Rejection of Recombination and Electron Collision Process in the Laser Plasma Generated by the Nd-YAG Laser Irradiation at Low Pressures

Transcription

1 Rejection of Recombination and Electron Collision Process in the Laser Plasma Generated by the Nd-YAG Laser Irradiation at Low Pressures Marincan Pardede 1 and Hendrik Kurniawan 1* 1 Applied Spectroscopy Laboratory, Graduate Program in Opto-Electrotechniques and Laser Application, Faculty of Engineering, The University of Indonesia, 4 Salemba Raya, Jakarta 10430, Indonesia. kurnia18@cbn.net.id * author to whom correspondence should be sent Abstract A simultaneous detection of the charge current and emission intensity during the laser plasma generation and expansion processes has been performed by means of a new and unique technique in which a partially transmitting metal mesh electrode was placed at an appropriate distance in front of the sample surface with the target serving as the counter electrode. The time evolutions of three Cu emission lines, namely the Cu I nm, Cu I nm and Cu I nm were measured along with that of the charge current at various applied voltages. Correlational analysis of the experimental data reveals that electron collision contributes to the excitation of Cu I nm and Cu I nm emission lines at the early stage, while recombination process is more responsible for the Cu I nm emission line. In the subsequent stage, however, the laser induced shock wave plasma model was clearly shown to be the dominant mechanism either directly for the thermal excitation, or indirectly by producing the electrons for the recombination process. Keywords: laser plasma, charge current, mesh electrode, shock wave plasma, reduced air pressure D I. INTRODUCTION espite the large number of studies that have been carried out on the practical applications of laserinduced atomic emission spectra, the physical mechanism underlying the excitation process of the ablated atoms remains less than completely understood. One of the main reasons for this impasse is the inherent nature of the ablation process which is highly transient in nature and occurring in a very restricted region near the target surface. Consequently, it is difficult to follow the plasma dynamics by using ordinary spectroscopic techniques. The employment of a high-speed streak camera would enable the detection of such dynamic process, but the instrumentation is costly, and the precision of data collected by this method is also limited because the data are usually obtained from a single shot of laser irradiation and hence do not allow for data quality improvement. In our previous works 1-2, experimental studies have been carried out on the dynamical process taking place in the secondary plasma generated by a Q- switched Nd:YAG laser (80 mj, 80 ns) on a copper target at reduced surrounding gas pressure. Accurate dynamical characterization of the cross sectional view of the plasma has been made possible by the unique combination of plasma confinement configuration and the time-resolved measurement technique along with the simultaneous detection of the density jump associated with the arrival of the shock wave. In addition to reaffirming the role of the shock wave mechanism in the laser plasma generation, the analysis of the time-resolved spatial distribution of plasma temperature further revealed the occurrence of twostage emission process, namely the shock excitation stage and the cooling stage. This two-stage mechanism has since been successfully applied to describe emission characteristics in shock wave laser plasmas generated under different experimental conditions. [?-?] Aside from the shock wave model developed and verified in those studies under certain conditions, other models have also been proposed for explaining the emission process of laser plasma under different conditions, such as the plasma diffusion process, the gas reservoir excitation process 5-6, the recombination process and the electron collision process 7. Among these only the last two are relevant to the experimental OL-10 ISBN :

2 condition to be considered in this study. Specifically, this experiment was undertaken to examine the roles of recombination process and electron collision process in the laser plasma emission generated at low surrounding gas pressure. For this purpose, a unique new technique has been developed for the observation of charged particles in the plasma. In this technique a partially transmitting metal mesh electrode was employed and located at a certain distance in front of the sample surface. An electric field between the mesh and the sample surface was then set up by applying DC high voltage between them. Incorporating this technique in our experiment, we were able to perform the simultaneous detection of charge current and emission profiles from the plasma. As a result, this experiment has provided new evidences which helps to clarify the contributions of those two physical processes as well as the shock wave mechanism to the laser plasma emission. II. EXPERIMENT SETUP The basic configuration of the experimental setup is shown in Fig. 1. In this experiment, the laser radiation from a 1,064 nm Nd:YAG (Quanta Ray, GCR, 400 mj, pulse duration 