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Abstrak :
ABSTRAK Telah dilakukan penelitian efek penguapan selektif (selective vaporation) plasma gelombang kejut (shock wave plasma) yang dibangkitkan dengan memfokuskan iradiasi laser TEA CO2 (130 mJ, 100 ns) atau laser Nd-YAG (50mJ, 8ns) pada bahan kuningan yang memiliki kandungan Zn (seng) dan Cu (tembaga) dalam udara tekanan rendah (1 Torr). Efek penguapan selektif akan terjadi apabila rapat daya laser yang digunakan tidak terlalu tinggi dibandingkan dengan nilai ambang batas pembangkitan plasma. Hasil pengamatan menunjukkan bahwa intensitas emisi Cu jauh lebih rendah dibandingkan dengan intensitas emisi Zn. Juga telah diamati bahwa titik awal (rising point) emisi Cu tertinggal dibandingkan dengan titik awal emisi Zn sepanjang daerah pengembangan plasma. Dalam kasus iradiasi laser CC2, penguapan selektif lebih dominan, dan kecepatan propagasi atom-atom Cu sangat rendah dibandingkan dengan atom-atom Zn. Hal ini menunjukkan bahwa gelombang kejut adalah dibentuk dari kumpulan atom-atom Zn, sedangkan atom-atom Cu yang datang terlambat tidak mampu membangkitkan gelombang kejutnya sendiri. Dilain pihak atom-atom Cu tersebut di atas juga tertinggal jauh dari muka gelombang kejut yang terbentuk oleh atorn-atom Zn, mengakibatkan atom-atom Cu ini tidak akan pernah tereksitasi.

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ABSTRACT TEA CO2 laser (130 mJ, 100 ns) and Nd-YAG laser (50 mJ, 8 ns) pulses were focused on brass samples under a reduced pressure of air at 1 Torr, and In and Cu emission characteristics were compared for the two cases. For the TEA CO2 laser the emission intensity of copper lines is extremely low, compared to zinc lines, indicating a serious selective-vaporization effect. By comparing the time-profile of the emission of In 1481.0 nm and Cu 1 327.4 nm near the surface, it was clearly shown that the gushing of Cu atoms occur later and continues for a long time with a rather low gushing speed, while In gushes faster at a high speed. Only In atoms form a shock front and Cu is left behind the shock-wave and does not undergo excitation. This type of selective vaporization takes place only when the power density of laser light is not high, compared to the threshold for plasma generation. The phenomenon of selective vaporization described in this paper also supports our laser-induced shock wave model.

Abstrak :
In spite of abundant experimental evidences supporting the viability of the laser induced shock wave plasma model for the explanation of the important features ofthe plasma and the associated spectroscopic characteristics, a controversy on the atomic excitation mechanism in the plasma has remained to be completely resolved. In this study the contributions of the shock wave model and two other most popular models, the electron-ion recombination model and thc electron collision model were thoroughly investigated. For that purpose, a special technique has been developed for the direct detection of the charge current in conjunction with plasma emission measurement dining the laser plasma generation and expansion. The current detection was performed by placing a partially transmitting metal mesh electrode at a distance in front of the sample surface with the sample target sewing as the counter electrode. The electric Held between the mesh and sample surface was set up and varied by applying a variable DC voltage (0-400 Volt) between them. The laser plasma was generated by a YAG laser (64 ml, 8 ns) tightly focused on a Cu target through the mesh electrode in low-pressure surrounding gas. It was found that the charge current time profiles obtained at various gas pressures invariably exhibit a lack of consistent correlation with the emission time profile of the plasma throughout most of the emission period. The result of this study has thus practically eliminated any significant roles ofthe electron-ion recombination and electron collision models in the excitation process. We are therefore led to conclude that the shock wave model proposed earlier is most plausible for the consistent explanation of the secondary plasma emission, while the other two models may have some contribution only at the very initial stage ofthe secondary plasma generation.

Abstrak :
ABSTRACT
In spite of abundant experimental evidences supporting the viability of the laser induced shock wave plasma model for the explanation of the important features ofthe plasma and the associated spectroscopic characteristics, a controversy on the atomic excitation mechanism in the plasma has remained to be completely resolved. In this study the contributions of the shock wave model and two other most popular models, the electron-ion recombination model and thc electron collision model were thoroughly investigated. For that purpose, a special technique has been developed for the direct detection of the charge current in conjunction with plasma emission measurement dining the laser plasma generation and expansion. The current detection was performed by placing a partially transmitting metal mesh electrode at a distance in front of the sample surface with the sample target sewing as the counter electrode. The electric Held between the mesh and sample surface was set up and varied by applying a variable DC voltage (0-400 Volt) between them. The laser plasma was generated by a YAG laser (64 ml, 8 ns) tightly focused on a Cu target through the mesh electrode in low-pressure surrounding gas. It was found that the charge current time profiles obtained at various gas pressures invariably exhibit a lack of consistent correlation with the emission time profile of the plasma throughout most of the emission period. The result of this study has thus practically eliminated any significant roles ofthe electron-ion recombination and electron collision models in the excitation process. We are therefore led to conclude that the shock wave model proposed earlier is most plausible for the consistent explanation of the secondary plasma emission, while the other two models may have some contribution only at the very initial stage ofthe secondary plasma generation. Key words: charge current, shock wave, electron-ion recombination and electron collision. Praiseci is to the Lord for He is my reason in everything I do. This manuscript is never be done without the guidance by Pro£ Tjia May On, to whom I am extremely grateful. He also provided the support without which this thesis would not possible. He is more than just a teacher for me for his words have deeply touched me. Moreover, he also introduced me that knowledge is something we should share among others and to improve the education in my country. I am also indebted to Prof. Kiichiro Kagawa at the Fukui University for providing the atmosphere and the physical resources to make thesis writing in these times of fast paced research. I am also thankful for the opportunity which is given to me to join research together with him in his laboratory in Japan. Extra special thanks go to Dr. Hendrik Kurniawan for providing me with encouragement and support for this project. He is the first one who encouraged me to take Doctor Cotuse Program which seemed impossible at the beginning. His companion during research at Applied Spectroscopy Laboratory at University of Indonesia is a leading experience in research for me. I am particularly grateful to the excellent team of referees who provided critical comments on this thesis. Their feedback was a great benefit to me. I gratefully acknowledge all my colleagues: Rinda Hedwig, Mangasi A. Marpaung, Hery Suyanto, MM. Suliyanti, Wahyu S. Budi, and Emon in Applied Spectroscopy Laboratory at University of Indonesia, for their assistance and support during my study. My never-ending thanks to my beloved family, especially to my parents who exhibited thoughtful patience over extended periods of time when I seemed to be invisible. Thanks also to Loviana who helped me in all situations which I no longer can resist by myselfl Finally, I apologize to all those who helped that I did not acknowledge specifically. I know there were many and greatly appreciate your assistance. August, 2002 Author