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Time resolved optical emission spectroscopy of cross-beam pulsed laser ablation on graphite targets
Physics Letters A 367 (2007) 351–355 www.elsevier.com/locate/pla
Time resolved optical emission spectroscopy of cross-beam pulsed laser ablation on graphite targets R. Sanginés a,b,∗ , C. Sánchez Aké a , H. Sobral a , M. Villagrán-Muniz a a Laboratorio de Fotofísica, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México,
Apartado Postal 70-186, México Distrito Federal 04510, Mexico b School of Physics, University of Sydney, New South Wales 2006, Australia
Received 19 December 2006; received in revised form 7 February 2007; accepted 16 March 2007 Available online 20 March 2007 Communicated by F. Porcelli
Abstract Cross-beam pulsed laser ablation with two delayed lasers is performed on two perpendicular graphite targets. The time delay between lasers is varied by up to 5 µs, and physical changes on the second plasma, due to the interaction with the first generated one, are determined by time resolved optical emission spectroscopy. © 2007 Elsevier B.V. All rights reserved. PACS: 79.20.Ds; 52.70.La Keywords: Plasma ablation; Plasma diagnostic techniques
The interaction between two laser induced plasmas has been recently applied for different purposes. The plasmas are generated by two synchronized laser pulses and the effects produced by the plumes’ interaction depend on the spatial overlap between them, the beam incidence configuration and the time delay between pulses. Double pulse configurations provide an improvement of the laser induced breakdown spectroscopy (LIBS) technique, rising its sensitivity by increasing the emission lines’ intensity [1–4]. This technique is also useful for thin film growth for a great diversity of materials, as it reduces the films’ splashing [5]. An alternative configuration is the crossbeam pulsed laser ablation technique (CBPLA) where the two plasmas are generated on two perpendicular targets and smooth films of different materials have been obtained with this method [6–8]. Diverse studies have been performed in order to obtain plasma properties using emission spectroscopy. Plasmas have shown changes in the kinetic energy of the species [9,10], and * Corresponding author at: School of Physics A28, University of Sydney, New South Wales 2006, Australia. Tel.: +61 2 9351 2553; fax: +61 2 9351 7725. E-mail address: [email protected] (R. Sanginés).
0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.03.016
it is possible to control this energy by varying the temporal delay between pulses [10,11]. The most important changes in the kinetic energy of the species, in the perpendicular targets arrangement, have been observed in the range of 2 to 4 µs time delay between lasers. For a time delay of 2 µs, the velocity of species in the second plume is reduced compared to the distance to the target by up to a factor of two, with respect to the velocity reached by the species in the single-beam generated plasma. The gradual reduction in the kinetic energy of the second plume is due to the collisions with the remaining particles of the first plume, and it is modeled using the drag model [12]. Under this framework, the ejected species are considered as an ensemble that suffers a viscous force, proportional to its expansion velocity caused by the background gas. Therefore, at early expansion stages (up to 2 cm from the target surface) the second plasma species interact with the remnant particles from the first plasma, loosing their kinetic energy. Once those particles leave this interacting zone, they have a similar behavior to a single-beam generated plume expanding at constant velocity. Furthermore, for two perpendicular interacting plumes, electron and neutral densities have been determined by the two-color interferometry technique [13]. Results have showed that densities do not
352
R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
Table 1 Emission transitions employed for the electron temperature determination; λ: central wavelength, Aki : transition probability from the level k to the level i, E: level’s energy, g: level’s degeneracy
Fig. 1. Experimental setup for time resolved optical emission spectroscopy used to analyze the plasmas induced by cross-beam pulsed laser ablation; DG: delay generator, L: lens, M: mirror, T1 and T2: rotating graphite targets, LCS: lens collector system, OF: optical fiber, Spect: spectrometer.