8 ns) was operated in a Q-switched mode with a repetition rate of 5 Hz and output energy of 64 mj. The laser beam was focused by a multilayer lens (f = 100 mm) through a quartz window onto the surface of the sample. A copper plate (Rare metallic Co., 99.9%, 0.2 mm thickness) sample were placed in vacuum-tight metal chamber which could be evacuated with a vacuum pump and filled with air or helium at desired pressure. The chamber pressure was measured and monitored by a digital pirani gauge (Diavac, PT-1DA). The target was rotated at 2 rpm during the radiation to secure uniformity of the emission intensity. For the detection of the charge current, a nickel mesh electrode with 60% transmission of the laser energy was placed in front of the sample and oriented parallel to the sample surface. As a consequence the laser energy reaching the target surface was reduced to around 40 mj, yielding a power density of around 15 GW/cm 2. For the spectrum measurement, the total plasma radiation (spatially integrated) was imaged onto the entrance slit of the monochromator (SPEX M-750, Czerny-Turner configuration, f = 750 mm with 1,200 grooves/mm blazed at 500 nm). The slit was set at 15 mm in height and 200 µm in width in order to collect the entire plasma radiation via the back mirror of the monochromator. The electric signal output of the photomultiplier (Hamamatsu IP-28) was fed through a 500 Ω resistor (RC time constant was about 30 ns) to channel 1 of a digital sampling storage scope (HP-54600B). A part of the laser beam was detected by a PIN photodiode, and the output was used to trigger the digital sampling storage scope. Data recording was carried out by using a printer. In order to measure the charge current in the laser generated plasma, the mesh was connected to a DC high voltage which was adjustable in the range of Volt while the sample holder was grounded to a resistor which also served as a voltage divider. The charge current measured was then fed into channel 2 of the digital sampling storage scope as shown in Fig.1. motor monochromator PMT mesh sample 50 Ω 500 Ω lens (f=100 mm) E digital storage scope window channel 2 photo diode laser light Fig. 1 Diagram of the experimental setup. III. EXPERIMENTAL RESULTS In order to examine the contributions of electron collisions and recombination processes to the atomic excitation mechanism in the secondary plasma, measurement of the time profile of emission intensity and that of charge current were performed simultaneously in this experiment. Fig. 2 shows the emission profiles of spatially integrated emission intensities of Cu spectral lines together with that of the detected charge current. Three Cu I emission lines, Cu I nm, Cu I nm, and Cu I nm corresponding to excitation energies of 6.2 ev, 3.8 ev, and 7.8 ev respectively were recorded while the laser irradiation of the target was carried out through the nickel mesh placed at 7 mm from the target surface in a surrounding air of 1.7 Torr. The emission profiles of Cu I nm and Cu I nm share basically the same basic pattern as those observed in our previous experiment 1, showing initial increase with time and reaching the maximum at around 1 µs, which is followed by a more gradual decline thereafter. channel 1 trigger OL-11

3 However, the previous result differs in detail from what is shown in Fig. 2 where all the emission profiles are marked by small initial spikes right after the laser irradiation. While exhibiting the same feature of initial spike, the emission profile of Cu I nm spectral line does not share the same trend displayed by the other Cu emission lines. To be specific, the emission of Cu I nm does not display a second peak after the spike, it decays monotonically instead, with a time constant of around a few micro second. Fig.2 Emission profile of spatially integrated total emission intensity of Cu I nm, Cu I nm and Cu I nm together with that of charge current. The laser irradiation of 64 mj was made through the mesh placed at 7 mm from the Cu target under the reduced air pressure of 1.7 Torr. the Cu target under the reduced He pressure of 40 Torr. The charge current detected in conjunction with Cu emission as displayed in the same figure clearly shows the dynamical changes in the amount of electron and ion in the laser plasma during the emission process. The current rises abruptly to its peak right after the laser irradiation, virtually coincident with the occurrence of the emission spike. This sharp rise of the current immediately drops to about half of its maximum value before continuing to decrease at a much slower rate in the rest of the profile referred as the slow-decay component in this work. We also observed that the peak value of the charge current decreased with increasing pressure of the surrounding gas. In connection with that, the decay time of the slow component in the charge current profile becomes correspondingly shorter. On the other hand, the intensity of the three Cu I emission lines increased with increasing pressure of surrounding gas. The emission profile of the Helium surrounding gas was also measured in this experiment, since it is useful for clarifying the excitation mechanism in the secondary plasma. For this