A comparative spectroscopic study of single and dual pulse laser produced UV tin plasmas

Optics & Laser Technology 45 (2013) 443–452

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A comparative spectroscopic study of single and dual pulse laser produced UV tin plasmas Ahmed A.I. Khalil a,b,n a b

National Institute of Laser Enhanced Sciences (NILES), Cairo University, Giza 12613, Egypt Physics Department, Faculty of Science for Girls, Dammam University, Dammam 31113, Saudi Arabia

a r t i c l e i n f o

abstract

Article history: Received 30 March 2012 Received in revised form 31 May 2012 Accepted 11 June 2012 Available online 10 July 2012

A comparative study of single-pulse (SP) and dual-pulse laser-induced breakdown spectroscopy (DPLIBS) using two Q-switched Nd:YAG lasers emitting at 532 nm is presented. Both lasers were combined in the same direction (collinear beam scheme) to focus on planar Sn targets at ambient pressure. The effect of the delay times between the incident laser pulse and the ICCD gate, placement of the laser beam focal position with respect to the illuminated surface, incident laser irradiance, and ambient argon pressure on the signal intensity enhancement for the dual pulse scheme have been studied. Atomic and ionic emission lines of Sn were recorded in the 272–296 nm UV spectral region. By using the DP-LIBS excitation technique, the intensity of Sn lines was enhanced by nearly seven times as compared to the single pulse signal that could help the analytical performance of the LIBS technique in terms of increasing sensitivity and reducing self-absorption effects for Sn targets. In the case of the DPLIBS scheme, the intensities of the atomic Sn I at 283.9 nm were recorded at different optimal angles of 451 and 901 and were compared. This comparison was done at different positions of the laser beam focus with respect to the illuminated surface (at 2.45 mm in front of the surface, on the surface, at 1.7 mm and 4.7 mm behind the surface). Furthermore, in the DP-LIBS scheme, an intensity enhancement of the atomic Sn I line at 283.9 nm occurs when the signal was recorded at an angle of 901 to the plasma expansion along the direction of the incident laser beam and the detector set at short delay times. The investigation proved that an optimized value of short delay times between the incident laser pulse and the ICCD gate is required. Variations in the electron temperature (Te) and electron number density (Ne) as a function of gate delay time and laser irradiance have been studied by using the emission lines of neutral tin. Special attention was paid to possible self-absorption of the different transitions. The micro-craters created by SP and DP laser ablation were compared using a reflection optical microscopy (ROM). & 2012 Elsevier Ltd. All rights reserved.

Keywords: LIBS Dual pulse Tin target

1. Introduction Recently dual-pulse laser-induced breakdown spectroscopy (DP-LIBS) has become a modern fast-response analytical technique for qualitative and quantitative elemental investigations in a variety of research fields and disciplines like industrial, defense and medical applications. Two concentric shock waves are generated by the two pulses: the first one follows the dynamics of spherical shock waves propagating into a homogeneous atmosphere; the second shock wave shows a significantly different dynamic evolution. Plasma volumes were approximately a factor of two larger for dual-pulse (DP) than for single-pulse (SP) n Correspondence address: National Institute of Laser Enhanced Sciences (NILES), Cairo University, Giza 12613, Egypt. Tel.: þ20 03 8469800; fax: þ 20 03 8460622. E-mail addresses: [email protected], [email protected]

0030-3992/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.optlastec.2012.06.011

operation. DP-LIBS has been studied by several groups [1–9] for better understanding of the physical mechanisms and to find the optimal experimental conditions for enhanced emission in various applications. The analytical performance of the LIBS technique is strongly dependent on the choice of the experimental conditions. The main parameters that can affect its performance are the target properties, laser wavelength, pulse duration, pulse energy, delay time of observations, ambient background gas pressure and geometrical setup of the optics. Several research groups focused on optimizing these parameters by introducing DP laser systems [10–15] to improve the sensitivity of LIBS. It is well known that DP-LIBS gives enhanced signal intensity, while the exact reason of this enhancement is not yet clearly understood [16,17]. Interest in the UV, EUV and X-ray domains has increased recently due to several important applications [18]. These are quite diverse, including high-resolution microscopy for machining of microelectromechanical devices,