have measurable changes, with respect to those obtained for the single-beam array, at distances up to 1.5 mm to the second pulse target surface; thus, it could be expected that physical changes produced by the interaction between both plumes would be accumulative with the distance according to the dynamical model. This work reports the diagnostic of a cross-beam pulsed laser induced plasma on two perpendicular graphite targets using optical emission spectroscopy. Time resolved electron density and temperature were measured for time delays up to 5 µs between lasers and for single beam. The aim of this work is to determine how the first plasma affects the second one as a function of the distance to the second pulse target. The plasma was assumed to be in local thermodynamic equilibrium (LTE); it was also assumed that the main broadening mechanism of the emission lines is the Stark effect [14–17]. Laser sources used to induce the ablation plasmas were a KrF excimer laser emitting at 248 nm, with a 30 ns pulse width at a fluence of 25 J/cm2 (Lambda Physik, COMPex102) and an Nd:yttrium–aluminum-garnet (YAG) laser emitting at 1064 nm, with a 7 ns pulse width and fluence at 15 J/cm2 (Continuum, Surelite I). Fig. 1 shows the experimental setup used in the cross-beam pulsed configuration. Laser beams were directed towards two perpendicular rotating graphite targets (T1 and T2 in Fig. 1) and focused by 30 cm focal length plano-convex lenses normal to the targets surfaces. The distance (direct line of sight) between the ablation sites was 3.5 mm. Ablation was performed in a vacuum chamber equipped with quartz windows and kept at 5 × 10−6 Torr. Synchronization of lasers was controlled by a pulse/delay generator (Stanford, DG535) and the pulse delay between lasers (τ ) was varied by up to 5 µs. The first pulse, which corresponds to the Nd: YAG laser, was directed along
λ [nm]
Aki [108 s−1 ]
Ek –Ei [eV]
gk –gi
391.90 392.07 515.11 564.81 566.25 657.81 658.29 723.13 723.64
0.636 1.27 0.416 0.197 0.293 0.363 0.362 0.352 0.422
19.49454–16.33174 19.49454–16.33312 23.11607–20.70979 22.89876–20.70421 22.89876–20.70979 16.33312–14.44883 16.33174–14.44883 18.04581–16.33174 18.04599–16.33312
2–2 2–4 4–6 4–4 4–6 4–2 2–2 4–2 6–4
the z-axis (see Fig. 1) and focused onto T1. Subsequently, the UV laser was delivered along the x-axis onto the perpendicular graphite target T2. Time-resolved emission spectra were obtained at a distance range of 3 to 10 mm on the normal direction to the target T2 (x in Fig. 1). At distances less than 3 mm, the spectrum is dominated by the Bremsstrahlung radiation and the spectral transitions are not defined at all; on the other hand, at distances greater than 10 mm the signal to noise ratio of the useful lines is poor and the measurements are not reliable. Light emitted by the plasma was collected by an inverse telescope arrangement on the y-axis direction (LCS in Fig. 1) and sent to a 50 cm focal length spectrometer with a 2400 lines mm−1 diffraction grating (Acton Research, SpectraPro 500i); this system has a resolution of 0.01 nm. The dispersed light was analyzed by a gated intensified charge coupled device (ICCD) camera (Princeton Instruments, PI-MAX 1024 × 1024), which was triggered by a second pulse/delay generator (Stanford, DG535) synchronized with the lasers (DG-2 in Fig. 1). The camera gate width was varied within the range of 20 to 40 ns according to the distance in which the spectra were recorded. Results obtained for the second plasma in the CBPLA experiments at different delays between lasers were systematically compared with the single-beam approach generated by the KrF excimer laser only. The most important changes were observed with a 2 µs time delay between pulses in accordance with results obtained in previous work [12]; for clarity, only results corresponding to this time delay are reported. Under the assumption of LTE and an optically thin plasma, its electron temperature can be determined by relative line intensity measurements from the same atomic or ionic species [18,19]. In this work, nine different C II emission lines (see Table 1) were used to determine the temperature employing Boltzmann plots and the required equations parameters were found on the NIST Atomic Spectra Database [20]. These transitions were chosen because they appeared isolated and lasted long enough in the emission spectra. Fig. 2 shows the time resolved electron temperature for the CBPLA arrangement with a 2 µs delay between lasers and for the single-beam experiment, obtained at 3 and 10 mm from the T2 surface; here, time zero is defined by the second plasma onset. All the reported lines in Table 1 have been used in the Boltzmann plots except for the 391.90 and 392.07 nm transi-
R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
(a)
353
(b)
Fig. 2. Time resolved electron temperature for single- and cross-beam (τ = 2 µs) laser ablation plasma on perpendicular graphite targets. Emission spectra were obtained at (a) 3 mm and (b) 10 mm from the second pulse target (T2).
