Detection of the Density Jump in the Laser-Induced Shock Wave Plasma Using Low Energy Nd: YAG Laser at Low Pressures of Air

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1 Detection of the Density Jump in the Laser-Induced Shock Wave Plasma Using Low Energy Nd: YAG Laser at Low Pressures of Air Hendrik KURNIAWAN1, Tjung Jie LIE1, Nasrullah IDRIS1, May On TJIA2, Masahiro UEDA3 and Kiichiro KAGAWA31 Applied Spectroscopy Laboratory, Graduate Program in Opto-Electrotechniques and Laser Applications, The University of Indonesia, 4 Salemba Raya, Jakarta 10430, Indonesia. kurnia18@cbn.net.id 2Department of Physicș Faculty of Mathematics and Natural Sciences, Bandung Institute of Technology, 10 Ganesha, Bandung, Indonesia. motjia@melsa.net.id 3Faculty of Education and Regional Studieș Fukui University, 9-1 bunkyo 3-chome, Fukui 910, Japan. kagawa@edu00.f-edu.fukui-u.ac.jp (Received September 11, 2000) Synopsis A special interferometric technique with high sensitivity has been devised on the basis of rainbow refractometry for the detection of density jump signal withouthe use of an additional and delicate amplitude-splitting setup. This new technique was used for the characterization of shock wave plasma induced by a low energy of Q-sw Nd: YAG laser (14-64mJ) on zinc samples under reduced air pressure (3-30 Torr). An unmistakable signal of density jump was detected simultaneously with the observation of emission front signal, implying that the emission front and the front of the shock wave coincided and moved together with time at the initial stage of the secondary plasma expansion. The result of measurement also showed that at a later stage, the emission front began to separate from and left behind the blast wave front propagating the surroundin gas. This result is a direct proof of our shock wave model to explain the excitation process of the laser-induced plasma. Keywords: rainbow interferometric method, laser-induced shock wave plasma, excitation stage, cooling stage. 1. Introduction The technique of laser ablation emission spectrometric analysis (LAESA) is well known as one of the typical applications of lasers1-3). In this technique, a Q-switched laser is focused on the sample in air at 1atm. Consequently, a high-temperature and high-density plasma is generated, invariably giving rise to a high-intensity continuous emission spectrum, further aggravated by undesirable self-absorption processes. These disadvantages constitute the major obstacle in yielding the linearity and sensitivity required for an accurate spectroscopic calibration. More recently, however, some of the shortcomings faced by the conventional LAESA techniques have been overcome to some extent, by the use of new spectral detection systems. Presently, two general strategies are being pursued in LAESA development. One of these new development, adhering to atmospheric surrounding pressure has been led by Radziemski and colleagues4-7), and commonly called laser-induced breakdown spectroscopy (LIBS). In this method a pulsed laser with high peak power and short duration, such as the Nd: YAG laser, is focused onto the sample at atmospheric pressure. In order to remove the interfering background from the high-intensity continuous emission due to the high-density plasma, a gated optical multichannel analyzer (OMA) is incorporated into the detection system. Another direction of development in LAESA method involves the use of low surrounding gas pressures. Being produced under reduced gas pressure, the laser plasma is practically free from strong continuous background emission8-10) In the studies of plasma generation process under reduced pressure, we have showed that a plasma having characteristics favorable to spectrochemical anal-

2 14 J. Spectrosc. Soc. Japan Vol.50, No.1 (2001) ysis can actually be generated by using a pulsed gas laser with short duration, such as the nitrogen laser11), carbon dioxide laser12-13), and excimer laser14), when the pressure of the surrounding gas is reduced to around 1 Torr. In these cases, the plasma invariably consists of two distinct parts. The first part, which is called the primary plasma, occupies a small area and gives off intense continuous emission spectra for a short time just above the surface of the target. The other part, called the secondary plasma, expands with time around the primary plasma with near-hemispherical shape, emitting sharp atomic spectral lines with negligibly low background, and free from undesirable self-absorption effect. As such, the secondary plasma is clearly endowed with favorable characteristics for elemental detection and even quantitative analysis because of the linear relationship between the emission line intensities and