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biological imaging, and optical devices. It is well known that the refractive index of the optical materials depends on the wavelength producing changes in the lens focal length, which is important in the UV region. In the EUV domain, standard X-ray tubes are not optimally efficient and synchrotron sources, though very brilliant, offer only a costly alternative. Therefore, several schemes based on the emission from small dense Sn plasmas produced either in electric discharges or by dual laser pulses have been proposed [19]. The energy levels of atomic Sn have been the subject of a few theoretical and experimental studies. Thin film coatings and Au–Sn solders containing tin compounds have many important applications in nanotechnology, microelectronics, optoelectronics, electroplating, advanced sensors, telecommunication, semiconductor packaging, solid state lighting and microwave devices [20–22]. A laser-produced Sn plasma is one of the most promising candidates for the next generation extreme ultraviolet lithography (EUVL) light source used in the industry development to produce nano-size single-memory semiconductor chips with a capacity highly above the Gbit. The EUV source produced from Sn targets has a high conversion efficiency of laser light to the required small band of EUV light [23,24], and the plasma debris may also be controlled. By employing a suitable tin target, a conversion efficiency of more than 2% has been achieved for the conversion of Nd:YAG (neodymium doped yttrium aluminum garnet) radiation to 13.5-nm EUV radiation [25,26]. Many authors have shown that the EUV emission characteristics of laser produced Sn plasmas depend strongly on many parameters such as laser wavelength, target material, target geometry, and laser intensity [27–29]. Nd:YAG laser excitation was explored thoroughly for the EUV source development. Previous works observed that laser-produced Sn plasma is optically thick to 13.5 nm EUV light [30]. Comparisons of atomic and ionic emission from plasmas produced by CO2 and Nd:YAG lasers to obtain optimum EUVL sources has been thoroughly done as well [31,32]. AlonsoMedina determined the transition probabilities of 97 spectral lines of Sn I from the intensities of emission lines in laser-induced breakdown spectrometry [33]. In the present work special attention is paid to the analysis of the temporal evolution of the spectral emission in the ultraviolet of a Sn plasma generated by a Nd:YAG laser operating at 532 nm wavelength and to better understand the mechanisms of the single and dual-pulse excitation using the LIBS technique at ambient pressure. Parameters studied in this work include the position of the laser beam focal point with respect to the surface of illumination, the delay time between laser pulse and opening of the ICCD gate, the incident laser pulse energy, ambient argon pressure, and optimum scattering angle between the direction of the plasma flare along the incident laser beam and the detector. The effects of these parameters on the line emission intensity by SP and DP excitation schemes and on the plasma plume lifetime are reported by using the time-resolved LIBS technique.

2. Experimental setup Two Q-switched Nd:YAG pulsed lasers (I and II, Model Ultra CFR, Big Sky Laser) were used having 8 ns pulse width operated at 10 Hz repetition rate and yielding beams 5 mm in diameter in a collinear scheme as shown in Fig. 1a. The wavelengths of the first and second laser were fixed at 532 nm. The collinear geometry brings two laser pulses onto the sample surface along the same optical path. Both laser beams were focused through 20 cm A/Rcoated fused silica plano-convex lenses on the target surface placed in ambient air. The pure Sn target (5  5  9 cm3) was mounted on a three-dimensional rotating stage, which was used

Fig. 1. (a) Schematic diagram of the experimental setup and (b) arrangements of laser focuses, collection optics, and a sample surface.

to provide a fresh surface after each laser pulse. Ten laser pulses were shone onto the target surface to have a complete measurement. A simple Galilean telescope was used to expand both laser beams by a ratio of 3:1, to produce a more defined focus, before the irradiation on the target. A delay generator (Stanford Research System, model DG535) was applied for controlling the time sequence and synchronization of both the lasers. The delay time between two laser beams was fixed at 0.5 ms during the whole experiment. The time jitter between the firing of two lasers was o10 ns. The influence of the position of the laser beam focal point with respect to the illuminated surface (at 2.45 mm in front of the surface, on the surface at 1.7 mm and 4.7 mm behind the surface) was investigated as shown in Fig. 1b. The emitted radiation from the plume was collected by using two A/R-coated fused silica plano-convex lenses with a focal length of 75 mm onto the quartz optical fiber bundle (1 m long, core diameter of 200 mm) entrance. The output of the fiber bundle is connected to an entrance slit of a 30 cm, f/4 aperture spectrometer (Acton Research Spectra-Pro 300i) equipped with a 2400 groove/mm holographic grating blazed at 500 nm to record the Sn plasma spectra in the UV region. An Intensified Charge Coupled Device array (ICCD) camera 690  128 pixels was coupled to the spectrometer. The ICCD gate delay (time between laser excitation and opening of camera shutter for plasma emission registering) was varied to investigate the temporal evolution of the plasma, while the gate width was kept constant at 50 ns. The software present in the spectrometer could read the data from the chip and reconstruct the spectrum. To optimize the signal-to-noise ratio and spectral reproducibility,

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the acquisition of the spectra was achieved by an average of data from ten individual shots with identical experimental conditions. The emitted signal was corrected by subtracting the dark signal of the detector through the LIBS software. Intensity (a.u.)