tions in the cross-beam configuration at 3 mm, which were not resolved by the spectrometer; and the 564.81 and 566.25 nm for the single-beam experiment at 10 mm, which had a poor signal to noise ratio. It should also be stated that in each plot the same set of lines were employed during the reported time evolution. Electron temperature evolution at 3 mm from the target T2 surface is shown in Fig. 2(a); data collection starts at 100 ns because prior to this the Bremsstrahlung radiation is dominant and the emission lines are strongly self-absorbed [18,21]. Since the light is collected at the early stages of plasma expansion, it is only possible to get the emission spectrum with high signal to noise ratio for a short period of time (up to 160 ns) because the plume-emitting region is small and it is moving quickly away from the target surface [12]. At this distance the electron temperature has similar values for both experiments after taking the experimental uncertainty into account. Fig. 2(b) shows the time resolved electron temperature at 10 mm from the KrF laser target surface T2. Here, the electron temperature of the cross-beam arrangement is greater than the single-beam experiment values, as is the arrival time of the plume to this distance. In both experiments, the temperature rises until it reaches the maximum value corresponding to the plume displacement and the fact that the edges of the plume are colder than the inner regions. However, in the cross-beam generated plasma, the maximum temperature value is 50% greater than that in the single pulse experiment. Furthermore, the hottest plume region in the first case reaches the studied distance at t = (480 ± 20) ns, while, in the second case, it is reached at time t = (320 ± 20) ns. Line intensification is observed in the cross-beam experiment emission spectra; therefore, it is possible to observe those transitions for a longer period of time. For singly ionized non-hydrogenic ions, Stark broadening is caused mainly by electron impact and the full width at halfmaximum (FWHM) of the lines is related to the electron density [21]. Three broadening mechanisms are likely to contribute
to line widths observed in laser induced plasmas: Doppler broadening, resonance pressure broadening and Stark broadening. The first mechanism contributes in less than 3% of the total measured line intensity for our C II expansion velocities and it can be ignored. The resonance pressure one is proportional to the ground state could be neglected under our experimental conditions [14]. Thus the electron density (ne ) in the plasma can be obtained from the Stark-broadening measurement of the individual lines. However, the instrumental contribution should be taken into account. These two significant line broadenings contributions have a Lorentzian profile; therefore, the FWHM of each line was measured by fitting this distribution over the spectral transition. The Stark contribution was obtained by subtracting the instrumental one, which is 0.02 nm, from the whole FWHM. This work employed four C II emission lines for this effect: 657.8, 658.2, 723.1 and 723.6 nm and the required parameters for the calculations were obtained from [18]. As a result, the experimental error on the reported electron density is about 10% due to the error propagation because of the different emission lines employed to make the calculations. Fig. 3 shows the time resolved electron density for the plasma generated by the cross-beam arrangement and for that generated by the single pulse. The electron density measurements at 3 mm from the target T2 surface are shown in Fig. 3(a). It is observed that, like electron temperature evolution at this distance [see Fig. 2(a)], both experiments have the same temporal behavior. However, the electron density in the cross-beam generated plasma is greater than that observed in the other experiment. The highest electron densities reached at this distance were 5.4 × 1017 and 4.6 × 1017 cm−3 for cross- and singlebeam arrangements respectively at t = 100 ns after the second plasma onset. At 10 mm from the target surface [see Fig. 3(b)] the ne evolution exhibits the same behavior as the electron temperature. The maximum ne values are reached, approximately, when the maximum electron temperature values occur [see Fig. 2(b)]. For all studied distances, the cross-beam gener-
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R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
(a)
(b)
Fig. 3. Time resolved electron density for single- and cross-beam (τ = 2 µs) laser ablation plasma on perpendicular graphite targets. Electron density was measured at (a) 3 mm and (b) 10 mm from the second pulse target (T2).