the contents of associated elements in the target. By means of time-resolved experiments using a carbon-dioxide laser12-13), excimer laser14) and Nd: YAG laser15), our groups have demonstrated that this secondary plasma was excited by the shock wave, while the primary plasma acted as an initial explosion energy source. According to our model proposed earlier on the excitation mechanism of the secondary plasma, the ablated target atoms begin to gush out from the primary plasma at supersonic speed right after the cessation of the primary plasma. It is envisioned that the surrounding gas plays the role of damping material, impeding the free expansion of the propelling atoms by forming a walllike layer against which compression is taking place. As a result of this compression, a shock wave is generated in the surrounding gas. The most important point of the shock wave model is that the energy required to produce the secondary plasma is supplied in the form of kinetic energy from the propelling atoms. By means of this compression, the kinetic energy of the propelling atoms is converted into thermal energy in the plasma, by which atoms are excited. However, the shock wave model was previously demonstrated mainly by detecting the emission profiles only, either of the neutral or ionic lines. Recently, we have introduced a technique for the detection of density jump accompanying the shock wave front by using a modified shadowgraph method, which involves the use of a He-Ne laser as a probe light. The application of this new technique allows the detection of time difference between the density jump and the starting point of atomic emission. The results confirmed unequivocally the simultaneous occurrence of the two events, and thereby constitute a direct evidence for the shock wave mode116). This highly successful technique for the case of moderately high pressure surrounding gas, turns out to be less than adequate in detecting the relatively weak deflection signal on the density jump in the case of low surrounding gas pressure. In an effort to enhance the detection sensitivity, we have devised a new rainbow interferometric technique. Recently, using this rainbow interferometric technique and a relatively high energy TEA CO2 laser of 600mJ, we have proved for the first time the coincidence between the density jump and the emission front at relatively low surrounding air of 3 Torr17). This study was undertaken to take advantage of this highly sensitive interferometer for the study of density jump in our laser-induced shock-wave plasma generated by a relatively low energy Nd: YAG laser (14-64mJ) in the surrounding air in the range between 3 and 30 Torr. The result of this study is expected to provide further information needed for the elucidation and establishment of the plasma generation mechanism induced by low energy laser at reduced pressures. 2. Experimental Setup The experimental method employed in this work is basically the same as that used in the previous paper17). Figure 1 shows the experimental set-up used in this work. A Nd: YAG laser (Quanta Ray, GCR-12S, 400mJ, 8ns) was employed. The energy of the laser irradiation was varied using a set of filter and the laser was operated under Q-sw mode. The laser was run at a repetition frequency of 5Hz and the laser beam was focused by a multi-layer lens (f=100mm) through a quartz window onto the surface of the target (crater diameter=150Đm). the shot-to-shot fluctuation of the laser was approximately 3%. The samples were placed in a vacuum-tight metal chamber (11cm ~11cm ~12.5cm), which could be evacuated with a vacuum pump and filled with air or helium at the desired pressure. The gas flowing through the chamber was regulated by a needle valve in the gas line and a valve in the pumping line. The chamber pressure was measured precisely with the use of an absolute digital vacuum-meter. A zinc plate (Rare Metallic Co., 99.99%, 0.4mm thickness) was used as targets in all experiments. The emission of the laser-induced plasma was observed through the top side of the chamber via a mirror and imaged 1:1 by a quartz lens (L2, f=150mm) onto the entrance plane of the monochromator (SPEX M-750, Czerny-Turner configuration, focal length 750mm with 1,200 grooves mm-1 blazed at 500nm). The electrical signal from the photomultiplier (Hamamatsu IP-28) was fed through a 500- ohri resistor (its RC time constant was 30ns) into the first channel of a digital sampling-storage scope (HP, model A) and data collection was performed using a printer after passing through RS-232 interface module. The trigger signal of the digital sampling-storage scope was provided from the extracted fraction of the laser light by means of a PIN photodiode.