The dynamic, transient behavior of the laser-induced plasma is challenging for diagnostics since the plasma emission intensities for SP- and DP-schemes vary dramatically in time and space. The optimal experimental conditions were obtained prior to testing actual Sn samples. A set of measurements were extensively optimized to improve the sensitivity of the UV-LIBS system and extending the plasma life time. To achieve these objectives, various parameters like delay time, laser energy, focusing of laser beam with respect to the target surface and optimization of the collection lens for plasma emission were varied to enhance the level of the LIBS signal intensity. Fig. 2 elucidates typical LIBS spectra in the 272–296 nm spectral UV region for SP and DP schemes attributed to the emission of the atomic and singly ionized Sn. A signal enhancement of seven times in the DP collinear scheme as compared to the SP scheme was observed. The signal to noise (S/N) ratio values obtained from the measured spectra are depicted in Fig. 2. The S/N were computed and found to be 98.6 and 691.4 for SP and DP-configurations respectively. This signal enhancement has been mainly attributed to the increase in ablated mass [34,35] and later, the increase in the plasma temperature was identified as another source for that [36]. The laser energy of SP was 40 mJ and that of lasers used in DP-scheme were 20 mJ each. Most of the lines in this region belong to transitions in neutral tin (Sn I). The strongest Sn I lines in this region were observed at 278.79, 281.26, 283.99 and 286.3 nm. The Sn II lines were at 282.55 and 291.98 nm. The various spectral transitions of the investigated atomic and singly ionized emission lines of Sn were identified as well; all transitions are listed in the National Institute of Standards (NIST) database [37]. These measurements allowed the determination of the position and the delay time of the most intense spectral lines. Fig. 3a–c elucidates the relative intensities of several Sn atomic lines at Sn I (278.79), Sn I (281.26), and Sn I (283.99) nm as a function of delay time from 0 to 6000 ns for different single and dual pulses, respectively, in ambient pressure. At the early stage of the plasma in the SP-scheme, the emission of all Sn atomic and ionized lines is sharply decreased and slightly decreased at the 3500 3000

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3. Results and discussion

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longer time delays. This phenomenon clearly took place when the laser was focused at 2.54 mm in front of the target, on the target and 1.7 mm behind the target surface except at 4.7 mm behind the surface, where the intensity of all emissions lines became nearly equal or changed only with time. While the intensity of all atomic and ionized Sn lines in the DP-scheme increased initially as the gate delay changed from 0 to 4000 ns, having maximum signals at 1500, 2000, 3000 and 3500 ns at different laser focus positions, focusing on the surface, 2.54 mm front of the target, 1.7 and 4.7 mm behind the target surface, respectively, and started to decay monotonically for gate delays beyond 1500 ns as shown in Fig. 3a–c. To obtain the maximum signal enhancement of the Sn atomic and singly ionized lines in the DP-scheme, the time delay was optimized at a position when the laser was

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(1.19, 1.36, 2.04)  1010 W/cm2 for the SP and DP schemes. The error bar is about 7%. The relative intensity of atomic spectral Sn lines at 2.04  1010 W/cm2 is higher than that obtained at 1.19  1010 W/cm2 by factor of about 7 and 10 times at 1.7 mm for SP and DP-schemes, respectively. This implies that the ideal position to obtain higher spectral intensity is 1.7 mm behind the surface. The measured signal intensity increases slowly with the laser irradiance in the low irradiance range and rapidly at higher irradiances for both SP- and DP-schemes as shown in Fig. 4a–c. The difference in the intensity values at lower irradiances in the range (1.19–1.36)  1010 W/cm2 is less than that at higher irradiances in the range (1.36–2.04)  1010 W/cm2 for SP- and DPschemes. In addition, the obtained intensity at 2.04  1010 W/cm2 is about 3 and 4 times larger than that at 1.36  1010 W/cm2 for both schemes. It is worth to mention that the length of the plasma

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focused well on the target surface. The short detection gate delay was required for sufficient intensity of the Sn I atomic lines at 278.79, 281.26, and 283.99 nm in the case of the SP-scheme. The signal enhancement for the DP-scheme became larger with respect to those in the SP scheme as the detection gate was delayed longer for different above mentioned spectral lines as shown in Fig. 3a–c. Fig. 4a–c depicts the relative intensities of the atomic (a) Sn I (278.79 nm), (b) Sn I 281.26 nm) and (c) Sn I (283.99 nm) spectral lines as a function of different laser focus positions (2.54 mm in front of the surface, on the surface, 1.7 mm and 4.7 mm behind the surface) for SP- and DP-schemes at various laser irradiances from 1.19  1010 W/cm2 to 2.04  1010 W/cm2. It was observed that the relative intensity has a maximum at the laser focus position 1.7 mm behind the surface for 3 above mentioned lines at various laser irradiances of

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Fig. 4. Relative intensity of (a) Sn I (278.79 nm), (b) Sn I (281.26 nm) and (c) Sn I (283.99 nm) spectral lines as a function of laser focus positions (2.54 mm in front of the surface, on the surface, 1.7 mm behind the surface and 4.7 mm behind the surface) at various laser irradiance for SP- and DP schemes. Relative intensity IDP/ISP for atomic spectral lines Sn I 278.79, 281.26, and 283.99 nm as a function of the above mentioned laser focus positions at different laser irradiance (a’) 1.19  1010 W/cm2 (b’) 1.36  1010 W/cm2 and (c’) 2.04  1010 W/cm2. The positive numbers state for ‘‘behind the target’’.