ated plasma has a maximum ne 20% higher than that achieved in the single pulse experiment. The electron temperature values were obtained under the assumption that the plasma is in LTE and is optically thin. The first condition is reached when particle densities are high enough to dominate the de-excitation mechanisms via collisional processes [18]. A necessary criterion for LTE is given 1/2 by ne 1.4 × 1014 Te (Eki )3 [21], where ne is in cm−3 , Te is the electron temperature in eV and Eki is the energy difference between upper and lower energy levels in eV. Applying this rule to the less favorable case in this work, that is, for the transition at 391.90 nm with Eki = 3.1628 eV and Te ≈ 32 000 K, the lowest limit for ne is 7.4 × 1015 cm−3 . The obtained ne values are much greater than this limit implying that LTE approximation for this analysis is valid. The second factor to consider is the absorption of radiation within the plasma (plasma opacity). The spectral absorption, in the center of one transition characterized by the levels i and k, can be estimated from κik (λ0 ) = 8.85 × 10−13 ni fik λ20 Lik (λ0 ) [18,19], where κik is the absorption coefficient in cm−1 , fik is the absorption oscillator strength, λ0 is the central wavelength in cm, ni is the population density of the lower-level energy in cm−3 and Lik (λ0 ) = 1/πλ is the normalized Lorentzian profile at the central wavelength, where λ is the FWHM. The partition function values necessary for ni calculations and the oscillator strengths were obtained from Ref. [20]. Considering a quasineutral plasma and the approximation n ≈ ne , with n the total C II density, the estimated population density of the lower level is ∼ 1014 cm−3 for all employed transitions; as a result, the estimated self-absorption is less than 5%, which could be neglected. Results obtained in the cross-beam experiment imply an interaction between the second generated plasma and the remnant particles from the first one, increasing the electron temperature and density of the second plasma. Collisional processes, as the dynamical model suggests [12], induce an enhancement of the
emission lines intensity and a decrease in the plume kinetic energy. These collisions produce a re-excitation and ionization of the ablated material from the second target, which results in an increase in the electron density and temperature. Changes produced in the interaction region are accumulative with the distance to the second pulse target surface. Acknowledgements This work was supported by National Autonomous University of Mexico, DGAPA UNAM: IN100706-3 and IN104806-3 and the National Council of Science and Technology of Mexico CONACyT: 44758-F and P47272-F. References [1] M. Corsi, G. Cristoforetti, M. Giuffrida, M. Hidalgo, S. Legnaioli, V. Palleshi, A. Salvetti, E. Tognoni, C. Vallebona, Spectrochim. Acta, Part B 59 (2004) 723. [2] L. St-Onge, M. Sabsabi, P. Cielo, Spectrochim. Acta Part, B 53 (1998) 407. [3] F. Colao, V. Lazic, R. Fantoni, S. Pershin, Spectrochim. Acta, Part B 57 (2002) 1167. [4] R. Noll, R. Sattmann, V. Sturm, S. Winkelmann, J. Anal. At. Spectrom. 19 (2004) 419. [5] S. Witanachchi, K. Ahmed, P. Sakthivel, P. Mukherjee, Appl. Phys. Lett. 66 (1995) 1469. [6] A. Tselev, A. Gorbunov, W. Pompe, Appl. Phys. A 69 (1999) 353. [7] L. Lambert, F. Grangeon, M. Autric, Appl. Surf. Sci. 138–139 (1999) 574. [8] E. Camps, L. Escobar-Alarcón, E. Haro-Poniatowski, M. FernándezGuasti, Appl. Surf. Sci. 197–198 (2002) 239. [9] A. Tselev, A. Gorbunov, W. Pompe, Rev. Sci. Instrum. 72 (2001) 2665. [10] C. Sánchez-Aké, H. Sobral, E. Sterling, M. Villagrán-Muniz, Appl. Phys. A 79 (2004) 1345. [11] C. Sánchez-Aké, H. Sobral, P. Ramos-Alvárez, C. Lemen, M. VillagránMuniz, Ion kinetic energy control in dual-pulsed laser ablation on graphite targets, J. Phys.: Conference Series, in press. [12] C. Sánchez Aké, R. Sanginés de Castro, H. Sobral, M. Villagrán-Muniz, J. Appl. Phys. 100 (2006) 053305. [13] R. Sanginés de Castro, H. Sobral, C. Sánchez-Aké, M. Villagrán-Muniz, Phys. Lett. A 357 (2006) 351.
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[14] S.S. Harilal, C.V. Bindhu, R.C. Issac, V.P.N. Nampoori, C.P.G. Vallabhan, J. Appl. Phys. 82 (1997) 2140. [15] F.J. Gordillo-Vázquez, A. Perea, J.A. Chaos, J. Gonzalo, C.N. Afonso, Appl. Phys. Lett. 78 (2001) 7. [16] F.J. Gordillo-Vázquez, A. Perea, A.P. McKiernan, C.N. Afonso, Appl. Phys. Lett. 86 (2005) 181501. [17] K.J. Saji, N.V. Joshy, M.K. Jayaraj, J. Appl. Phys. 100 (2006) 043302. [18] H.R. Griem, Plasma Spectroscopy, McGraw–Hill, USA, 1964.
