Hendrik Kurniawan

Effect of Different Atmospheres on the Excitation Process of TEA-CO2 Laser-Induced Shock Wave Plasma

The plasma characteristics and excitation process of laser-induced plasma with the use of a TEA CO2 laser of 750 mJ pulse energy and 100 ns pulse width are studied in different surrounding gases at reduced pressures. From the time-resolved spatial distribution, it is clear that in helium and argon atmospheres, two different excitation processes take place in forming the plasma. The first excitation process is due to the blast wave, while the second process is due to the metastable state of the noble gases. It is believed that this second process transfers metastable energy to the vaporized atoms of the target for emission, even long after the laser bombardment ends, thus giving total emission intensity that is higher in the noble gases than in air. The displacement of the front of the emission line under different atmospheres is also presented.

Laser-Induced Shock Wave Plasma in Glass and Its Application to Elemental Analysis

The characteristics of a laser-induced shock wave plasma which was induced by focusing a laser pulse on the surface of glass samples were examined by using radiation from a XeCl excimer laser and a TEA CO2 laser under reduced pressure of around 1 Torr. It was observed that shock wave plasma could not be generated by the TEA CO2 laser on low-melting-point glass because of the lack of expulsion from the sample surface. On the other hand, with the use of an excimer laser, shock wave plasma can be generated, even in low-melting-point glasses, thus making it amenable for spectrochemical analysis. Initial quantitative analysis was performed on a number of glass samples, and a linear calibration curve with a slope of near unity was obtained at a certain pressure. Furthermore, light elements such as Li and B, which are usually difficult to observe by the X-ray fluorescence method, were also successfully detected with a very low detection limit of less than 10 ppm. Other detection limits and background equivalent concentrations of almost all elements usually contained in glass, such as Na, Mg, Al, K, Ca, Ti, Zn, Zr, and Ba, were also presented. These results showed that the detection limit is much lower than those usually required for glass analysis.

Shock Excitation and Cooling Stage in the Laser Plasma Induced by a Q-Switched Nd:YAG Laser at Low Pressures

An experimental study has been carried out on the dynamical process taking place in the secondary plasma generated by a Q-switched Nd:YAG laser (80 mJ, 8 ns) on a copper target at reduced pressure. Accurate dynamical characterization of the cross-sectional view of the plasma has been made possible by the unique combination of a plasma confinement configuration and the time-resolved measurement technique. In addition to reaffirming the role of the blast-wave mechanism in the generation of secondary plasma, an analysis of the time-resolved spatial distributions of emission intensities and the time-resolved spatial distributions of temperature was made. As a result, the occurrence of two-stage emission processes, the “shock excitation stage” and “cooling stage,” has been proved. For instance, at 2 Torr it is shown that the emission process is initiated by a brief shock excitation process (∼ 1 μs) and followed by a longer cooling process (∼ 3 μs). The experimental results concerning the characteristics of the plasma can be well understood by considering the two-stage processes.

Spectrochemical Analysis of Metal Elements Electrodeposited from Water Samples by Laser-Induced Shock Wave Plasma Spectroscopy

We have succeeded in applying laser-induced shockwave plasma spectroscopy (LISPS) to the problem of the detection and analysis of metal elements deposited from water samples by means of electrolysis. It is shown that metal elements are generally deposited in the form of a thin film on the electrode surface, while the electrode also conveniently serves as a subtarget for the relatively soft metal film, thereby providing the necessary conditions for the generation of shockwave plasma, which is favorable for highly sensitive spectrochemical analysis. It is shown that the detection sensitivity of this method reaches its highest value at low surrounding air pressure of around 1 torr. The lowest detection limit attained for various metal elements investigated in this experiment varies from around ten to a few tens of ppb. This limit can be readily improved upon by incorporating an optical multichannel analyzer into the detection system. We have thus presented a promising method for the realization of a compact mobile monitoring system for the accurate control of water and soil quality.

A time-resolved spectroscopic study on the shock wave plasma induced by the bombardment of a TEA CO2 laser

A TEA CO2 laser (500 mJ, 100 ns) has been focused on a Zn plate in a surrounding gas at 0.5 Torr. The characteristics of the laser plasma were analysed using a unique time-resolved spectroscopic method. Time-resolved spatial distributions of Zn I 481.0 nm and Zn II 492.4 nm emission lines were observed. It was clearly shown that the resulting plasma has a thin shell structure and expands with time, and the ionization of Zn atoms precedes at slower rate than the excitation of neutral Zn atoms. The displacement of the neutral Zn emission was proportional to the two-fifths power of time. These experimental results proved that the plasma was excited by a blast wave induced by the laser bombardment.