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plume increases with the increase of laser irradiance for both schemes. Fig. 4a’–c’ depicts the relative intensity IDP/ISP for the atomic Sn I (278.79, 281.26 and 283.99 nm) spectral lines as a function of different laser focus positions (2.54 mm in front of the surface, on the surface, 1.7 mm and 4.7 mm behind the surface) at various laser irradiance ((1.19, 1.36 and 2.04)  1010 W/cm2). It was found that an increase in IDP/ISP for the above mentioned atomic spectral lines if the laser focused very closed or behind the target surface and also if the laser irradiance increased. In addition, the atomic line Sn I 281.26 nm was found more sensitive than the other atomic lines to the increase of the laser irradiance.

3.2. Comparison between LIBS emission at y ¼451 and at y ¼901 of incident laser beam angle Fig. 5a–d elucidates the spectral emission for the DP excitation scheme recorded at different positions of the laser focus from the target surface: 2.45 mm in front of the surface, on the surface, and 1.7 mm and 4.7 mm behind the surface, respectively. The spectra were recorded at various time delays between laser pulse and ICCD camera gate delay ranging from 0 to 6000 ns. The spectral lines appear markedly widened at the initial state of the plasma. Emission lines are broad mainly due to the Stark effect caused by the high density of free electrons. After a few hundreds of nanoseconds, characteristic atomic lines can obviously be distinguished as the free electrons start to be captured by ions and neutral atoms and the highly excited species decay to lower energy levels. At the same time, line narrowing is noted as the main effects causing line broadening (collision and Stark)

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becoming weaker. As the plasma emission expands, the relative intensities of the emission lines can vary due to energy distribution among the plasma species [38]. The intensity of the continuum emission from the plasma around 276.17, 278.79, 291.35 and 291.98 nm is highest when the laser is focused at 1.7 and 4.7 mm behind the surface. This continuum arises around the zero delay and is due to free–free bremsstrahlung and electron– ion recombination [39–45]. At further distances from the target surface, the spectra are dominated by emission from neutral atoms and low-lying state of the ions as shown in Fig. 5a–d. Fig. 6 depicts the recorded intensity of the DP-emission of the Sn I 283.99 nm spectral line for both 901 (IDP 90) and 451 (IDP 45) detection angles at different laser focus positions. At early times the spectral emission line (Sn I 283.99 nm) is more intense when the laser is focused at 4.7 mm behind the surface, i.e. IDP 90 4 IDP 45. However at longer gate delays, IDP 45 reaches higher values and seems to dominate in comparison with IDP 90.

3.3. Effect of background gas pressure on the Sn plasma intensity In this set of experiments, the Sn sample was enclosed in a stainless steel vacuum vessel which could be evacuated using rotary and diffusion pumps. The emission of the Sn plasma, in front of the target after irradiation by the laser, was recorded at a 901 angle. The plasma was propagated in the direction of the incident laser beam. The plasma expanded about 9 mm away from the surface. The collisional excitation rate is large in front of the target surface (1 mm) to create a high density plasma. The emitted plasma will tend to diffuse towards low density regions.

Fig. 5. Typical spectra recorded for DP-excitation scheme: (a) 2.45 mm in front of the surface, (b) on the surface, (c) 1.7 mm behind the surface and (d) 4.7 mm behind the surface at ambient pressure, with 901 detection angle on the Sn target.

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laser shielding. At low pressure (1.4 Torr); laser shielding is very low, hence most of the laser power reaches the target surface, resulting in a large mass removal and consequently, increase of the number of released atoms. Low absorption of the laser power by the plasma results in low plasma temperature, and the resulting intensities of the spectral lines are also relatively low. At high pressure (7.4 Torr), more laser absorption by the plasma gives rise of the plasma temperature, but the ablated mass is lower due to the laser shielding effect in the SP scheme. However, in the DP scheme, the laser shielding is reduced. Therefore, the second laser pulse is less absorbed and most of its laser power reaches the target surface, producing higher mass removal that in turn increases the LIBS intensities of the Sn atomic spectral lines. Furthermore, no emission lines from argon were observed at the different pressures at the present experimental conditions. 3.4. Plasma parameters

Fig. 6. Time dependence of the emission intensities of Sn (I) line at 283.99 nm recorded at 451 and 901 detection angles for DP-scheme at different laser focusing positions relative to the target surface.

Fig. 7. Variation of the intensities of atomic spectral Sn I lines 278.79, 281.25 and 283.99 nm as a function of argon pressure for SP- and DP-schemes by focusing the laser beam on the target surface with a 451 detection angle.