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[19] A. Thorne, U. Litzén, S. Johansson, Spectrophysics, Springer-Verlag, Germany, 1999. [20] Y. Ralchenko, F.C. Jou, D.E. Kelleher, A.E. Kramida, A. Musgrove, J. Reader, W.L. Wiese, K. Olsen, NIST Atomic Spectra Database, Gaithersburg, MD, 3.0.2, 2005. Available online: http://physics.nist.gov/ PhysRefData/ASD/index.html. National Institute of Standards and Technology. [21] G. Bekefi, Principles of Laser Plasmas, Wiley, USA, 1976.
Time resolved optical emission spectroscopy of cross-beam pulsed laser ablation on graphite targets R. Sanginés a,b,∗ , C. Sánchez Aké a , H. Sobral a , M. Villagrán-Muniz a a Laboratorio de Fotofísica, Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México,
Apartado Postal 70-186, México Distrito Federal 04510, Mexico b School of Physics, University of Sydney, New South Wales 2006, Australia
Received 19 December 2006; received in revised form 7 February 2007; accepted 16 March 2007 Available online 20 March 2007 Communicated by F. Porcelli
Abstract Cross-beam pulsed laser ablation with two delayed lasers is performed on two perpendicular graphite targets. The time delay between lasers is varied by up to 5 µs, and physical changes on the second plasma, due to the interaction with the first generated one, are determined by time resolved optical emission spectroscopy. © 2007 Elsevier B.V. All rights reserved. PACS: 79.20.Ds; 52.70.La Keywords: Plasma ablation; Plasma diagnostic techniques
The interaction between two laser induced plasmas has been recently applied for different purposes. The plasmas are generated by two synchronized laser pulses and the effects produced by the plumes’ interaction depend on the spatial overlap between them, the beam incidence configuration and the time delay between pulses. Double pulse configurations provide an improvement of the laser induced breakdown spectroscopy (LIBS) technique, rising its sensitivity by increasing the emission lines’ intensity [1–4]. This technique is also useful for thin film growth for a great diversity of materials, as it reduces the films’ splashing [5]. An alternative configuration is the crossbeam pulsed laser ablation technique (CBPLA) where the two plasmas are generated on two perpendicular targets and smooth films of different materials have been obtained with this method [6–8]. Diverse studies have been performed in order to obtain plasma properties using emission spectroscopy. Plasmas have shown changes in the kinetic energy of the species [9,10], and * Corresponding author at: School of Physics A28, University of Sydney, New South Wales 2006, Australia. Tel.: +61 2 9351 2553; fax: +61 2 9351 7725. E-mail address: [email protected] (R. Sanginés).
0375-9601/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2007.03.016
it is possible to control this energy by varying the temporal delay between pulses [10,11]. The most important changes in the kinetic energy of the species, in the perpendicular targets arrangement, have been observed in the range of 2 to 4 µs time delay between lasers. For a time delay of 2 µs, the velocity of species in the second plume is reduced compared to the distance to the target by up to a factor of two, with respect to the velocity reached by the species in the single-beam generated plasma. The gradual reduction in the kinetic energy of the second plume is due to the collisions with the remaining particles of the first plume, and it is modeled using the drag model [12]. Under this framework, the ejected species are considered as an ensemble that suffers a viscous force, proportional to its expansion velocity caused by the background gas. Therefore, at early expansion stages (up to 2 cm from the target surface) the second plasma species interact with the remnant particles from the first plasma, loosing their kinetic energy. Once those particles leave this interacting zone, they have a similar behavior to a single-beam generated plume expanding at constant velocity. Furthermore, for two perpendicular interacting plumes, electron and neutral densities have been determined by the two-color interferometry technique [13]. Results have showed that densities do not
352
R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
Table 1 Emission transitions employed for the electron temperature determination; λ: central wavelength, Aki : transition probability from the level k to the level i, E: level’s energy, g: level’s degeneracy
Fig. 1. Experimental setup for time resolved optical emission spectroscopy used to analyze the plasmas induced by cross-beam pulsed laser ablation; DG: delay generator, L: lens, M: mirror, T1 and T2: rotating graphite targets, LCS: lens collector system, OF: optical fiber, Spect: spectrometer.