3 Kurniawan et al.: Detection of the Density Jump in the Laser-Induced 15 Fig. 1 Diagram of the experimental setup used in this study. For density jump measurements, a He-Ne laser was used with its beam directed perpendicular to the YAG light into the expansion region of the laser plasma by means of a mirror. The outgoing probe beam was then sent into the cylindrical glass of diameter 60mm. The probe beam emerges from the cylindrical glass appears as interference fringes. This interference effect is readily explained18,19) by the fact that the actual laser light is not a single ray, but a beam consisting of many rays with a cross section of around 1 to 2mm. Except the central ray, all the other rays are slightly shifted from the condition of minimum deviation. These rays spread out, expanding the beam to some extent as they emerge from the cylindrical glass. Interference takes place among these rays and produces bright fringes at certain positions where the phase matching condition is satisfied. Before carrying out the measurement, the mirror in front of the cylindrical glass should be properly adjusted so that the He-Ne laser light will undergo a minimum deviation in passing through the cylindrical glass. Under this condition, the highest visibility of the interference fringes will be secured as illustrated in inset of Fig. 1. The separation between adjacent fringes in the absence of laser plasma is about 100ƒÊm on the entrance plane of the slit when the slit is placed 80cm from the cylindrical glass. The slit width was set at about 20ƒÊm. A phase change in the He-Ne probe beam due to the arrival of shock front will alter the deviation shift of the fringes. A photomultiplier tube (Hamamatsu R- 1104) set behind the slit was used to detect the fringe change, and the electrical signal coming out from the PMT was fed into the second channel of digital sampling-storage scope. A vertical slit was placed in front of the PMT for detecting the probing He-Ne laser light in order to avoid disturbances from other unwanted light sources which were further blocked by an interference filter for He-Ne laser wavelength placed behind the slit. 3. Results and discussion Basov et al.20,21) studied for the first time the expansion of the laser-plasma induced by Nd-glass laser pulse bombardment (6J, 15ns) in air at 2 Torr. They confirmed the generation of shock wave by observing a thin shell structure using shadow-photograph technique and schlieren-photograph technique and also by interferometric method. Bobin et al.22) demonstrated the detection of transient shell emission structure and its development with time, proving the shock wave generation in the pressure range from 0.2 to 3 Torr under the bombardment of ruby laser (0.5J, 40ns), where they employed an image converter camera to record the expansion of the emission front. Similar experiments were conducted by Hall23), Emmony and Irving24). These laser-induced shock wave experiments were reviewed by Huhges25). However, it should be noted that all of the experiments made by other researchers were carried out either for the detection of the density jump front only or the emission front only. Also the laser pulse energies used in their experiments were as high as the order of several hundreds mj or more. In our previous experiment using TEA CO2 laser

4 16 J. Spectrosc. Soc. Japan Vol.50, No.1 (2001) (600mJ, 100ns) detection of density jump was made for the first time simultaneously with the observation of emission front. The laser light was focused on quartz sample at low pressure of around 10 Torr. Since the focusing spot area of the TEA CO2 laser was relatively large, the power density was rather low to give about 0.6GW/cm2. Therefore, in the experiment no plasma generation was observed when the pulse energy was reduced below 400mJ. As the result, we could not confirm that shock wave plasma can be produced even when the laser energy is as low as several tens mj. In this study we clearly detected the density jump and confirmed the coincidence between the density jump and the emission front even when 14mJ YAG laser pulse was focused on the target, where the power density was 10GW/cm2. In general, the detection of density jump become difficult with decreasing pressure of the surrounding gas. Fig. 2 shows the intensity variation of the density jump signal as a function of air pressure which was observed at 2mm from the target when 37mJ YAG laser pulse was used. It is clearly seen that the density jump signal decreases with decreasing air pressure. With this experimental setup, we were able to detect the density jump signal with surrounding gas pressure as low as around 3 Torr. Below 3 Torr, however, the density jump signal became partially swamped in the noise signal. For this density jump experiment, careful adjustment was made on the position of the slit attached to the photomultiplier so that density jump signal can be obtained with highest sensitivity. Namely, the fringe intensity was measured with the slit placed at the inflexion point of the fringe, middle point between the minimum and maximum in the fringe intensity distribution. When the shock wave arrive the probing position, the fringe intensity changes due to the occurrence of phase shift. However, the signal intensity never exceed the certain value, the intensity difference be- tween the middle point and the maximum point (or minimum point) of the fringe. This is the reason why the curve in Fig. 2 shows non-linear saturation like curve. Fig. 3 (a) displays the density jump signal along with the emission intensity of Zn I 481.0nm spectral line when a Nd: YAG laser of 37mJ was focused onto the pure zinc sample at reduced pressure of 10 Torr. In this figure, a clear deflection signal is seen standing out of the probing He-Ne laser light background, representing an unmistakable signal of density jump, This signal was detected with the probing He-Ne laser light positioned at 2mm in front of the sample surface. It is seen from the figure that the occurrence of the emission intensity (Zn I 481.0nm) took place at the same times with the arrival of shock waves which cause the density jump. Fig. 3 (b) shows the density jump and emission of Zn I 481.0nm measured at 6mm above the sample surface at air pressure of 10 Torr. In this case, the appearance of rising emission of Zn I 481.0nm line took place a bit later than the arrival of shock waves. In order to correlate the occurrence of density jump with the subsequent observation of emission Fig. 2 Relationship between the intensity of the density jump signal as a function of air pressures. The laser energy was set at 37mJ and density jump measurement was made at 2mm from zinc target. Fig. 3 The appearance of the density jump signal, emission intensity of Zn I 481.0nm at (a) 2mm and (b) 6mm above sample surface. The air pressure was kept constant at 10 Torr. Zinc plate was used as a sample.