In the presence of argon gas, the plasma was confined, and the energy transferred from the front of the plume to the surrounding gas. It is worth to mention that the expanding plume is spread out at 2.6 Torr argon pressure as a result of the pressure gradient. The Sn particles undergo frequent collisions with the neutral atoms of the argon gas; in this stage the collision rate is high and the density of the argon atoms is high to enhance the plasma emission. The lifetime of the resultant plasma is decreased with increasing pressure. Strong radiation (with and without background gas) was detected, when the laser beam was focused onto the tin target with a laser intensity of around 109 W/cm2 [46]. The intensities of the Sn I lines at 278.7, 281.2 and 283.9 nm were recorded at different argon pressures in the range 1.4–7.4 Torr for SP and DP schemes as shown in Fig. 7. The intensity initially rises at lower values of the argon pressure and slowly increases remarkably at higher values up to 7.4 Torr. This is due to the argon atmosphere rapidly cooling and confining the plasma plume, thus enhancing electron collisional excitation. Therefore, the visible emission from the excited states is increased. The behavior obtained in Fig. 7 can be attributed to the influence of

The determination of plasma parameters (electron temperature (Te) and electron density (Ne)) is essential for understanding the mechanisms underlying the DP-LIBS technique. The plasma parameters represent the most fundamental physical quantities, whose knowledge is useful for characterization of the plasma and for its efficient use for analytical purposes [47]. 3.4.1. Excitation temperature The electron temperature Te of the plasma plume is a crucial parameter for determining the elemental composition and depends on the measurements of line intensities. These include relative and absolute line intensities, and intensity ratios of line to continuum. Two methods have been mainly applied to determine Te, namely the assumption of a Saha–Boltzmann distribution and the line pair ratio method. Te extracted by the line pair method is less accurate than the Saha–Boltzmann plot method due to the main sources of error in the line pair ratio method such as uncertainties of transition probabilities and the measurements of the integrated intensity ratios of the spectral lines. The local thermodynamic equilibrium (LTE) approximation is often utilized for modeling the plasma in both schemes. A number of spectral lines of the Sn element at the same ionization stage should be taken into consideration. Selected lines should have known transition probabilities, be in a narrow wavelength range, and the excitation energy of their upper levels should be greatly different to enhance the accuracy of the Te measurement. The intensity of a spectral line emitted from an element under LTE (in LTE the population of all levels must be according to a Boltzmann distribution) is given by [48].   Il hcNe E  i , lnð Þ ¼ ln ð1Þ Aij g i 4pUðTÞ kT exc where I is the spectral line intensity, l is the wavelength of the line in nm, Ne is the electron number density, k is the Boltzmann constant, U(T) is the partition function of the species, Aij is the transition probability, gi is the statistical weight for the upper level, Ei is the excited level energy in cm  1. Instead of the transition probability Aij, the oscillator strength fij can also be used whereas; the fij value is directly proportional to Aij as given by Ladenburg formula, f ij ¼

l2 mc 8p2 e2

Aij ,

ð2Þ

where m is the mass and e is the charge of electron. The plot of ln (Il/Aijgi) versus Ei for Sn atomic and ionic spectral lines would give a straight line if the populations of the excited states follow the Boltzmann law as shown in the inset of Fig. 9. By using Eq. (1), the plasma temperature can be calculated from the slope of the line

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drawn. In the present work, Te was determined by using 5 atomic lines of Sn I at 276.1 nm, 281.2 nm, 285.0 nm, 291.3 nm, and 563.1 nm and one ionic line of Sn II at 579.9 nm. Fig. 8 depicts the typical LIBS spectra in the region 550–600 nm for the SP scheme, and the two spectral lines at 563.1 nm and 579.9 nm were used from Fig. 8. The spectroscopic details of these transitions are taken from the NIST database [37]. Fig. 9 depicts the temporal evolution of the determined plasma temperature for Sn lines recorded with our LIBS setup at sequential distances from target surface. Te for Sn lines slightly increases for SP and largely increases for DP-scheme at the early stages from 0 to 500 ns due to absorption of the laser energy by means of electrons via the inverse Bremsstrahlung absorption process. A peak Te of 15,300, 16,500, and 13,500 K was observed if the laser was focused on the surface, 1.7 and 4.7 mm behind the Sn surface, respectively and for  500 ns after the laser hit the target in the DP-scheme. After a delay time of 500 ns, a decay in Te was observed for different focus positions at 2.54 mm, on the surface, 1.7 and 4.7 mm behind the Sn surface for both SP and DPschemes, respectively. Te varies from 5500 to 12,000 K for the SP and from 7150 to 16,500 K for the DP-scheme at different laser focal point positions. Consistently, TDP is comparable to or slightly less than TSP for long delays when the laser is focused on the surface. The difference, TDP  TSP, increases as the gate delay

