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Shock wave plasma induced by TEA CO2 laser bombardment on glass samples at high pressures
Spectrochimica Acta Part B 55 Ž2000. 1591᎐1599
Shock wave plasma induced by TEA CO 2 laser bombardment on glass samples at high pressures A.M. Marpaung a , R. Hedwig a , M. Pardede a , T.J. Lie a , M.O. Tjiab, K. Kagawac , H. Kurniawan a,U a
Applied Spectroscopy Laboratory, Graduate Program in Optoelectrotechniques and Laser Application, Faculty of Engineering, The Uni¨ ersity of Indonesia, 4, Salemba Raya, Jakarta 10430, Indonesia b Department of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, 10 Ganesha, Bandung, Indonesia c Department of Physics, Faculty of Education and Regional Studies, Fukui Uni¨ ersity, 9-1 Bunkyo 3-Chome, Fukui 910, Japan Received 23 January 2000; accepted 13 July 2000
Abstract An experimental study has been carried out on the dynamical process taking place in laser plasma, generated by TEA CO 2 laser Ž400 mJ, 100 ns. irradiation on glass samples surrounding by air of high pressures up to 760 torr. Accurate dynamical characterization was performed by simultaneous observation of the plasma emission front and the shock wave front. The shock wave front was detected by a modified shadowgraph technique while the emission front was detected by observing the rising time at various slit positions. In spite of the occurrence of a new feature uncommon to laser plasma, generated in low air pressures, it is found that the two fronts coincide and move together at the initial stage of the laser plasma, but eventually separate from each other, with the emission front being left behind the shock wave front at a later stage. These characteristics hold for the atomic emission lines of all elements contained in the glass samples examined, regardless of their different atomic weights. It is therefore strongly indicative of the shock wave mechanism in the laser plasma generation and the emission in the high-pressure surrounding air. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: TEA CO 2 laser; Density jump; Emission front; Laser induced shock wave plasma spectroscopy
U
Corresponding author. 51G᎐51H, Tomang Raya, Jakarta Barat 11440, Indonesia. Tel.: q62-21-330188; fax: q62-213918115r5809144. E-mail addresses: [email protected] ŽM.O. Tjia ., [email protected] ŽK. Kagawa ., kurnia18@ cbn.net.id ŽH. Kurniawan.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 2 6 4 - 0
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1. Introduction The widely adopted technique of laser-ablationemission spectrometric analysis ŽLAESA. is one of the important results accomplished in laser applications w1᎐3x. In its early development for spectrochemical analysis, this technique was confronted with problems of high-intensity continuous background and contamination by undesirable self-absorption processes due to the use of high pressure surrounding air. These problems were expected in turn to cause serious impairment of the detection sensitivity and linearity. In its more recent development led by Cremers and his colleagues w4᎐11x, those technical hurdles have been overcome to a good extent by the use of a better laser system, such as a modern high power Nd-YAG laser and the incorporation of gatedoptical-multichannel analyzer ŽOMA. into the detection system. This line of development has resulted in a much-improved spectroscopic technique widely known as laser-induced breakdown spectroscopy ŽLIBS. in connection with the predicted role of a gaseous breakdown mechanism in the emission process. An alternative route of development has been proposed by Kagawa et al. w12᎐21x, who suggested the use of low-pressure surrounding gas instead of surrounding gas at atmospheric pressure adopted in the other line of development. It has been shown that a plasma having characteristics favorable to spectrochemical analysis can actually be generated by using a pulsed laser with a short duration, such as a nitrogen laser w12,13x, a carbon-dioxide laser w14x, and an excimer laser w15x, when the pressure of the surrounding gas is reduced to approximately 1 torr. In these cases, the generated plasmas invariably consist of two distinct parts. The first part, which is called the primary plasma, occupies a small space just above the sample surface, which gives off intense continuous emission spectra for a short time. The other part, called the secondary plasma, expands with time around the primary plasma in a nearhemispherical shape, emitting sharp atomic spectral lines with negligibly low background. In the time-resolved experiments conducted previously using a carbon-dioxide laser w14x, and
an excimer laser w15x, our group demonstrated that this secondary plasma was produced by a shock wave in the surrounding gas, while the primary plasma acted as an initial explosion energy source. A model has also been proposed for the explanation of the excitation mechanism in the secondary plasma. According to this model, right after the cessation of the primary plasma, atoms gushed out from the primary plasma at supersonic speeds, pushing the surrounding gas like a piston. These fast moving atoms, being impeded by the surrounding gas, gave rise to a compression process. As a result of this compression, a shock wave was generated in the surrounding gas, leading to the formation of secondary plasma. The most important point in the shock wave model was that the energy required to produced the secondary plasma was supplied in the form of kinetic energies from the propelling atoms. Due to the compression process, the kinetic energies of the propelling atoms were converted into the thermal energy in the plasma, by which the atoms were excited. We have further presented in our previous work w18x, a result supporting the idea that the emission front of the plasma and the front of the shock wave moved together with time at the initial stage of the secondary plasma expansion, during which the excitation process took place most effectively. At a later stage, the emission front separated from the shock wave front and left it behind, marking the cooling stage of the plasma. In spite of the detailed understanding described above, about laser plasma generated at low surrounding gas pressures, the fundamental mechanism of laser plasma generated at the atmospheric pressure has yet to be unraveled despite its remarkable popularity as a tool for spectrochemical analysis. In order to look for clues on this mechanism, we have carried out a detailed experiment employing a surrounding gas at a relatively high pressure, ranging from 100 to 760 torr. Time resolved measurements were performed on the laser plasma produced at these air pressures in the hope of revealing new dynamical features which were hitherto unheeded. It will be shown that the analysis of these features led to a
A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599
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Fig. 1. Diagram of the experimental set-up used in this study.
strong indication of the occurrence of secondary plasma even in the high-pressure surrounding air.
