma to be in local thermal equilibrium (LTE) and using the relative intensity of two different transitions of the 7

ACCEPTED MANUSCRIPT same ionic species of CII (426.7 nm and 657.8 nm) [7, 27]. The electron number density of plasma is calculated using the Stark broadened profile of C II at 426.7 nm. The width of the

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spectral transitions depends on various broadening mechanisms, with Doppler and Stark

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broadening predominating in laser ablated plasmas [7]. In our case, the plasma is primarily

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composed of singly ionized species and hence electron collisions dominates. In Stark broadening, the full width half maximum (FWHM; ∆) of the profile is related to the electron

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density by the relation [28]

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 n   (nm)  2W  e16   10 

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where W is electron impact parameter and ne is electron number density (cm-3).

The

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contribution of Doppler broadening (~0.002 nm) is very small compared to Stark broadening

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(~0.3 nm, inset Fig. 9(b)) and hence can be neglected. The required parameters are taken

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from available literature [26, 29]. Figure 7 shows the spatial variation of the electron temperature and electron number density in the interaction zone of the colliding plumes at 100 ns (just after interpenetration), 200 ns (after zone splitting) and 300 ns (at later time).

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Each data point is taken as average of 10 datasets: error bars correspond to the observed standard deviations. The temperature at all temporal delays increases at spatial positions from 0.5 mm to 1.5 mm. Beyond 1.5 mm, the temperature at 100 ns decreases with distance. However, at 200 and 300 ns, it first decreases and then shows slight increase in its value. At larger distances (> 4.5 mm) away from target, the temperature becomes higher at 300 ns as compared to temporal delay of 100 and 200 ns (Fig. 7(a)). Similar behaviour has been observed in the spatial variation of electron number density at these temporal delays (Fig. 7(b)).

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ACCEPTED MANUSCRIPT The observed behaviour is attributed to the dominance and variation in various collisional processes and three body recombination occurring in the colliding zone. Both electron

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temperature and electron number density attain their maximum values in the colliding zone at

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1.5 mm away from the target surface. Therefore, the rest of the analysis has been done at this

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particular distance. Figure 8 shows the temporal evolution of emission spectra of C II centred at 426.7 nm at distance 1.5 mm from the target, in single and colliding plumes, at 100, 200, and 300 ns. The emission intensity in the case of colliding plumes keeps increasing with time

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and show the presence of the C2 Swan band in the colliding region, see inset in Fig. 8(c).

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However, for a single plume, the intensity first increases attaining a maximum value and then decreases with further increase in time.

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Figure 9 shows the temporal variation of the electron temperature (Fig. 9(a)) and electron

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number density (Fig. 9(b)), respectively, in single plume and colliding plumes at 1.5 mm away from the target. The temperature in the colliding plumes decays exponentially at a

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relatively slower rate [exp (-0.013t)] as compared to [exp (-0.046t)] of the single plume. This is because of continuous influx of energetic particles from the seed plasma in the interaction

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region which causes prolonged collisional excitation in the interaction region. In the colliding plasma, the electron number density decreases exponentially as exp (-0.03t) compared to exp (-0.12) in case of the single plume. The electron number density decreases with time because of plasma expansion and recombination processes [30]. Our calculated electron number density is of the order of 1016 cm-3 throughout the experiment which is larger than 5x1015 cm-3 needed to satisfy the McWhirter criterion for LTE [7]. The McWhirter criterion essentially assumes that collisional processes dominate over radiative processes. The condition is not strictly applicable to transient plasmas like ours where temperature and density varies temporally and spatially. It is worthwhile to examine whether LTE / partial LTE condition is valid in of our experiment. Following Cristoforetti et 9

ACCEPTED MANUSCRIPT al. [31], τrel and the detector gate width τgate should be smaller than τchar  T temporally varying plasma and variation length d  T

dT

dT dt for

dx should be greater than ten

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times the diffusion length ldiff for spatially varying plasma. In our case, both McWhirter and

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Cristoforetti et al.[31] conditions are satisfied, with τchar (~210 ns) being greater than τrel (~3

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ns); τgate (10 ns) < τchar and d (~5 mm) greater than ldiff. (~8.8 µm) and calculated electron number density ~ 1016 cm-3 throughout the experiment, thus justifying the assumption made.

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4. Conclusion

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In this work, the dynamic behaviour of laser ablated colliding carbon plasmas in collinear geometry and single plume plasma was investigated. Two plumes interact with each

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other ~70 ns giving rise to an interaction zone, which evolves with time and splits into slower

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and faster components at ~180 ns, whereas plume splitting is observed at ~80 ns for single plume. These findings are attributed to the different thermal velocities of electrons, ions,

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neutrals molecular/dimer species present in the plasma plumes, due to their different masses and charge states. It was observed that the slower component in the colliding plasma persists

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for a longer time (~800 ns) as compared to the single plasma (300 ns). Physical movement and the plume evolution of the slower component of colliding plasma saturates at later time delays with respect to the ablating pulse. Optical time of flight analysis confirms that plasma species slow down in the colliding region. The interaction zone of the two collinear colliding carbon plasma is reported to have larger temperature and density along with the signature of C 2 band emission at later time delays with respect to the ablating pulse.

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ACCEPTED MANUSCRIPT Figure captions Fig. 1: Schematic of the experimental setup Fig. 2(a): 2-D images of temporal evolution of single plume along with respective intensity

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plots.

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Fig. 2(b): Temporal variation of plume front position of slow (■) and fast component (●) in

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single plasma plume. Inset 1 shows various regions of colliding plumes and inset 2 gives the temporal variation of the slow component at an expanded scale.

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Fig. 2(c): Lateral expansion of single plume with time.

Fig.3: 2-D images of temporal evolution of colliding plumes along with respective intensity

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plots.

Fig. 4(a): Temporal variation of plume front position of slow (■) and fast component (●) of

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the colliding zone. Inset 1 shows various regions of colliding plumes and inset 2

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gives the temporal variation of slow component at an expanded scale.

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Fig. 4(b): Temporal variation in length of slow moving component of colliding plasma

Fig. 5: Spatio-temporal evolution of emission intensity of C II transition at 426.7 nm for (a)

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Single plume; (b) Colliding plume. Fig. 6: (a) Spatial variation of observed maximum intensity of C II transition at 426.7 nm in single (●) and colliding (■) plasma; (b) Temporal variation of CII emission intensity maximum in single and colliding plumes. Fig. 7: Spatial evolution of (a) temperature; (b) electron density in colliding plasma at 100 ns (■); 200 ns (●); 300 ns (▲) with respect to ablating pulse. Fig. 8: Optical emission spectrum of single and colliding plumes in spectral range 410- 450 nm at (a) 100 ns; (b) 200 ns; (c) 300 ns with respect to ablating pulse at 1.5 mm away from the target. Inset shows C2 band in colliding plasma at 300 ns.

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ACCEPTED MANUSCRIPT Fig. 9: Temporal evolution of (a) temperature; (b) electron density in single (●) and colliding (■) plasma at 1.5 mm away from the target surface. Inset shows the Stark broadened

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profile of C II transition at 426.7 nm at 100 ns with respect to ablating pulse.

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ACCEPTED MANUSCRIPT Highlights

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Collinearly colliding carbon plume Comparative study of single and colliding plumes Spatio-temporal variation of plasma electron temperature and electron density in colliding region Presence of C2 swan band at later time in colliding region

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