May On Tjia

Review of Laser-Induced Plasma, Its Mechanism, and Application to Quantitative Analysis of Hydrogen and Deuterium

Abstract A comprehensive review of important progress achieved over the last 30 years regarding knowledge of laser-induced plasmas generated by CO2 and Nd:YAG lasers in a variety of ambient gases is presented in this article, as well as research results on the extension of laser-induced breakdown spectroscopy (LIBS) for quantitative analysis of light elements, especially hydrogen and deuterium. First, the formation of shock wave–induced expanding secondary plasma in low-pressure ambient gases is discussed along with the dynamic characteristics of the secondary plasma expansion process. The unique advantages of low-pressure gas plasma are explained in relation to the successful detection of the sharp H and D emission lines. The experimental results using helium ambient gas are presented with emphasis on the role of He gas plasma in introducing an additional delayed excitation mechanism involving the helium metastable excited state, which resulted in the complete resolution of H and D emission lines, separated by only 0.18 nm. The development of a laser precleaning treatment and special double-pulse techniques further produced a linear calibration line with zero intercept applicable to quantitative H and D analyses of zircaloy sample, with either low- or high-pressure ambient He gas. More recent use of a transversely excited atmospheric (TEA) CO2 laser in place of an Nd:YAG laser has demonstrated the much desired larger excited helium plasma and thereby resulted in significant emission enhancement and improved detection sensitivity.

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.

Planar waveguides of PPV derivatives: attenuation loss, third-harmonic generation and photostability

Abstract We studied MEH-PPV and MEH-PPB which are derivatives of poly( p -phenylenevinylene) (PPV), synthesized via the so-called HORNER-polycondensation route. The residual sub-bandgap absorptions of MEH-PPV were found to be extremely low at λ ⩾1064 nm as compared to other PPVs. We attribute this fact to its very low defect density. MEH-PPV enables the fabrication of slab waveguides with propagation losses which are well below 1 dB/cm at 1064 nm. Comparative studies of the cubic nonlinearity χ (3) , the photodegradation and the damage thresholds at 1064 nm of PPV, MEH-PPV and MEH-PPB demonstrate that polycondensation-type MEH-PPV has the greatest potential for the realization of nonlinear optical waveguide applications.

Shock wave plasma induced by TEA CO2 laser bombardment on glass samples at high pressures

Abstract An experimental study has been carried out on the dynamical process taking place in laser plasma, generated by TEA CO2 laser (400 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.

Hydrogen analysis in solid samples using laser-induced helium plasma at atmospheric pressure

A special technique for the modification of laser-induced breakdown spectroscopy (LIBS) has been developed to improve the spectral quality of hydrogen emission from a solid sample in helium gas at atmospheric pressure. In this technique, the plasma was generated by focusing a fundamental Nd-YAG (yttrium aluminum garnet) laser into a surrounding helium gas. The helium atoms excited to their metastable states would then serve to excite the atoms of the solid material vaporized by using another Nd-YAG laser. When properly synchronized, the resulting hydrogen emission line of H I 656.2 nm shows a dramatic improvement of the emission intensity and the spectral quality over what was obtained by conventional LIBS technique. This study further reveals that this improvement is mainly due to the role of the metastable excited state in a helium atom, which allows the delayed detection to be performed at a favorable moment when the charged particles responsible for the strong Stark broadening effect in the plasma hav...

Hydrogen analysis of zircaloy tube used in nuclear power station using laser plasma technique

It is shown that remarkable improvements essential to a quantitative spectrochemical analysis of hydrogen emissions from the zircaloy samples were achieved when the low-pressure surrounding air used in the previous experiment of Nd-YAG laser-induced shockwave plasma was replaced by an inert gas. Using the high-purity (99.999%) nitrogen gas at 1.5 Torr, a linear calibration curve of the HI 656.2 nm emission line was obtained with a zero intercept from the zircaloy samples prepared with various hydrogen concentrations. Further, when the surrounding nitrogen gas was replaced by a helium gas, more than an order of magnitude enhancement was obtained on the signal-to-noise ratio, yielding a detection limit of less than 5 ppm.

Detection of Density Jump in Laser-Induced Shock Wave Plasma Using A Rainbow Refractometer

A special interferometric technique with high sensitivity has been devised on the basis of rainbow refractometry without the use of an additional and delicate amplitude-splitting setup. This new technique was used for the characterization of shock wave plasma induced by a Q-switched Nd:YAG laser on various metal samples under reduced surrounding gas pressures. An unmistakable signal of the density jump was detected simultaneously with the observation of the emission front signal. It proved that the emission front and the front of the blast wave coincided and moved together with time at the initial stage of the secondary plasma expansion. However, at a later stage, the emission front began to separate from and left behind the blast wave front propagating in the surrounding gas at low pressures. With the use of Cu and Zn samples, the experimental results showed that the separation of the emission front and blast wave front took place at about 5 mm above sample surface for laser energy of 140 mJ.

Laser-induced shock wave plasma spectrometry using a small chamber designed for in situ analysis ☆

Abstract Direct spectrochemical analyses on large bulk samples such as metal plates have been performed by using a small vacuum chamber, which was attached directly to the sample surface through an o-ring. This technique allowed the in situ generation of laser plasma and hence overcome to a good extent the inconvenient and sometime clumsy sample preparation procedure required in Laser-Induced Shock Wave Plasma Spectrometry. Additionally, the presence of the o-ring near the target surface effectively shielded off the surrounding area from the undesirable continuum emission from the primary plasma, and thereby enhanced the detection sensitivity of this technique. Using zinc plate and Pb glass as samples, it was further demonstrated in this experiment that even the time-integrated spectra, obtained by employing an OMA system, still exhibited a lower background than those obtained by ordinary time-resolved Laser-Induced Breakdown Spectroscopy.

Comprehensive study on the pressure dependence of shock wave plasma generation under TEA CO2 laser bombardment on metal sample

An experimental study has been carried out on the dynamical process taking place in the plasma generated by a TEA CO2 laser (400 mJ, 100 ns) on a zinc target when surrounded by helium gas of pressure ranging from 2 Torr to 1 atm. Plasma characteristics were examined in detail on the emission lines of Zn I 481.0 nm and He I 587.6 nm by means of an unique time-resolved spatial distribution technique in addition to an ordinary time-resolved emission measurement technique. The results reveal, for the first time, persistent shock wave characteristics in all cases throughout the entire pressure range considered. Further analysis of the data has clarified the distinct characteristics of laser plasmas generated in different ranges of gas pressure. It is concluded that three types of shock wave plasma can be identified; namely, a target shock wave plasma in the pressure range from 2 Torr to around 50 Torr; a coupling shock wave plasma in the pressure range from around 50 Torr to 200 Torr and a gas breakdown shock wave plasma in the pressure range from around 200 Torr to 1 atm. These distinct characteristics are found to be ascribable to the different extents of the gas breakdown process taking place at the different gas pressures. These results, obtained for a TEA CO2 laser, will provide a useful basis for the analyses of plasmas induced by other lasers.