2. Experimental procedure The diagram of the experimental arrangement is shown in Fig. 1. In this experiment, a TEA CO 2 laser ŽShibuya Kogyo, SQ-2000, 3 J of full power, 100 ns of pulse width as can be seen in Fig. 2 with wavelengths of 10.6 m., as the irradiation source, was operated shot by shot with the output energy fixed at 400 mJ by an aperture. The laser beam was focused by a Ge lens Ž f s 100 mm. through a ZnSe window onto the surface of the sample. A museum glass containing silicon Ž; 12%., calcium Ž; 2.5%. and sodium Ž; 4%., and a lead glass containing lead Ž; 17%., silicon Ž; 6%., calcium Ž; 2%. and sodium Ž; 0.5%. were used as samples.
The sample was placed in a vacuum tight metal chamber Ž12.5 cm = 10 cm = 10 cm., which could be evacuated to 10 ᎐ 2 torr and filled with a desired gas at a certain pressure. Precisely a digital manometer ŽNishiyama Seisakusho, DM760. measured the chamber pressure. The airflow through the chamber was regulated by needle valves in the air line and in the pumping line. The sample, together with the whole chamber and the
Fig. 2. A TEA CO 2 laser pulse waveform.
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Ge lens could be moved by a step motor, for the movement in the laser beam direction and a micrometer for the perpendicular movement to the laser beam direction. The sample could be rotated with another step motor to avoid repeated irradiation on the same surface spot. Emission of the laser-induced plasma was observed through an optical window at right angle to the laser beam, by an imaging quartz lens Ž f s 100 mm. with an aperture of 10 mm= 10 mm. The plasma was imaged, with an enlargement ratio of 1:3, onto the entrance slit of a monochromator ŽNikon P-250, plane grating 1200 groovesrmm blazed at 500 nm, focal length for concave mirror s 250 mm.. The entrance slit was set at 2 mm in height and 100 m in width so that the observation was restricted to a limited area. The output signal from a photomultiplier ŽHamamatsu R-585., attached with a 500-⍀ resistance for a RC time constant of 30 ns, was fed into a digital-sampling-storage scope ŽHP 545616B.. For the detection of the shock wave front associated with a density jump, a modified shadowgraph technique was employed using a He᎐Ne laser Ž632.8 nm and 1 mm beam diameter., as a separate probe light. The probe light was sent to the expansion region of the plasma. A sudden deviation in the intensity of the probe light due to the density jump was detected by a photomultiplier ŽHamamatsu R-1104. after the light has passed through an interference filter. The output signal of the photomultiplier was then sent to another channel of the digital-sampling-storage scope. A photon-drag detector to provide a trigger signal for digital-sampling-storage scope detected a part of the TEA CO 2 laser beam. In order to reduce the undesired continuous emissions taking place near the primary plasma, the observation region is shifted 1 mm upward along the z-axis from the center of the laser plasma, as shown in the insert of Fig. 1.
3. Results and discussion As we have reported in our previous works w12᎐18x, the secondary plasma and the primary
plasma could be clearly observed by the naked eye in surrounding air below 10 torr. The secondary plasma exhibited a characteristic hemispherical shape with brilliant colors associated with emissions from the constituent atoms, while the primary plasma displayed an intense white color associated with the continuum emission. The hemispherical shape of the secondary plasma was indicative of the dominant role of the shock wave in its generation, connected with the typical hemispherical shape of the shock wave. In the present experiment which is conducted in surrounding air at pressure above 50 torr, the secondary plasma appears to be dominated by sodium emission at its later stage. It is also observed that the strong sodium emission region exhibits a cone-like structure instead of the hemispherical shape common to shock wave-induced plasma, with the sodium emission process lasting longer than those from other atoms such as calcium, silicone, etc. At its earlier stage however, the use of a blue filter to cut off the sodium emission does reveal the hemispherical secondary plasma shape. This can be explained as a consequence of the use of a broad laser pulse Ž100 ns., giving rise to a longer vaporization process. In such a relatively prolonged process, only the cluster of atoms initially ablated from the target surface are involved in the compression process responsible for the formation of a shock wave as well as the kinetic-to-thermal energy conversion process. Therefore, only these atoms will benefit from the large supply of thermal energy from the high temperature plasma for their excitations. The atoms vaporized by the later part of the laser pulse are not supposed to generate an effective compression process in the relatively rarefied region left behind the initial shock wave. As a result, the energy available for the atomic excitation is accordingly reduced. This energy reduction is expected, however, to have a highly preferential effect in favor of the sodium atoms, simply because the Na I 589.0 nm and Na I 589.6 nm emission lines are associated with relatively low excitation energy of 2.1 eV. We note further that the primary plasma can still be readily distinguished from the secondary plasma by the naked eye even in the surrounding
A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599