purpose, the Helium pressure was increased to 40 Torr because the emission at 7 Torr was too weak to measure. As shown in Fig. 3, the emission profile of He I nm is similar to that of Cu I nm in its growth and the decay pattern. An initial spike also appears in the emission profile even without application of external voltage. The spike is seen to grow when the voltage is applied, whereas the corresponding emission profile retains more or less the same general shape. Fig.4 The log-log plot of the time development of 100 Laser energy : 64 mj Surrounding gas : air at 1.7 Torr position (mm) 10 Cu I nm Cu I nm Cu I nm Fig. 3 Emission profile of spatially integrated total emission intensity of He I nm together with that of charge current. The laser irradiation of 64 mj was made through the mesh placed at 7 mm from time (ns) the front position of Cu I nm, Cu I nm and Cu I nm emissions lines. The laser irradiation of 64 mj was made through the mesh placed at 7 mm from the Cu target under the reduced air pressure of 1.7 Torr. OL-12

4 In our previous paper 1, we have shown how the emission front moved with time, which provided a crucial evidence for the shock wave mechanism. As shown in Fig. 2, the emission profile of Cu I nm is different from those of Cu I nm and Cu I nm. Therefore it is important to examine the movement of the related emission fronts and the result was presented in Fig. 4 in the log-log plot depicting the time evolution of the emission front positions. These data were collected by reading the rising times of each of the emission lines by varying the position of the slit of the monochromator. It is seen that emission fronts of Cu I nm and Cu I nm were found at nearly the same position all along with the emission of Cu I nm slightly lagging behind. The lack of data in the range between 7 and 10 mm is due to disturbance caused by the presence of the mesh. The slope of this curve at the initial stage is about 1.7, and it drops to about 0.3 at the later stage. Similar result was obtained for the emission front movement when no voltage was supplied between the electrodes. IV. DISCUSSION As we described in the introduction, different models have been proposed in the literature to explain the excitation mechanism of the luminous plasma induced by laser irradiation. For plasma produced at low pressure, the gas reservoir model is irrelevant simply because the shielding effect due to the gas is negligibly. The diffusion model is also rendered ineffective in such a highly transient process due to its relatively slow response at low gas pressure. We are thus left in this case with the shock wave plasma model, electron collision and the recombination models for the study of plasma emission processes. In fact, the last two models are still being widely held among researchers in the field, without experimental clarification of their detailed roles in the excitation process. In view of the assumed central roles of electrons in these models, information on the correlation between the time profile of the atomic emission and that of the charge current should provide important clues on the validity of the assumption. The mechanism responsible for the occurrence of charge current is similar with the working principle of an ionization chamber used to detect ionizing radiation. Electrons and ions are injected from the target following the laser irradiation, or when they are produced during the rapid plasma expansion process as described in shock wave laser plasma model. It must be added, however that not only the number of electrons and ions ejected from the target that counts, but the speed of the charge particles will also effect the magnitude of the induced current. Although it is difficult to measure precisely the magnitude of the induced current, but even the time profile of the charge current will provide useful information as to when the electrons and ions are produced and when they disappeared through their recombination in the plasma. We shall now compare the time profile of the atomic emission and that of the charge current presented in Fig. 2. If the electron recombination process or electron collision process were the major mechanisms for secondary plasma emission, a clear correlation would be observed between those two time profiles. Such a correlation is indeed observable at the initial part of the time profile corresponding to the state right after the laser irradiation. This is marked by the near coincident appearance of the emission spike and the initial surge of charge current, which actually slightly preceded the former. This observation suggests the role of electron collision in all the Cu emission lines. This interpretation is consistent with the decline of charge current following its peak value due to speed reduction of the charge particle as a consequence of the collisional process. Beyond this early stage of the laser plasma evolution (t < 5 µs), we are entering the period of full development of the secondary plasma, and we clearly see in Fig. 2 the rise of Cu I nm and Cu I nm emission intensities and their slower decay after reaching the maxima. This pattern of time evolution resembles closely what have been reported and described