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become shorter, correspondingly, the IDP/ISP ratio increases. The Sn I emission at 283.99 nm with a higher upper level is enhanced further. These observations indicate that the principle factor for signal enhancement in the present experiment is the increase in Te. As in the ns range, a significant fraction of the laser energy is absorbed by the plasma due to inverse bremsstrahlung so it should increase Te as well as Ne at an early stage of plasma production. It was also observed that Te decreased at long time delays for various laser focal positions as shown in Fig. 9. This may be due to the fact that thermal energy is rapidly converted into kinetic energy, by attaining maximum expansion velocities and leading to cooling of the plasma. In addition, the maximum Te is higher in the case of the DP-scheme by factor of  1.5 than that obtained in the SP scheme. 3.4.2. Electron density measurements There are two crucial line broadening mechanisms in laserproduced plasmas, i.e. Stark broadening and Doppler broadening. Stark broadening induced by collisions with charged particles dominates at higher plasma densities. The electron density Ne can thus be derived from width of the neutral Sn emission lines [49,50] using the approximate formula         Ne N e 1=4 3 1=3 Ne Dl1=2 ¼ 2o 1 ND o þ 3:5A , ð3Þ 16 16 16 4 10 10 10 where Dl1/2 is the experimental full width at half maximum (FWHM) of the emission line, A is the ion broadening parameter, o is the theoretical Stark width parameter for neutral Sn atoms, and ND is the number of particles in the Debye sphere. The dependence of the Stark coefficient on the plasma temperature is weak and can usually be neglected: the variations expected are typically of the order of 10%. The contribution to the width is almost due to electron impact. The ion broadening parameter A is small for many cases and Eq. (3) reduces to   N ðcm3 Þ DlFWHM ðAÞ ¼ 2o e 16 , ð4Þ 10 The observed line widths were corrected by subtracting the contribution of the instrumental width using the relation

Dltrue ¼ Dlobserved Dlinstrument ,

Fig. 8. Typical visible spectra recorded at 1500 ns delay for the SP scheme by focusing the laser directly on the Sn target surface.

which holds when all profiles have a Lorentzian shape. This was verified by fitting a Lorentzian profile to the observed lines. The LTE conditions are examined for selected spectral lines by using the McWhirter criterion [51] Ne ðcm3 Þ Z1:6  1012 ½TðKÞ1=2 ½DEðeVÞ3 ,

Fig. 9. Temporal evolution of Te at sequential distances from target. Error bars are due to least squares fit to the measurements made at multiple wavelengths.

ð5Þ

ð6Þ

where DE is the energy difference between upper and lower levels of the transition, which are expected to be in LTE. Although a high electron density is essential for establishing the equilibrium, the derivation of the electron density from line widths of lines does not depend on LTE. In principle, LTE conditions should prevail over the major part of plasma life time. By knowing the Ne and Te one can check if the LTE assumption is valid using the criterion suggested by McWhirter [51]. It is worth mentioning that the plasma under study was assumed to be optically thin for the utilized lines, i.e. not subject to self-absorption. The measured Ne for SP and DP-schemes of the Sn I spectral line at 283.99 nm as a function of delay time at different laser focus positions (2.45 mm in front of the target surface, on the surface, 1.7 and 4.7 mm behind the target surface) are depicted in Fig. 10. It can be observed that the values of the measured Ne for the DP-scheme are higher than that obtained for the SP-scheme, which is the result of more ablated species from the target. From 0 to 6000 ns, Ne drops after plasma initiation and has values ranging from (2.3, 3, 1.9, and 1)  1016 cm  3 to (1.1, 0.5, 0.45, and 0.7) 1016 cm  3 for

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SP- and from (0.68, 3.1, 5, and 1.3)  1017 cm  3 to (3.9, 2.5, 3, and 2.6 )  1016 cm  3 for DP at different positions, 2.54 mm in front of the surface, on the surface, 1.7 mm and 4.7 mm behind the surface, respectively. In general, the highest value of the Ne is observed when the laser is focused on the target surface in the case of SP and at 1.7 mm behind the surface in the case of DP; then, it decreases in both schemes towards longer times and distances from the Sn target. The fast decay rate of the Ne can be attributed to the plasma expansion while slowing and leveling off (saturation behavior) at longer times are probably due to recombination. The most likely processes in laser-produced plasmas are radiative recombination and the three-body recombination [52]. In addition, it is observed that the temporal behavior of Ne displays a nearly similar behavior for the examined laser focus positions from Sn target surface in the case of SP, where the Ne suffers a slow decrease as the laser is out of focus from the Sn target surface. During the early stages of plasma evolution, Ne reaches a value of up to 3  1016 cm  3 in SP the scheme and 5  1017 cm  3 in the DP scheme at laser focus position on the Sn target surface and 1.7 mm behind the surface, respectively. The inset shows the Sn (I) 283.99 nm line to measure the full width at half-maximum (Dl1/2) in the case of DP scheme. All the data points were fitted with Lorentzian fitting function to determine (Dl1/2). Substituting the values of Dl1/2 in Eq. (4) and the corresponding value of Stark width parameter (the electron impact parameter) (o) is equal to 0.034 70.005 (A1; normalized 1016 cm  3) [24,53] one can find the Ne values. Te and Ne have

Fig. 10. Temporal evolution of Ne at sequential distances from target surface. Error bars due to uncertainty in electron impact parameter and theoretical fit to Lorentzian profile. The inset shows the Sn I 283.99 nm line to measure the full width at half-maximum (Dl1/2) in the case of DP scheme. Solid points represent the experimental data and smooth curves are the Lorentz fits.