gas with a pressure of up to 300 torr. As the pressure increases beyond 300 torr, the highly dense white color region of the primary plasma expands accordingly, blurring the boundary between the two plasma regions and making it practically indistinguishable to the naked eye. Nevertheless, the presence of secondary plasma can still be clearly detected by time-resolved spectroscopic measurements, even with surrounding gas at 760 torr. Whereas only a high-intensity continuum emission spectrum is detected in a region restricted around the primary plasma, distinct atomic lines become clearly observable as the detection region is shifted 2 mm off the primary plasma and after a considerable delay Žof the order of micro-seconds. with respect to the irradiation pulse. It is amply confirmed therefore that the laser-induced plasma still consists of a primary as well as secondary parts even at a pressure as high as 760 torr. Another feature supporting the occurrence of a shock wave is the density jump associated with the presence of a thin shell structure at the shock wave front. The observation of this density jump has been reported by Basov et al. w22,23x from their experiments using a shadowgraph technique and a Schlieren photograph technique, consecutively. The detection of a density jump was also reported at about the same time and independently by Bobin et al. w24x Hall w25x and Emmony et al. w26x employing an image converter camera in their recording of the expanding plasma and the detection of emission fronts. These evidences were further confirmed by the observation made by Hohla et al. w27x using a shadow photograph technique. In contrast to those techniques employed in previous experiments, a modified shadowgraph technique proposed earlier w17x, is used in this experiment to simplify the simultaneous detection of the density jump and the arrival of the emission front. Fig. 3 shows the result of the simultaneous detection of the density jump and the plasma emission of a calcium neutral line ŽCa I 422.6 nm. at various probing distances from the sample surface. The plasma is generated by a TEA CO 2 laser, which is focused onto the museum glass sample in air at 100 torr. The coincidence between
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Fig. 3. The relationship between the arriving of the density jump Žshown by an arrow. and rising of emission front of the calcium neutral line ŽCa I 422.6 nm. at various positions from the sample surface when the TEA CO 2 laser beam was focused onto an ordinary glass sample in air at 100 torr.
the density jump and rising point of the emission front is clearly seen for the probe light position is as far away as 4 mm from the sample surface, beyond which the two fronts begin to separate. It must be stressed that the emission front can be clearly detected by the detector as far as 7 mm from the sample surface, while the density jump can be detected as far as 20 mm from the sample surface. The same detection limits also hold for the sodium neutral line ŽNa I 589.0 nm. from the same samples. It should be further noted that the density jump signal in Fig. 3 is generally followed by additional late-coming ‘signals’ with an irregular pattern. These ‘signals’ appear to weaken as the probing position moves away from the target,
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and finally disappear at a probing position, 10 mm from the sample surface. These irregular ‘signals’ are apparently unrelated to the shock wave because they have no corresponding signal in the emission channel. It is conceivable that these signals are the signs of disturbance caused by atoms vaporized from the target behind the shock wave. For a TEA CO 2 laser with energy as high as 400 mJ and a pulse width as broad as 100 ns, the atoms ablated during the later part of the laser pulse may still accumulate to significant enough amounts to yield such a disturbance. We have found indeed that the windows of the chamber always turn cloudy after several hundred shots of laser irradiations. In order to substantiate the elucidation of the excitation mechanism in the secondary plasma at high pressures, one also needs to know how the shock wave front and the emission front move with time. These dynamical characteristics are
depicted in Fig. 4, where the variations of those front positions are plotted with respect to time at different air pressures Ž100 torr, 400 torr and 760 torr.. These plots are actually obtained by reading the rising points of the corresponding timeprofiles of the Ca I 422.6 nm emission line and the starting points of the density jumps of the curves given in Fig. 3. It is seen that the plot generally consists of two linear segments with different slopes. As we have mentioned above, it is remarkable that the coincidence between the emission front and the shock wave front is clearly sustained, as far as 4 mm from the sample surface Ž4.1 mm from the plasma center. at 100 torr, and up to 3 mm from the sample surface Ž3.2 mm from the plasma center. at 400 torr, before the curves corresponding to the two fronts branch off. These observations constitute a rather convincing evidence for our shock wave plasma model as described earlier. According to this model, the
Fig. 4. Relationship between the propagation length of the emission front of Ca I 422.6 nm and arrival of the density jump, as a function of time, at surrounding air pressure of 100 torr, 400 torr and 760 torr.