have measurable changes, with respect to those obtained for the single-beam array, at distances up to 1.5 mm to the second pulse target surface; thus, it could be expected that physical changes produced by the interaction between both plumes would be accumulative with the distance according to the dynamical model. This work reports the diagnostic of a cross-beam pulsed laser induced plasma on two perpendicular graphite targets using optical emission spectroscopy. Time resolved electron density and temperature were measured for time delays up to 5 µs between lasers and for single beam. The aim of this work is to determine how the first plasma affects the second one as a function of the distance to the second pulse target. The plasma was assumed to be in local thermodynamic equilibrium (LTE); it was also assumed that the main broadening mechanism of the emission lines is the Stark effect [14–17]. Laser sources used to induce the ablation plasmas were a KrF excimer laser emitting at 248 nm, with a 30 ns pulse width at a fluence of 25 J/cm2 (Lambda Physik, COMPex102) and an Nd:yttrium–aluminum-garnet (YAG) laser emitting at 1064 nm, with a 7 ns pulse width and fluence at 15 J/cm2 (Continuum, Surelite I). Fig. 1 shows the experimental setup used in the cross-beam pulsed configuration. Laser beams were directed towards two perpendicular rotating graphite targets (T1 and T2 in Fig. 1) and focused by 30 cm focal length plano-convex lenses normal to the targets surfaces. The distance (direct line of sight) between the ablation sites was 3.5 mm. Ablation was performed in a vacuum chamber equipped with quartz windows and kept at 5 × 10−6 Torr. Synchronization of lasers was controlled by a pulse/delay generator (Stanford, DG535) and the pulse delay between lasers (τ ) was varied by up to 5 µs. The first pulse, which corresponds to the Nd: YAG laser, was directed along
λ [nm]
Aki [108 s−1 ]
Ek –Ei [eV]
gk –gi
391.90 392.07 515.11 564.81 566.25 657.81 658.29 723.13 723.64
0.636 1.27 0.416 0.197 0.293 0.363 0.362 0.352 0.422
19.49454–16.33174 19.49454–16.33312 23.11607–20.70979 22.89876–20.70421 22.89876–20.70979 16.33312–14.44883 16.33174–14.44883 18.04581–16.33174 18.04599–16.33312
2–2 2–4 4–6 4–4 4–6 4–2 2–2 4–2 6–4
the z-axis (see Fig. 1) and focused onto T1. Subsequently, the UV laser was delivered along the x-axis onto the perpendicular graphite target T2. Time-resolved emission spectra were obtained at a distance range of 3 to 10 mm on the normal direction to the target T2 (x in Fig. 1). At distances less than 3 mm, the spectrum is dominated by the Bremsstrahlung radiation and the spectral transitions are not defined at all; on the other hand, at distances greater than 10 mm the signal to noise ratio of the useful lines is poor and the measurements are not reliable. Light emitted by the plasma was collected by an inverse telescope arrangement on the y-axis direction (LCS in Fig. 1) and sent to a 50 cm focal length spectrometer with a 2400 lines mm−1 diffraction grating (Acton Research, SpectraPro 500i); this system has a resolution of 0.01 nm. The dispersed light was analyzed by a gated intensified charge coupled device (ICCD) camera (Princeton Instruments, PI-MAX 1024 × 1024), which was triggered by a second pulse/delay generator (Stanford, DG535) synchronized with the lasers (DG-2 in Fig. 1). The camera gate width was varied within the range of 20 to 40 ns according to the distance in which the spectra were recorded. Results obtained for the second plasma in the CBPLA experiments at different delays between lasers were systematically compared with the single-beam approach generated by the KrF excimer laser only. The most important changes were observed with a 2 µs time delay between pulses in accordance with results obtained in previous work [12]; for clarity, only results corresponding to this time delay are reported. Under the assumption of LTE and an optically thin plasma, its electron temperature can be determined by relative line intensity measurements from the same atomic or ionic species [18,19]. In this work, nine different C II emission lines (see Table 1) were used to determine the temperature employing Boltzmann plots and the required equations parameters were found on the NIST Atomic Spectra Database [20]. These transitions were chosen because they appeared isolated and lasted long enough in the emission spectra. Fig. 2 shows the time resolved electron temperature for the CBPLA arrangement with a 2 µs delay between lasers and for the single-beam experiment, obtained at 3 and 10 mm from the T2 surface; here, time zero is defined by the second plasma onset. All the reported lines in Table 1 have been used in the Boltzmann plots except for the 391.90 and 392.07 nm transi-
R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
(a)
353
(b)
Fig. 2. Time resolved electron temperature for single- and cross-beam (τ = 2 µs) laser ablation plasma on perpendicular graphite targets. Emission spectra were obtained at (a) 3 mm and (b) 10 mm from the second pulse target (T2).