about 10% uncertainties due to the uncertainties in the width measurement, the instrumental width de-convolution, integrated intensity of the transition line, transition probability and the electron impact parameter. The atomic and ionic lines chosen for estimating Te are listed in Table 1. In our experiment, the optimum integration time is 500 ns to record the well known Sn plasma emission spectrum. Figs. 11 and 12 depict the obtained values of Te and Ne of the Sn plasma dependence on the laser intensity at different laser focused positions relative to the target surface for SP- and DPschemes. It is observed that Te is in the range (3500–12300) K for SP and (4160–17740) K for DP, and Ne is in the range (2.17  1015–7  1016) cm  3 for SP and (2  1015–4  1017) for DP. When increasing the laser intensity from 0.5  1010 to 2.5  1010 W/cm2¸ Te increases by a factor of 3.5 and 4.2, respectively, and Ne increases by a factor of 32.2 and 200, respectively. By increasing the laser irradiance, more excited species, free electrons and ions are produced which interact with the incoming laser photons and cause more heating and ionization which increases the absorption of more laser photons. The incident energy absorbed by the plasma is used to increase the internal energy of the plasma. Because of the high expansion velocity of the leading edge of the plasma, Ne rapidly decreases, which makes the plasma transparent for the incoming laser radiation. In addition, the target surface is constantly absorbing laser photons. Hence, a dynamic equilibrium will exist between the transfer of the thermal energy into kinetic energy and plasma absorption. It was observed that the plasma life time for both SP and DP are slightly different, however, the DP scheme obviously generated

Fig. 11. Dependence of Te of the Sn plasma with laser intensity at different laser focal positions relative to the target surface for SP- and DP-excitation scheme.

Table 1 Spectroscopic data of various Sn atomic and ionic lines investigated in the present work and used for the calculations of plasma parameters. Wavelength, lij (nm)

Transition probability, Aij (s  1)

Lower energy level, Ei (cm  1)

Upper Energy level, Ej (cm  1)

Transition

Statistical weight, gi

Statistical weight, gj

276.17 278.79 281.25 283.99 285.06 286.33 291.35 563.17 597.03

3.7  105 1.4  107 2.3  107 1.7  108 3.3  107 5.4  107 8.3  107 2.4  106 9.6  106

3427.673 17,162.499 17,162.499 3427.673 8612.955 0.000 17,162.499 17,162.499 38,628.876

39,625.506 53,020.952 52,706.832 38,628.876 43,682.737 34,914.282 51,474.718 34,914.282 55,373.835

5s25p2-5s5p3 5s25p2-5s28p8s 5s25p2-5s25p7s 5s25p2-5s25p6s 5s25p2-5s25p5d 5s25p2-5s25p6s 5s25p2-5s25p6d 5s25p2-5s25p6s 5s25p6s-5s25p7p

5 1 1 5 5 1 1 1 5

5 3 3 5 5 3 3 3 3

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higher Te and denser plasmas for the same amount of laser energy. With increase of the laser irradiance, Te and Ne are observed to increase up to the 2.25  1010 W/cm2 laser irradiance and then to saturate. The saturation in Te and Ne at higher laser irradiance for different above mentioned laser focus positions for SP and DP excitation, respectively, are due to plasma shielding, i.e., reflection and/or absorption of the laser photons by the plasma plume, which depends upon the plasma frequency. The prompt electrons released from Sn target collisionally excite and ionize the ambient air molecules, which give enhancement in the emission intensity of spectral lines especially in the DP-scheme. 3.5. Self-absorption analysis Self-absorption leads to a reduction of the line intensities. It depends on the oscillator strength, level energies degeneracy, broadening parameters as well as on the plasma parameters. In homogeneous plasma self-absorbed lines have a flat top, in plasmas with gradients a dip at the central frequency develops. The self-absorption is probably noticeable in the resonance line at 286.33 nm because of the high population of the lower level and large oscillator strengths. However, in the present work no dip at the central frequency of the emission lines was observed. With the determination values of Ne and Te it is easy to calculate the absorption coefficient for the selected spectral lines, using the following equation [54,55], expressed in m-1: " # Nj gi pe2 gðoÞ, f N 1 ð7Þ ko ¼ 2e0 mc ij i Ni gj where gi and gj are the statistical weights of the states and g(o) is the normalized profile of the line. At the maximum, o ¼ 0, and for a

Fig. 12. Dependence of Ne of the Sn plasma with laser intensity at different laser focal positions relative to the target surface for SP- and DP-excitation scheme.