previously in terms of the shock wave plasma model 7. This part of the emission time profiles obviously lacks the correlation with the practically monotonous time profile of the charge current as the source of atomic excitation, either by way of collision or recombination process. We shall return to this point later. Turning our attention to the Cu I nm emission line, we notice a clear correlation between the time profile of its intensity and that of the charge current as shown in Fig. 2. It must be recalled at this point, that the emission of the Cu I nm spectral line is associated with an excited state of much higher energy, close to the ionization level of Cu 8. This emission line is therefore likely to have its origin in the recombination process taking place in Cu ion by trapping the free electrons in the plasma. Indeed, even at the initial stage of the charge current time profile OL-13

5 some of the electrons and Cu ions directly ejected from the target are likely to recombine with each other near the target surface in addition to being involved in the collision process. This combined process readily explains the sharp drop of charge current after its initial surge to the peak value. The gradual reduction of emission intensity at a later stage is therefore understandably caused by similarly decreasing supply of electron and ion in the plasma due to their limited production during the following plasma expansion as predicted by the shock wave model. Returning to the role of the shock wave excitation in the emission of Cu lines alluded above, we refer to Fig. 4 for further evidence regarding the propagation of the shock fronts as denoted by the associated emission fronts. According to the shock wave model, the emission front of Cu I nm and that of Cu I nm spectral lines should coincide despite their difference in excitation energy. The result shown in Fig.4 unequivocally demonstrates just that characteristics which supports our shock wave model. Additionally, we have shown in our previous paper 9, that the production of ions and electrons during the plasma expansion generally lags behind the excitation of neutral atoms. This observation explains very nicely the slight delay of Cu I nm emission with respect to other Cu neutral emission line owing to the different excitation mechanism mentioned above. A further evident of the role of shock wave is provided by data presented in Fig. 3 for plasma generated in helium surrounding gas at 3 Torr. It is noted that charge current detected in the later stage is obviously lower during the plasma expansion in this case compared to that in air even though the densities of the gases are not so much different. This difference is readily understood in terms of the shock wave plasma model according to which ionization of the surrounding gas is induced by the shock wave model during the high speed plasma expansion process. The higher charge current in air is simply due to the considerably lower ionization energy of air compared to that of helium. It is also worth nothing that the time profile of Helium gas emission described in Fig. 7 also supports our shock wave model as it exhibits a growth characteristics similar to that of Cu I nm emission. This feature on the contrary, can not be understood by electron collision model as the observed charge current is virtually featureless, lacking the correlation needed to explain the characteristic behavior of the emission time profile. V. CONCLUSION The role of shock wave mechanism in the generation of the laser-induced plasma was experimentally established using a unique technique of charge current detection during the plasma generation, which is accomplished by placing a metal mesh between the laser and the sample surface. It is shown that the electron collision and recombination process play the dominant role for plasma emission from the neutral atoms and ions respectively within the brief initial stage of the plasma generation. Throughout the rest and most of the emission process however, the physical mechanism involved is either the thermal excitation induced directly by the shock wave or the relatively less significant electron-ion recombination process following the ionization process due to shock wave plasma expansion. For a more comprehensive understanding of these mechanisms, a further study involving different metal samples is in progress and will be reported elsewhere in the near future. REFERENCE 1. W.S. Budi, H. Suyanto, H. Kurniawan, M.O. Tjia and K. Kagawa, Appl. Spectrosc. 53, 6, (1999). 2. H. Kurniawan, T.J. Lie, M.O. Tjia and K. Kagawa, Appl. Spectrosc., 54, 12 (2000). 3. W.T. Silfvast, O.R. Wood, J. Appl. Phys. Lett., 25, 274 (1974). 4. J.M. Green, W.T. Silfvast and O.R. Wood, J. Appl. Phys., 48, 2753 (1977). 5. F. Leis, W. Sdorra, J.B. Ko and K. Niemax, Mikrochim. Acta (Wien) II, 185 (1989). 6. W. Sdorra and K. Niemax, Mikcrochim. Acta, 107, 319 (1992). 7. H. Kurniawan, Y. Ishikawa, S. Nakajima and K. Kagawa, Appl. Spectrosc., 51, 12, 1769 (1997). 8. M. Autlin, A. Brian and P. Mauchien, Spectrochim. Acta Part B. 48, 6/ (1993). 9. H. Kurniawan, M.O. Tjia, M. Barmawi, S. Yokoi, Y. Kimura and K. Kagawa, J. Phys.D: Appl. Phys., 28, (1995). OL-14