[Show abstract][Hide abstract] ABSTRACT: We report in this paper the results of an experimental study on the spectral and dynamical
characteristics of plasma emission induced by 1 mJ picoseconds (ps) Nd-YAG laser using spatially
resolved imaging and time resolved measurement of the emission intensities of copper sample.
This study has provided the experimental evidence concerning the dynamical characteristics of the
excitation mechanisms in various stages of the plasma formation, which largely consolidate the
basic scenarios of excitation processes commonly accepted so far. However, it is also clearly shown
that the duration of the shock wave excitation process induced by ps laser pulses is much shorter
than those observed in laser induced breakdown spectroscopy employing nanosecond laser at
higher output energy. This allows the detection of atomic emission due exclusively to He assisted
excitation in low pressure He plasma by proper gating of the detection time. Furthermore, the
triplet excited state associated with He I 587.6 nm is shown to be the one most likely involved
in the process responsible for the excellent spectral quality as evidenced by its application to
spectrochemical analysis of a number of samples. The use of very low energy laser pulses also
leads to minimal destructive effect marked by the resulted craters of merely about 10 micro meter diameter
and only 10 nm deep. It is especially noteworthy that the excellent emission spectrum of deuterium
detected from D-doped titanium sample is free of spectral interference from the undesirable
ubiquitous water molecules without a precleaning procedure as applied previously and yielding
an impressive detection limit of less than 10 micro gram/g. Finally, the result of this study also shows a
promising application to depth profiling of impurity distribution in the sample investigated.
[Show abstract][Hide abstract] ABSTRACT: We report the results of experimental study on CN 388.3 nm and C I 247.8 nm emission
characteristics using 40 mJ laser irradiation with He and N2 ambient gases. The results obtained
with N2 ambient gas show undesirable interference effect between the native CN emission and the
emission of CN molecules arising from the recombination of native C ablated from the sample
with the N dissociated from the ambient gas. This problem is overcome by the use of He ambient
gas at low pressure of 2 kPa, which also offers the additional advantages of cleaner and stronger
emission lines. The result of applying this favorable experimental condition to emission
spectrochemical measurement of milk sample having various protein concentrations is shown to
yield a close to linear calibration curve with near zero extrapolated intercept. Additionally, a low
detection limit of 5 lg/g is found in this experiment, making it potentially applicable for
quantitative and sensitive CN analysis. The visibility of laser induced breakdown spectroscopy
with low pressure He gas is also demonstrated by the result of its application to spectrochemical
analysis of fossil samples. Furthermore, with the use of CO2 ambient gas at 600 Pa mimicking the
Mars atmosphere, this technique also shows promising applications to exploration in Mars.
[Show abstract][Hide abstract] ABSTRACT: A systematic study has been performed on the spectral characteristics of the full spectrum of He emission lines and their time-dependent behaviors measured from the He gas plasmas generated by a nanosecond neodymium-doped yttrium aluminum garnet laser. It is shown that among the major emission lines observed, the triplet He(I) 587.6 nm emission line stands out as the most prominent and long-lasting line, associated with de-excitation of the metastable triplet (S = 1) excited state (1s1 3d1). The role of this metastable excited state is manifested in the intensity enhancement and prolonged life time of the Cu emission with narrow full width half-maximum, as demonstrated in an orthogonal double-pulse experiment using a picosecond laser for the target ablation and a nanosecond laser for the prior generation of the ambient He gas plasma. These desirable emission features are in dire contrast to the characteristics of emission spectra observed with N2 ambient gas having no metastable excited state, which exhibit an initial Stark broadening effect and rapid intensity diminution typical to thermal shock wave-induced emission. The aforementioned He metastable excited state is therefore responsible for the demonstrated favorable features. The advantage of using He ambient gas in the double-pulse setup is further confirmed by the emission spectra measured from a variety of samples. The results of this study have thus shown the potential of extending the existing laser-induced breakdown spectroscopy application to high-sensitivity and high-resolution spectrochemical analysis of wide-ranging samples with minimal destructive effect on the sample surface.
[Show abstract][Hide abstract] ABSTRACT: A systematic study has been performed on the spectral characteristics of the full spectrum of He emission lines and their time-dependent behaviors measured from the He gas plasmas generated by a nanosecond neodymium-doped yttrium aluminum garnet laser. It is shown that among the major
emission lines observed, the triplet He(I) 587.6 nm emission line stands out as the most prominent and long-lasting line, associated with de-excitation of the metastable triplet (S = 1) excited state (1s1 3d1). The role of this metastable excited state is manifested in
the intensity enhancement and prolonged life time of the Cu emission with narrow full width half-maximum, as demonstrated in an orthogonal double-pulse experiment using a picosecond laser for the target ablation and a nanosecond laser for the prior generation of the ambient He gas plasma.
These desirable emission features are in dire contrast to the characteristics of emission spectra observed with N2 ambient gas having no metastable excited state, which exhibit an initial Stark broadening effect and rapid intensity diminution typical to thermal shock wave-induced
emission. The aforementioned He metastable excited state is therefore responsible for the demonstrated favorable features. The advantage of using He ambient gas in the double-pulse setup is further confirmed by the emission spectra measured from a variety of samples. The results of this study
have thus shown the potential of extending the existing laser-induced breakdown spectroscopy application to high-sensitivity and high-resolution spectrochemical analysis of wide-ranging samples with minimal destructive effect on the sample surface.