Characteristics of a laser plasma induced by irradiation of a normal-oscillation YAG laser at low pressures

The generation of a laser-induced shock wave plasma in air at reduced pressure was achieved by focusing a normal-oscillation Nd:YAG laser on a brass sheet. The characteristics of the plasma were analysed under three different conditions: free expansion of the plasma, confinement of the plasma by limiting the plasma region using two parallel glass plates and interruption of the plasma by placement of a wedge in front of the plasma, in order to interrupt the expansion of the plasma. Spatially and temporally integrated methods were also developed in order to develop a more detailed understanding of the excitation mechanism of the plasma. Furthermore, a new technique of shadow graphing which involves the use of a He - Ne laser as a probe light was also developed as well as a measurement of the time profile of the temperature. By using this new method the relationship between the density jump and the starting point of atomic emission was detected simultaneously. The results show that the plasma was also generated by the shock wave as for the case of the short-pulse laser (Q switched). The extremely low ion and background emission, as low as compared with neutral emission, makes this plasma potentially useful for spectrochemical analysis.

Characteristics of the Secondary Plasma Induced by Focusing a 10-mJ XeCl Laser Pulse at Low Pressures

A XeCl excimer laser (20 ns, 10 mJ) was focused on a copper target at a relatively high power density (1010 W/cm2) in a surrounding gas of air in the pressure range from 1 Torr to about 10 Torr. Plasma characteristics were examined in detail with the use of a time-resolved spatial distribution technique on the emission lines Cu(I) 5218 Å, Cu(I) 5105 Å, and Cu(II) 4674 Å. The results show that the majority of atoms are ablated in the form of neutral atoms, and that the temperature of the plasma increases with time in the initial explosion stage. These facts suggest that the secondary plasma is excited by a shock wave and the recombination emission process is not predominant. It was also demonstrated that the plasma produced at 1 Torr is due to another excitation mechanism; namely, the collision of ultrafast atoms with surrounding gas molecules. This plasma is also suitable for analytical spectrochemical application.

Emission Spectrochemical Analysis of Glass Containing Li and K in High Concentrations Using a XeCl Excimer Laser-Induced Shock Wave Plasma

A XeCl excimer laser (20 ns, 25 mJ) was focused on glass samples containing high concentrations of Li and K (10%, 15%, and 20%) in a surrounding gas of air and He. Plasma characteristics are compared in two cases: at 1 atm and at reduced pressure (1–10 Torr). It is shown that the plasma produced at 1 atm cannot be applied to the elemental analysis of solid samples because of strong background emission and self-absorption in the plasma. The spectrum width of the emission line is very wide due to strong interaction in the plasma. In contrast to this, the shock-wave-induced plasma produced at reduced pressure gives a high S/B (ratio of the intensity of the emission line to that of the background) in the measurement of analytical emission lines, and the slope of the calibration curve is near unity in a log-log plot, thus making it possible for emission spectrochemical analysis.

Liquid refractometry by the rainbow method

A new method for measuring the refractive index of liquid, proposed in a previous paper [Appl. Opt. 36, 5552-5556 (1997)], has been developed. The minimum deviation of a laser beam deflected by a liquid-filled cylindrical cell was calculated by use of geometric optics. These theoretical results were compared with experimental results, with excellent agreement. As a result, the unknown refractive index of a liquid could be obtained by use of a computer calculation to give a best fit. The computer calculation showed that the sensitivity of the refractometer increases with the cell-wall thickness until total reflection takes place. A small refractive-index difference can be detected within a precision of 1 x 10(-6) by use of a metal-oxide semiconductor linear image sensor. We show how to calibrate the refractometer with pure water at 3.98 degrees C.

Characteristics of Hydrogen Emission in Laser Plasma Induced by Focusing Fundamental Q-sw YAG Laser on Solid Samples

Hydrogen emission has been studied in laser plasma by focusing a Nd-YAG laser (1,064 nm, 50 mJ, 8 ns) on various types of samples, such as copper plate, zinc plate and glass plate. Several parameters influencing the emission were varied, such as the type of gas (air, nitrogen and helium), gas pressures (ranging from 2 up to 760 Torr) and laser power density. It was found that Hα emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. It was also confirmed that the conventional well-known laser-induced breakdown spectroscopy (LIBS), which usually carried out at atmospheric air pressure, cannot be applied for the analysis of hydrogen as impurity. This specific characteristic of the pressure dependence of hydrogen is interpreted based on our shock wave model, taking account of the fact that the hydrogen mass is extremely light compared to that of the host elements.