5 Kurniawan et al.: Detection of the Density Jump in the Laser-Induced 17 front propagation, we have measured the shock front position and emission front position with respect to time for laser energy of 14mJ and 64mJ and the results are presented in Fig. 4 in log-log plot. We note that at both of the earlier stage and the later stage, the curves associated with the density jump for 14mJ and 64mJ share almost the same slope of 0.48 and 0.35, respectively. These value are close to the value of 0.4, which was derived theoretically for the shock wave induced by a point explosion26). It is noteworthy from the figure that the front of the propelled atoms as indicated by their emission front and the front of the shock waves virtually coincide with each other and move together at the same speed during the initial stage of the plasma expansion. At the later stage, after about 800ns, the front of the propelled atoms begins to fall behind and separates from the shock wave. To complete the picture, the separation time is also presented in Fig. 5 as a function of surrounding air pressure. It is clearly seen that the separation time decreases with increasing air pressures. In our previous paper15) we have proved a previously proposed hypothetical model, in which the dynamical process of the secondary plasma was separated into two stages, namely, the shock excitation stage, which was followed by the cooling stage. In the shock excitation region, atoms were excited by the heat arising from the strong compression between the shock front and propelled atoms. The cluster of propelling atoms was supposed to move in a manner somewhat similar that of piston pushing the surrounding gas and thereby had their kinetic energy converted to thermal energy responsible for the excitation process. We called this stage the shock excitation stage. In this stage, the front of the shock front coincided with the emission front of the propelled atoms. Soon after reaching their maximum emission, the cluster of atoms began to slow down while losing its energy to the surrounding gas, and the compression between the shock front and propelled atoms could no longer be sustained due to Fig. 4 Correlation between the arrival of the shock front and emission front of Zn I 481.0nm as a function of time at different laser energies of 14mJ and 64mJ under the surrounding air pressure of 10 Torr. Zinc plate was used as a sample. Fig. 5 Relationship between the separation time and surrounding air pressure. Data was taken at laser energy of 37mJ and zinc plate was used as a sample. the increasing separation between them. We called this part of the process the cooling stage. This scenario as described in the shock excitation model has been directly proved in the present study. The experimental fact shown in Fig. 5 in which the separation time decreases with increasing air pressure is readily understood on the basis of the shock wave model, at least qualitatively. Namely, It is naturally supposed that the atoms pushing the gas in the early stage of the plasma expansion lose the energy faster for high pressure and separate faster than the case for low pressures. In addition to zinc target, we also repeat the experiment with plastic sample. We detected C I 247.8nm to investigate the plasma expansion. As the result, the same result was obtained. This is hence further confirming the shock wave plasma model for the case of low energy laser source and reduced surrounding air pressure, which is favorable for its practical applications. 4. Conclusion We have taken advantage of the unusually high sensitivity of the rainbow interferometric method to extend our previous study of shock wave plasma generated by high power laser to the present case involving the use of low power laser. The shock wave plasma characteristics are unequivocally verified by the observation of detailed correlation between the plasma emission front and the density jump detected by the rainbow interferometer. There are still many people who doubt the shock wave model which has been presented by the present authors. However, this experimental results reported in this paper becomes one of the strong support for the shock wave model.

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