A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599
shock wave front and the emission front are expected to coincide and move along together at the initial stage of formation of the secondary plasma. This stage is soon followed by the cooling stage, starting at the moment when the two fronts begin to separate. At this stage, the emission front is expected to move at a lower speed than the shock wave front, as evidenced by those two plots in Fig. 4. It must be pointed out nevertheless that no such a coincidence is indicated in Fig. 4 for the case of 760 torr air pressure. It is suspected, however, that gas breakdown also takes place to some extent at this air pressure due to absorption of part of the laser energy by the air. It is then natural to expect that the emission from the plasma is disturbed by the light coming from the gas breakdown, leading to distorted emission data. It is therefore important to examine the dynamical characteristics of plasma formation from a different angle. To this end, a direct measurement is performed for the time-profiles of the spatially integrated emission intensity of Ca I 422.6 nm at various pressures Ž50 torr, 100 torr, 400 torr,and 760 torr. and the result is presented in Fig. 5. In this measurement, the imaging lens in Fig. 1 is removed, while the slit height and width of the monochromator are set at 15 mm and 100 m, respectively. It is seen in Fig. 5 that each curve undergoes a steep initial climb with time up to the maximum points and declines gradually thereafter. The growth segment of the emission curve corresponds to the excitation stage in our shock wave plasma model and the decay segment corresponds to the cooling stage. It is remarkable that the transition point from the growth region to the decay region Žmaximum point. in these curves practically coincide with the corresponding bending points of the two linear segments found in Fig. 4, including the case occurring at 760 torr air pressure. The same feature was already noted in the low-pressure case w18x. This result is at least consistent with the scenario consisting of the compression process, the thermal energy generation process, as well as cooling off process typical of the shock wave plasma model proposed for the low-pressure case. Fig. 6 shows the propagation lengths of the
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Fig. 5. Time-profiles of the spatial integrated emission intensity of Ca I 422.6 nm, at different pressures of surrounding air.
emission front of Si I 288.1 nm as a function of time at different air pressures of 100 torr, 400 torr, and 760 torr. These data are taken by focusing the laser beam onto the surface of a lead glass. It is seen that at all air pressures, the plots share nearly the same slope, which is a typical feature for a shock wave. Similar results are also obtained for the other constituent elements in the sample such as, Na, Ca and Pb. This observation clearly lends further support to our shock wave model, according to which atoms are excited in a limited region just behind the shock front in spite of the differences in their atomic weight w28x.
4. Conclusion We have shown in this experiment that laser plasma generated, by TEA CO 2 laser irradiation on glass samples in high-pressure surrounding air, generally exhibits features of secondary plasma well known in low-pressure surrounding air. Analysis of the dynamical characteristics further reveals the occurrence of two-stage process in the
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Fig. 6. Relationship between the propagation length of the emission front of Si I 288.1 nm as a function of time, at different air pressures.
plasma formation corresponding respectively to the shock excitation and the cooling stages described in our shock wave plasma model. While this model was originally proposed to explain the formation and excitation processes of laser plasma generated in low-pressure surrounding air, this experiment has clearly demonstrated its validity for the elucidation of plasma emission induced at surrounding air pressure all the way up to 760 torr.
Acknowledgements A Research Team Grant and a Sandwich Research Grant under contract supported part of this work no: 001rADD.IrIIIrURGEr1999, University Research for Graduate Education Project, Ministry of Education and Culture, Republic of Indonesia. References w1x D.A. Cremers, L.J. Radziemski, R.W. Solarz, J.A. Paisner, Laser Spectroscopy and its Application, Marcel Dekker, New York, 1987. w2x K. Laqua, N. Omenetto, Analytical Laser Spectroscopy, Wiley, New York, 1979. w3x E.H. Piepmeier, Analytical Application of Laser, Wiley, New York, 1986. w4x L.J. Radziemski, T.R. Loree, Plasma Chem. Plasma Proc. 1 Ž1981. 281.
w5x L.J. Radziemski, T.R. Loree, D.A. Cremers, N.M. Hoffman, Anal. Chem. 55 Ž1983. 1246. w6x D.A. Cremers, L.J. Radziemski, Anal. Chem. 55 Ž1983. 1252. w7x I. Ahmad, B.J. Goddard, J. Friz. Mal. 14 Ž43. Ž1993. 43᎐54. w8x L.J. Radziemski, Microchem. J. 50 Ž3. Ž1994. 218᎐234. w9x M. Sabsabi, P. Cielo, Appl. Spectrosc. 49 Ž1995. 499. w10x K. Song, Y.I. Lee, J. Sneddon, Appl. Spectrosc. Rev. 32 Ž3. Ž1997. 183᎐235. w11x L.J. Radziemski, D.A. Cremers, K. Niemezyk, Spectrochim. Acta Part B 40 Ž1985. 517. w12x K. Kagawa, S. Yokoi, Spectrochim. Acta Part B 37 Ž1982. 789. w13x K. Kagawa, M. Ohtani, S. Yokoi, S. Nakajima, Spectrochim. Acta Part B 39 Ž1984. 525. w14x H. Kurniawan, T. Kobayashi, K. Kagawa, Appl. Spectrosc. 46 Ž1992. 581. w15x K. Kagawa, K. Kawai, M. Tani, T. Kobayashi, Appl. Spectrosc. 48 Ž1994. 198. w16x H. Kurniawan, M.O. Tjia, M. Barmawi, S. Yokoi, Y. Kimura, K. Kagawa, J. Phys. D: Appl. Phys. 28 Ž1995. 879. w17x H. Kurniawan, W.S. Budi, M.M. Suliyanti, A.M. Marpaung, K. Kagawa, J. Phys. D: Appl. Phys. 30 Ž1997. 3335. w18x W.S. Budi, H. Suyanto, H. Kurniawan, M.O. Tjia, K. Kagawa, Appl. Spectrosc. 53 Ž6. Ž1999. 55. w19x W. Harjoutomo, H. Munechika, H. Kurniawan, I. Hattori, T. Kobayshi, K. Kagawa, Opt. Laser Tech. 24 Ž1992. 273. w20x H. Kurniawan, K. Kagawa, M. Okamoto, M. Ueda, T. Kobayashi, S. Nakajima, Appl. Spectrosc. 50 Ž1996. 299. w21x H. Kurniawan, M. Pardede, K. Kagawa, M.O. Tjia, J. Spectrosc. Soc. Jpn. 47 Ž5. Ž1998. 220.