tions in the cross-beam configuration at 3 mm, which were not resolved by the spectrometer; and the 564.81 and 566.25 nm for the single-beam experiment at 10 mm, which had a poor signal to noise ratio. It should also be stated that in each plot the same set of lines were employed during the reported time evolution. Electron temperature evolution at 3 mm from the target T2 surface is shown in Fig. 2(a); data collection starts at 100 ns because prior to this the Bremsstrahlung radiation is dominant and the emission lines are strongly self-absorbed [18,21]. Since the light is collected at the early stages of plasma expansion, it is only possible to get the emission spectrum with high signal to noise ratio for a short period of time (up to 160 ns) because the plume-emitting region is small and it is moving quickly away from the target surface [12]. At this distance the electron temperature has similar values for both experiments after taking the experimental uncertainty into account. Fig. 2(b) shows the time resolved electron temperature at 10 mm from the KrF laser target surface T2. Here, the electron temperature of the cross-beam arrangement is greater than the single-beam experiment values, as is the arrival time of the plume to this distance. In both experiments, the temperature rises until it reaches the maximum value corresponding to the plume displacement and the fact that the edges of the plume are colder than the inner regions. However, in the cross-beam generated plasma, the maximum temperature value is 50% greater than that in the single pulse experiment. Furthermore, the hottest plume region in the first case reaches the studied distance at t = (480 ± 20) ns, while, in the second case, it is reached at time t = (320 ± 20) ns. Line intensification is observed in the cross-beam experiment emission spectra; therefore, it is possible to observe those transitions for a longer period of time. For singly ionized non-hydrogenic ions, Stark broadening is caused mainly by electron impact and the full width at halfmaximum (FWHM) of the lines is related to the electron density [21]. Three broadening mechanisms are likely to contribute
to line widths observed in laser induced plasmas: Doppler broadening, resonance pressure broadening and Stark broadening. The first mechanism contributes in less than 3% of the total measured line intensity for our C II expansion velocities and it can be ignored. The resonance pressure one is proportional to the ground state could be neglected under our experimental conditions [14]. Thus the electron density (ne ) in the plasma can be obtained from the Stark-broadening measurement of the individual lines. However, the instrumental contribution should be taken into account. These two significant line broadenings contributions have a Lorentzian profile; therefore, the FWHM of each line was measured by fitting this distribution over the spectral transition. The Stark contribution was obtained by subtracting the instrumental one, which is 0.02 nm, from the whole FWHM. This work employed four C II emission lines for this effect: 657.8, 658.2, 723.1 and 723.6 nm and the required parameters for the calculations were obtained from [18]. As a result, the experimental error on the reported electron density is about 10% due to the error propagation because of the different emission lines employed to make the calculations. Fig. 3 shows the time resolved electron density for the plasma generated by the cross-beam arrangement and for that generated by the single pulse. The electron density measurements at 3 mm from the target T2 surface are shown in Fig. 3(a). It is observed that, like electron temperature evolution at this distance [see Fig. 2(a)], both experiments have the same temporal behavior. However, the electron density in the cross-beam generated plasma is greater than that observed in the other experiment. The highest electron densities reached at this distance were 5.4 × 1017 and 4.6 × 1017 cm−3 for cross- and singlebeam arrangements respectively at t = 100 ns after the second plasma onset. At 10 mm from the target surface [see Fig. 3(b)] the ne evolution exhibits the same behavior as the electron temperature. The maximum ne values are reached, approximately, when the maximum electron temperature values occur [see Fig. 2(b)]. For all studied distances, the cross-beam gener-
354
R. Sanginés et al. / Physics Letters A 367 (2007) 351–355
(a)
(b)
Fig. 3. Time resolved electron density for single- and cross-beam (τ = 2 µs) laser ablation plasma on perpendicular graphite targets. Electron density was measured at (a) 3 mm and (b) 10 mm from the second pulse target (T2).