451

Lorentz profile, g(0)¼2/&n, where n is the FWHM of the line. Ni and Nj are the population densities of upper and lower levels, respectively. For the lines studied in this work, self-absorption was negligible. O’Shay et al. [56] reported nanosecond spectroscopy of expanding Nd:YAG laser-produced tin plasma. The laser parameters such as wavelength, focal spot size, pulse durations and laser intensity were 1064 nm, 58 mm, 10 ns and 3.8  1011 W/cm2 respectively, whereas in the present work these parameters were 532 nm, 50 mm, 8 ns, and (0.5–2.5)  1010 W/cm2 respectively. The analytical properties of the laser produced plasma are highly influenced by these parameters. In this work the shorter pulse duration and lower spot size were obtained. The most crucial factor that affects the lower values of the Te and Ne for the SP scheme is the laser wavelength, which influences the critical electron density (Ncr). The plasma becomes opaque to the laser radiation at Ncr, which is inversely proportional to the square of the incident laser wavelength. Therefore Ncr for 532 nm radiation is greater than that for the 1064 nm laser beam, whereas the optical depth at 1064 nm in the target surface is less than that at 532 nm. This difference in the laser absorption explains the lower Ne obtained with the 532 nm Nd:YAG laser in the SP-scheme. 3.6. Target surface morphology The surface topography of the Sn samples irradiated with the SP and DP schemes was investigated. The crater developed on the target surface and the structural damage surrounding the craters was recorded. Fig. 13a and b depicts micrographs obtained with a reflection optical microscope (ROM). 10 Nd:YAG laser pulses at 532 nm were focused onto the Sn targets at 451 with a magnification of 10 for the SP- and DP-scheme, respectively. The DP-scheme clearly produced a larger damage area at the center and a significant heat affected zone (HAZ) at the boundary in comparison to the SPscheme. In the DP-scheme a large crater formation of a ring-shaped structure could originate from residues of the molten material pushed out of the interaction region by laser irradiation which is the indication of a large mass ejection from the surface in comparison with the SP-scheme. Enough space between two adjacent craters was provided after each exposure by moving the target to a fresh area by the X–Y stage in order to prevent any interference of any two adjacent craters. A radial stream of solidified metal was identified on the target surface. This may be due to a large area of liquid Sn surrounding the crater during plasma expansion. The probable causes could be surface melting for reasons such as absorption of the X-rays from the plasma volume, quality of the laser beam or lateral heat transport. This surface deformation can scatter a small amount of radiation of the incident laser beam. This scattered light may propagate as a surface wave within or above the irradiated target and interfere with the incident beam producing an intensity distribution across the surface. Consequently, producing a positive feedback effect is being obtained. Photographs show craters of about 50–100 mm in diameter for the SP-scheme and of about 250–350 mm for the DP-scheme. The craters are circularly

Fig. 13. ROM micrographs of laser irradiated Sn target by focusing the laser beam on the target surface with ten laser pulses by (a) SP- and (b) DP-excitation schemes.

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symmetric, and the estimated mass removal for Sn is about 5 g/cm3 for the DP-scheme. A black hollow structure around the laser spot is caused by a thin layer of complex oxides created by the laser irradiation of the target surface. This oxidation layer includes larger amounts of SnO created by ns Nd:YAG laser ablation. As mentioned above, in the DP-scheme the second laser pulse enters the rarefied core  500 ns later. Thus the life time of the enclosed part of the DPscheme in the rarefied region becomes longer than that of the SPscheme. These observations indicate that one of the main factors for the signal enhancement in the present work is the mass removal.

4. Conclusions In this paper, the DP-LIBS technique employing the second harmonic of a Nd:YAG laser was utilized to analyze atomic and singly ionized spectral lines from Sn targets at different focus positions in ambient gas. The capabilities of DP-LIBS for investigating the temporal behavior of the plasmas were demonstrated successfully. The spectral intensity as a function of pulse delay time, laser pulse energy, ambient argon gas pressure and focal position of the laser beam with respect to the illuminated surface was measured. Parametric dependence studies were carried out for the optimization of the sensitivity of the LIBS spectrometer. This work revealed that the collinear DP-LIBS is superior as compared to SP-LIBS. However, it also should be noted that the obtained intensity at a laser irradiance of 2.04  1010 W/cm2 was 3 times larger than that at 1.36  1010 Wcm  2. On the other hand the intensification obtained using the DP-scheme was about 1.5 times larger than that in the SP-scheme. Based on our results one can conclude that employing a single pulse is a simpler and cheaper technique and adequate if the research tolerates a low resolution. On the other hand if high resolution is required the dual pulse technique is preferable. No self-absorption effects have been detected. It was also experimentally shown that the intensities of the atomic Sn spectral lines increase with the laser irradiance and with the ambient Ar pressure. Furthermore, the enhancement of the maximum signal intensity of the atomic Sn spectra is about seven times for a DP-scheme at 20 mJ as compared with a SP-scheme at 40 mJ laser energy. This enhancement may be attributed to thermal reheating of the Sn plasma plume. In addition, the spatial studies carried out in this work, by focusing the laser at different distances, in front, at and behind the surface, reveal that several Sn emission lines at Sn I (278.7), Sn I (281.2) and Sn I (283.9) nm for SP and DP-schemes in the UV spectral region are limited to within 5 mm behind the target surface at 0 delay between the laser pulse and triggering of ICCD camera in ambient pressure. This implies that the ideal position to obtain higher spectral intensity particularly of the Sn I line at 283.9 nm is 4.7 mm behind the surface at 901 at short delay times (0–1500 ns). This study can be a great benefit for optimization conditions of a number of collinear DP LIBS experiments to be adjusted in different applications. In order to optimize the analytical performance, short delay time, sufficient high laser irradiance and suitable argon pressure within a wide range should be adjusted.

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