A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599 w22x N.G. Basov, O.N. Krokhin, G.V. Skizkov, JETP Lett. 6 Ž1967. 168. w23x N.G. Basov, V.A. Gribkov, O.N. Krokhin, G.V. Skizkov, Sov. Phys.-JETP 27 Ž1968. 575. w24x J.L. Bobin, Y.A. Durand, P. Langer, G. Tonon, J. Appl. Phys. 39 Ž1968. 4184.
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w25x R.B. Hall, J. Appl. Phys. 40 Ž1969. 1941. w26x D.C. Emmony, J. Irving, Br. J. Appl. Phys. 2 Ž1969. 1168. w27x K. Hohla, K. Buchi, R. Wienecke, S. Wrkowiski, Z. Naturf. A. 24 Ž1969. 1244. w28x H. Kurniawan, T. Kobayshi, S. Nakajima, K. Kagawa, Jpn. J. Appl. Phys. 31 Ž1992. 1213.
Shock wave plasma induced by TEA CO 2 laser bombardment on glass samples at high pressures A.M. Marpaung a , R. Hedwig a , M. Pardede a , T.J. Lie a , M.O. Tjiab, K. Kagawac , H. Kurniawan a,U a
Applied Spectroscopy Laboratory, Graduate Program in Optoelectrotechniques and Laser Application, Faculty of Engineering, The Uni¨ ersity of Indonesia, 4, Salemba Raya, Jakarta 10430, Indonesia b Department of Physics, Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, 10 Ganesha, Bandung, Indonesia c Department of Physics, Faculty of Education and Regional Studies, Fukui Uni¨ ersity, 9-1 Bunkyo 3-Chome, Fukui 910, Japan Received 23 January 2000; accepted 13 July 2000
Abstract An experimental study has been carried out on the dynamical process taking place in laser plasma, generated by TEA CO 2 laser Ž400 mJ, 100 ns. irradiation on glass samples surrounding by air of high pressures up to 760 torr. Accurate dynamical characterization was performed by simultaneous observation of the plasma emission front and the shock wave front. The shock wave front was detected by a modified shadowgraph technique while the emission front was detected by observing the rising time at various slit positions. In spite of the occurrence of a new feature uncommon to laser plasma, generated in low air pressures, it is found that the two fronts coincide and move together at the initial stage of the laser plasma, but eventually separate from each other, with the emission front being left behind the shock wave front at a later stage. These characteristics hold for the atomic emission lines of all elements contained in the glass samples examined, regardless of their different atomic weights. It is therefore strongly indicative of the shock wave mechanism in the laser plasma generation and the emission in the high-pressure surrounding air. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: TEA CO 2 laser; Density jump; Emission front; Laser induced shock wave plasma spectroscopy
U
Corresponding author. 51G᎐51H, Tomang Raya, Jakarta Barat 11440, Indonesia. Tel.: q62-21-330188; fax: q62-213918115r5809144. E-mail addresses: [email protected] ŽM.O. Tjia ., [email protected] ŽK. Kagawa ., kurnia18@ cbn.net.id ŽH. Kurniawan.. 0584-8547r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 5 8 4 - 8 5 4 7 Ž 0 0 . 0 0 2 6 4 - 0
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A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599
1. Introduction The widely adopted technique of laser-ablationemission spectrometric analysis ŽLAESA. is one of the important results accomplished in laser applications w1᎐3x. In its early development for spectrochemical analysis, this technique was confronted with problems of high-intensity continuous background and contamination by undesirable self-absorption processes due to the use of high pressure surrounding air. These problems were expected in turn to cause serious impairment of the detection sensitivity and linearity. In its more recent development led by Cremers and his colleagues w4᎐11x, those technical hurdles have been overcome to a good extent by the use of a better laser system, such as a modern high power Nd-YAG laser and the incorporation of gatedoptical-multichannel analyzer ŽOMA. into the detection system. This line of development has resulted in a much-improved spectroscopic technique widely known as laser-induced breakdown spectroscopy ŽLIBS. in connection with the predicted role of a gaseous breakdown mechanism in the emission process. An alternative route of development has been proposed by Kagawa et al. w12᎐21x, who suggested the use of low-pressure surrounding gas instead of surrounding gas at atmospheric pressure adopted in the other line of development. It has been shown that a plasma having characteristics favorable to spectrochemical analysis can actually be generated by using a pulsed laser with a short duration, such as a nitrogen laser w12,13x, a carbon-dioxide laser w14x, and an excimer laser w15x, when the pressure of the surrounding gas is reduced to approximately 1 torr. In these cases, the generated plasmas invariably consist of two distinct parts. The first part, which is called the primary plasma, occupies a small space just above the sample surface, which gives off intense continuous emission spectra for a short time. The other part, called the secondary plasma, expands with time around the primary plasma in a nearhemispherical shape, emitting sharp atomic spectral lines with negligibly low background. In the time-resolved experiments conducted previously using a carbon-dioxide laser w14x, and