ated plasma has a maximum ne 20% higher than that achieved in the single pulse experiment. The electron temperature values were obtained under the assumption that the plasma is in LTE and is optically thin. The first condition is reached when particle densities are high enough to dominate the de-excitation mechanisms via collisional processes [18]. A necessary criterion for LTE is given 1/2 by ne 1.4 × 1014 Te (Eki )3 [21], where ne is in cm−3 , Te is the electron temperature in eV and Eki is the energy difference between upper and lower energy levels in eV. Applying this rule to the less favorable case in this work, that is, for the transition at 391.90 nm with Eki = 3.1628 eV and Te ≈ 32 000 K, the lowest limit for ne is 7.4 × 1015 cm−3 . The obtained ne values are much greater than this limit implying that LTE approximation for this analysis is valid. The second factor to consider is the absorption of radiation within the plasma (plasma opacity). The spectral absorption, in the center of one transition characterized by the levels i and k, can be estimated from κik (λ0 ) = 8.85 × 10−13 ni fik λ20 Lik (λ0 ) [18,19], where κik is the absorption coefficient in cm−1 , fik is the absorption oscillator strength, λ0 is the central wavelength in cm, ni is the population density of the lower-level energy in cm−3 and Lik (λ0 ) = 1/πλ is the normalized Lorentzian profile at the central wavelength, where λ is the FWHM. The partition function values necessary for ni calculations and the oscillator strengths were obtained from Ref. [20]. Considering a quasineutral plasma and the approximation n ≈ ne , with n the total C II density, the estimated population density of the lower level is ∼ 1014 cm−3 for all employed transitions; as a result, the estimated self-absorption is less than 5%, which could be neglected. Results obtained in the cross-beam experiment imply an interaction between the second generated plasma and the remnant particles from the first one, increasing the electron temperature and density of the second plasma. Collisional processes, as the dynamical model suggests [12], induce an enhancement of the
emission lines intensity and a decrease in the plume kinetic energy. These collisions produce a re-excitation and ionization of the ablated material from the second target, which results in an increase in the electron density and temperature. Changes produced in the interaction region are accumulative with the distance to the second pulse target surface. Acknowledgements This work was supported by National Autonomous University of Mexico, DGAPA UNAM: IN100706-3 and IN104806-3 and the National Council of Science and Technology of Mexico CONACyT: 44758-F and P47272-F. References [1] M. Corsi, G. Cristoforetti, M. Giuffrida, M. Hidalgo, S. Legnaioli, V. Palleshi, A. Salvetti, E. Tognoni, C. Vallebona, Spectrochim. Acta, Part B 59 (2004) 723. [2] L. St-Onge, M. Sabsabi, P. Cielo, Spectrochim. Acta Part, B 53 (1998) 407. [3] F. Colao, V. Lazic, R. Fantoni, S. Pershin, Spectrochim. Acta, Part B 57 (2002) 1167. [4] R. Noll, R. Sattmann, V. Sturm, S. Winkelmann, J. Anal. At. Spectrom. 19 (2004) 419. [5] S. Witanachchi, K. Ahmed, P. Sakthivel, P. Mukherjee, Appl. Phys. Lett. 66 (1995) 1469. [6] A. Tselev, A. Gorbunov, W. Pompe, Appl. Phys. A 69 (1999) 353. [7] L. Lambert, F. Grangeon, M. Autric, Appl. Surf. Sci. 138–139 (1999) 574. [8] E. Camps, L. Escobar-Alarcón, E. Haro-Poniatowski, M. FernándezGuasti, Appl. Surf. Sci. 197–198 (2002) 239. [9] A. Tselev, A. Gorbunov, W. Pompe, Rev. Sci. Instrum. 72 (2001) 2665. [10] C. Sánchez-Aké, H. Sobral, E. Sterling, M. Villagrán-Muniz, Appl. Phys. A 79 (2004) 1345. [11] C. Sánchez-Aké, H. Sobral, P. Ramos-Alvárez, C. Lemen, M. VillagránMuniz, Ion kinetic energy control in dual-pulsed laser ablation on graphite targets, J. Phys.: Conference Series, in press. [12] C. Sánchez Aké, R. Sanginés de Castro, H. Sobral, M. Villagrán-Muniz, J. Appl. Phys. 100 (2006) 053305. [13] R. Sanginés de Castro, H. Sobral, C. Sánchez-Aké, M. Villagrán-Muniz, Phys. Lett. A 357 (2006) 351.
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