an excimer laser w15x, our group demonstrated that this secondary plasma was produced by a shock wave in the surrounding gas, while the primary plasma acted as an initial explosion energy source. A model has also been proposed for the explanation of the excitation mechanism in the secondary plasma. According to this model, right after the cessation of the primary plasma, atoms gushed out from the primary plasma at supersonic speeds, pushing the surrounding gas like a piston. These fast moving atoms, being impeded by the surrounding gas, gave rise to a compression process. As a result of this compression, a shock wave was generated in the surrounding gas, leading to the formation of secondary plasma. The most important point in the shock wave model was that the energy required to produced the secondary plasma was supplied in the form of kinetic energies from the propelling atoms. Due to the compression process, the kinetic energies of the propelling atoms were converted into the thermal energy in the plasma, by which the atoms were excited. We have further presented in our previous work w18x, a result supporting the idea that the emission front of the plasma and the front of the shock wave moved together with time at the initial stage of the secondary plasma expansion, during which the excitation process took place most effectively. At a later stage, the emission front separated from the shock wave front and left it behind, marking the cooling stage of the plasma. In spite of the detailed understanding described above, about laser plasma generated at low surrounding gas pressures, the fundamental mechanism of laser plasma generated at the atmospheric pressure has yet to be unraveled despite its remarkable popularity as a tool for spectrochemical analysis. In order to look for clues on this mechanism, we have carried out a detailed experiment employing a surrounding gas at a relatively high pressure, ranging from 100 to 760 torr. Time resolved measurements were performed on the laser plasma produced at these air pressures in the hope of revealing new dynamical features which were hitherto unheeded. It will be shown that the analysis of these features led to a
A.M. Marpaung et al. r Spectrochimica Acta Part B: Atomic Spectroscopy 55 (2000) 1591᎐1599
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Fig. 1. Diagram of the experimental set-up used in this study.
strong indication of the occurrence of secondary plasma even in the high-pressure surrounding air.
2. Experimental procedure The diagram of the experimental arrangement is shown in Fig. 1. In this experiment, a TEA CO 2 laser ŽShibuya Kogyo, SQ-2000, 3 J of full power, 100 ns of pulse width as can be seen in Fig. 2 with wavelengths of 10.6 m., as the irradiation source, was operated shot by shot with the output energy fixed at 400 mJ by an aperture. The laser beam was focused by a Ge lens Ž f s 100 mm. through a ZnSe window onto the surface of the sample. A museum glass containing silicon Ž; 12%., calcium Ž; 2.5%. and sodium Ž; 4%., and a lead glass containing lead Ž; 17%., silicon Ž; 6%., calcium Ž; 2%. and sodium Ž; 0.5%. were used as samples.
The sample was placed in a vacuum tight metal chamber Ž12.5 cm = 10 cm = 10 cm., which could be evacuated to 10 ᎐ 2 torr and filled with a desired gas at a certain pressure. Precisely a digital manometer ŽNishiyama Seisakusho, DM760. measured the chamber pressure. The airflow through the chamber was regulated by needle valves in the air line and in the pumping line. The sample, together with the whole chamber and the
Fig. 2. A TEA CO 2 laser pulse waveform.
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Ge lens could be moved by a step motor, for the movement in the laser beam direction and a micrometer for the perpendicular movement to the laser beam direction. The sample could be rotated with another step motor to avoid repeated irradiation on the same surface spot. Emission of the laser-induced plasma was observed through an optical window at right angle to the laser beam, by an imaging quartz lens Ž f s 100 mm. with an aperture of 10 mm= 10 mm. The plasma was imaged, with an enlargement ratio of 1:3, onto the entrance slit of a monochromator ŽNikon P-250, plane grating 1200 groovesrmm blazed at 500 nm, focal length for concave mirror s 250 mm.. The entrance slit was set at 2 mm in height and 100 m in width so that the observation was restricted to a limited area. The output signal from a photomultiplier ŽHamamatsu R-585., attached with a 500-⍀ resistance for a RC time constant of 30 ns, was fed into a digital-sampling-storage scope ŽHP 545616B.. For the detection of the shock wave front associated with a density jump, a modified shadowgraph technique was employed using a He᎐Ne laser Ž632.8 nm and 1 mm beam diameter., as a separate probe light. The probe light was sent to the expansion region of the plasma. A sudden deviation in the intensity of the probe light due to the density jump was detected by a photomultiplier ŽHamamatsu R-1104. after the light has passed through an interference filter. The output signal of the photomultiplier was then sent to another channel of the digital-sampling-storage scope. A photon-drag detector to provide a trigger signal for digital-sampling-storage scope detected a part of the TEA CO 2 laser beam. In order to reduce the undesired continuous emissions taking place near the primary plasma, the observation region is shifted 1 mm upward along the z-axis from the center of the laser plasma, as shown in the insert of Fig. 1.
3. Results and discussion As we have reported in our previous works w12᎐18x, the secondary plasma and the primary
plasma could be clearly observed by the naked eye in surrounding air below 10 torr. The secondary plasma exhibited a characteristic hemispherical shape with brilliant colors associated with emissions from the constituent atoms, while the primary plasma displayed an intense white color associated with the continuum emission. The hemispherical shape of the secondary plasma was indicative of the dominant role of the shock wave in its generation, connected with the typical hemispherical shape of the shock wave. In the present experiment which is conducted in surrounding air at pressure above 50 torr, the secondary plasma appears to be dominated by sodium emission at its later stage. It is also observed that the strong sodium emission region exhibits a cone-like structure instead of the hemispherical shape common to shock wave-induced plasma, with the sodium emission process lasting longer than those from other atoms such as calcium, silicone, etc. At its earlier stage however, the use of a blue filter to cut off the sodium emission does reveal the hemispherical secondary plasma shape. This can be explained as a consequence of the use of a broad laser pulse Ž100 ns., giving rise to a longer vaporization process. In such a relatively prolonged process, only the cluster of atoms initially ablated from the target surface are involved in the compression process responsible for the formation of a shock wave as well as the kinetic-to-thermal energy conversion process. Therefore, only these atoms will benefit from the large supply of thermal energy from the high temperature plasma for their excitations. The atoms vaporized by the later part of the laser pulse are not supposed to generate an effective compression process in the relatively rarefied region left behind the initial shock wave. As a result, the energy available for the atomic excitation is accordingly reduced. This energy reduction is expected, however, to have a highly preferential effect in favor of the sodium atoms, simply because the Na I 589.0 nm and Na I 589.6 nm emission lines are associated with relatively low excitation energy of 2.1 eV. We note further that the primary plasma can still be readily distinguished from the secondary plasma by the naked eye even in the surrounding
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gas with a pressure of up to 300 torr. As the pressure increases beyond 300 torr, the highly dense white color region of the primary plasma expands accordingly, blurring the boundary between the two plasma regions and making it practically indistinguishable to the naked eye. Nevertheless, the presence of secondary plasma can still be clearly detected by time-resolved spectroscopic measurements, even with surrounding gas at 760 torr. Whereas only a high-intensity continuum emission spectrum is detected in a region restricted around the primary plasma, distinct atomic lines become clearly observable as the detection region is shifted 2 mm off the primary plasma and after a considerable delay Žof the order of micro-seconds. with respect to the irradiation pulse. It is amply confirmed therefore that the laser-induced plasma still consists of a primary as well as secondary parts even at a pressure as high as 760 torr. Another feature supporting the occurrence of a shock wave is the density jump associated with the presence of a thin shell structure at the shock wave front. The observation of this density jump has been reported by Basov et al. w22,23x from their experiments using a shadowgraph technique and a Schlieren photograph technique, consecutively. The detection of a density jump was also reported at about the same time and independently by Bobin et al. w24x Hall w25x and Emmony et al. w26x employing an image converter camera in their recording of the expanding plasma and the detection of emission fronts. These evidences were further confirmed by the observation made by Hohla et al. w27x using a shadow photograph technique. In contrast to those techniques employed in previous experiments, a modified shadowgraph technique proposed earlier w17x, is used in this experiment to simplify the simultaneous detection of the density jump and the arrival of the emission front. Fig. 3 shows the result of the simultaneous detection of the density jump and the plasma emission of a calcium neutral line ŽCa I 422.6 nm. at various probing distances from the sample surface. The plasma is generated by a TEA CO 2 laser, which is focused onto the museum glass sample in air at 100 torr. The coincidence between
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Fig. 3. The relationship between the arriving of the density jump Žshown by an arrow. and rising of emission front of the calcium neutral line ŽCa I 422.6 nm. at various positions from the sample surface when the TEA CO 2 laser beam was focused onto an ordinary glass sample in air at 100 torr.
the density jump and rising point of the emission front is clearly seen for the probe light position is as far away as 4 mm from the sample surface, beyond which the two fronts begin to separate. It must be stressed that the emission front can be clearly detected by the detector as far as 7 mm from the sample surface, while the density jump can be detected as far as 20 mm from the sample surface. The same detection limits also hold for the sodium neutral line ŽNa I 589.0 nm. from the same samples. It should be further noted that the density jump signal in Fig. 3 is generally followed by additional late-coming ‘signals’ with an irregular pattern. These ‘signals’ appear to weaken as the probing position moves away from the target,
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and finally disappear at a probing position, 10 mm from the sample surface. These irregular ‘signals’ are apparently unrelated to the shock wave because they have no corresponding signal in the emission channel. It is conceivable that these signals are the signs of disturbance caused by atoms vaporized from the target behind the shock wave. For a TEA CO 2 laser with energy as high as 400 mJ and a pulse width as broad as 100 ns, the atoms ablated during the later part of the laser pulse may still accumulate to significant enough amounts to yield such a disturbance. We have found indeed that the windows of the chamber always turn cloudy after several hundred shots of laser irradiations. In order to substantiate the elucidation of the excitation mechanism in the secondary plasma at high pressures, one also needs to know how the shock wave front and the emission front move with time. These dynamical characteristics are
depicted in Fig. 4, where the variations of those front positions are plotted with respect to time at different air pressures Ž100 torr, 400 torr and 760 torr.. These plots are actually obtained by reading the rising points of the corresponding timeprofiles of the Ca I 422.6 nm emission line and the starting points of the density jumps of the curves given in Fig. 3. It is seen that the plot generally consists of two linear segments with different slopes. As we have mentioned above, it is remarkable that the coincidence between the emission front and the shock wave front is clearly sustained, as far as 4 mm from the sample surface Ž4.1 mm from the plasma center. at 100 torr, and up to 3 mm from the sample surface Ž3.2 mm from the plasma center. at 400 torr, before the curves corresponding to the two fronts branch off. These observations constitute a rather convincing evidence for our shock wave plasma model as described earlier. According to this model, the
Fig. 4. Relationship between the propagation length of the emission front of Ca I 422.6 nm and arrival of the density jump, as a function of time, at surrounding air pressure of 100 torr, 400 torr and 760 torr.
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shock wave front and the emission front are expected to coincide and move along together at the initial stage of formation of the secondary plasma. This stage is soon followed by the cooling stage, starting at the moment when the two fronts begin to separate. At this stage, the emission front is expected to move at a lower speed than the shock wave front, as evidenced by those two plots in Fig. 4. It must be pointed out nevertheless that no such a coincidence is indicated in Fig. 4 for the case of 760 torr air pressure. It is suspected, however, that gas breakdown also takes place to some extent at this air pressure due to absorption of part of the laser energy by the air. It is then natural to expect that the emission from the plasma is disturbed by the light coming from the gas breakdown, leading to distorted emission data. It is therefore important to examine the dynamical characteristics of plasma formation from a different angle. To this end, a direct measurement is performed for the time-profiles of the spatially integrated emission intensity of Ca I 422.6 nm at various pressures Ž50 torr, 100 torr, 400 torr,and 760 torr. and the result is presented in Fig. 5. In this measurement, the imaging lens in Fig. 1 is removed, while the slit height and width of the monochromator are set at 15 mm and 100 m, respectively. It is seen in Fig. 5 that each curve undergoes a steep initial climb with time up to the maximum points and declines gradually thereafter. The growth segment of the emission curve corresponds to the excitation stage in our shock wave plasma model and the decay segment corresponds to the cooling stage. It is remarkable that the transition point from the growth region to the decay region Žmaximum point. in these curves practically coincide with the corresponding bending points of the two linear segments found in Fig. 4, including the case occurring at 760 torr air pressure. The same feature was already noted in the low-pressure case w18x. This result is at least consistent with the scenario consisting of the compression process, the thermal energy generation process, as well as cooling off process typical of the shock wave plasma model proposed for the low-pressure case. Fig. 6 shows the propagation lengths of the
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Fig. 5. Time-profiles of the spatial integrated emission intensity of Ca I 422.6 nm, at different pressures of surrounding air.
emission front of Si I 288.1 nm as a function of time at different air pressures of 100 torr, 400 torr, and 760 torr. These data are taken by focusing the laser beam onto the surface of a lead glass. It is seen that at all air pressures, the plots share nearly the same slope, which is a typical feature for a shock wave. Similar results are also obtained for the other constituent elements in the sample such as, Na, Ca and Pb. This observation clearly lends further support to our shock wave model, according to which atoms are excited in a limited region just behind the shock front in spite of the differences in their atomic weight w28x.
4. Conclusion We have shown in this experiment that laser plasma generated, by TEA CO 2 laser irradiation on glass samples in high-pressure surrounding air, generally exhibits features of secondary plasma well known in low-pressure surrounding air. Analysis of the dynamical characteristics further reveals the occurrence of two-stage process in the
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Fig. 6. Relationship between the propagation length of the emission front of Si I 288.1 nm as a function of time, at different air pressures.
plasma formation corresponding respectively to the shock excitation and the cooling stages described in our shock wave plasma model. While this model was originally proposed to explain the formation and excitation processes of laser plasma generated in low-pressure surrounding air, this experiment has clearly demonstrated its validity for the elucidation of plasma emission induced at surrounding air pressure all the way up to 760 torr.
Acknowledgements A Research Team Grant and a Sandwich Research Grant under contract supported part of this work no: 001rADD.IrIIIrURGEr1999, University Research for Graduate Education Project, Ministry of Education and Culture, Republic of Indonesia. References w1x D.A. Cremers, L.J. Radziemski, R.W. Solarz, J.A. Paisner, Laser Spectroscopy and its Application, Marcel Dekker, New York, 1987. w2x K. Laqua, N. Omenetto, Analytical Laser Spectroscopy, Wiley, New York, 1979. w3x E.H. Piepmeier, Analytical Application of Laser, Wiley, New York, 1986. w4x L.J. Radziemski, T.R. Loree, Plasma Chem. Plasma Proc. 1 Ž1981. 281.
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