[Show abstract][Hide abstract] ABSTRACT: An experimental study has been performed on the pressure-dependent plasma emission intensities in Ar, He, and N2 surrounding gases with the plasma induced by either nanosecond (ns) or picosecond (ps) yttrium aluminum garnet laser. The study focused on emission lines of light elements such as H, C, O, and a moderately heavy element of Ca from an agate target. The result shows widely different pressure effects among the different emission lines, which further vary with the surrounding gases used and also with the different ablation laser employed. It was found that most of the maximum emission intensities can be achieved in Ar gas plasma generated by ps laser at low gas pressure of around 5 Torr. This experimental condition is particularly useful for spectrochemical analysis of light elements such as H, C, and O, which are known to suffer from intensity diminution at higher gas pressures. Further measurements of the spatial distribution and time profiles of the emission intensities of H I 656.2 nm and Ca II 396.8 nm reveal the similar role of shock wave excitation for the emission in both ns and ps laser-induced plasmas, while an additional early spike is observed in the plasma generated by the ps laser. The suggested preference of Ar surrounding gas and ps laser was further demonstrated by outperforming the ns laser in their applications to depth profiling of the H emission intensity and offering the prospect for the development of three-dimensional analysis of a light element such as H and C.
[Show abstract][Hide abstract] ABSTRACT: A time-resolved orthogonal double pulse laser-induced breakdown spectroscopy (LIBS) with helium surrounding gas is developed for the explicit demonstration of time mismatch between the passage of fast moving impurity hydrogen atoms and the formation of thermal shock wave plasma generated by the relatively slow moving major host atoms of much greater masses ablated from the same sample. Although this so-called 'mismatching effect' has been consistently shown to be responsible for the gas pressure induced intensity diminution of hydrogen emission in a number of LIBS measurements using different ambient gases, its explicit demonstration has yet to be reported. The previously reported helium assisted excitation process has made possible the use of surrounding helium gas in our experimental set-up for showing that the ablated hydrogen atoms indeed move faster than the simultaneously ablated much heavier major host atoms as signaled by the earlier H emission in the helium plasma generated by a separate laser prior to the laser ablation. This conclusion is further substantiated by the observed dominant distribution of H atoms in the forward cone-shaped target plasma.
[Show abstract][Hide abstract] ABSTRACT: A crucial safety measure to be strictly observed in the operation of heavy-water nuclear power plants is the mandatory regular inspection of the concentration of deuterium penetrated into the zircaloy fuel vessels. The existing standard method requires a tedious, destructive, and costly sample preparation process involving the removal of the remaining fuel in the vessel and melting away part of the zircaloy pipe. An alternative method of orthogonal dual-pulse laser-induced breakdown spectrometry (LIBS) is proposed by employing flowing atmospheric helium gas without the use of a sample chamber. The special setup of ps and ns laser systems, operated for the separate ablation of the sample target and the generation of helium gas plasma, respectively, with properly controlled relative timing, has succeeded in producing the desired sharp D I 656.10 nm emission line with effective suppression of the interfering H I 656.28 nm emission by operating the ps ablation laser at very low output energy of 26 mJ and 1 μs ahead of the helium plasma generation. Under this optimal experimental condition, a linear calibration line is attained with practically zero intercept and a 20 μg/g detection limit for D analysis of zircaloy sample while creating a crater only 10 μm in diameter. Therefore, this method promises its potential application for the practical, in situ, and virtually nondestructive quantitative microarea analysis of D, thereby supporting the more-efficient operation and maintenance of heavy-water nuclear power plants. Furthermore, it will also meet the anticipated needs of future nuclear fusion power plants, as well as other important fields of application in the foreseeable future.
[Show abstract][Hide abstract] ABSTRACT: An experimental study of double pulse LIBS is performed for the development of highly sensitive and quantitative analysis of Cl and Ca for the strength evaluation of concrete. The two lasers employed are arranged in orthogonal geometry and operated for delayed ablation with delay time of 10 μs, using He ambient gas at atmospheric pressure. The very large intensity enhancement is obtained over those detected with single pulse operation without generating the He plasma. It is most remarkable that the same sharpness and intensity enhancement observed in the previous double pulse experiment with 37 mJ laser ablation energy is achieved in the present experiment with much lower ablation energy of merely 2.5 mJ, resulting in average crater size of about 10 μm in diameter. Further, a linear relation is obtained between the Cl concentration and its emission intensity in alumina samples, while an estimated limit of detection of 80 ppm is obtained by using concrete sample, which is adequate for highly sensitive quantitative Cl analysis in concrete.
Spectrochimica Acta Part B Atomic Spectroscopy 03/2012; 69:56–60. DOI:10.1016/j.sab.2012.03.002 · 3.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An experimental study on picosecond laser induced plasma spectroscopy of a zircaloy sample with low-pressure surrounding helium gas has been carried out to demonstrate its potential applicability to three-dimensional quantitative micro-analysis of deuterium impurities in zircaloy. This was achieved by adopting the optimal experimental condition ascertained in this study, which is specified as 7 mJ laser energy, 1.3 kPa helium pressure, and 50 μs measurement window, and which was found to result in consistent D emission enhancement. Employing these operational parameters, a linear calibration line exhibiting a zero intercept was obtained from zircaloy-4 samples doped with various concentrations of D impurity, regarded as surrogates for H impurity. An additional measurement also yielded a detection limit of about 10 μg/g for D impurity, well below the acceptable threshold of damaging H concentration in zircaloy. Each of these measurements was found to produce a crater size of only 25 μm in diameter, promising its application for performing less-destructive measurements. The result of this study has thus paved the way for conducting a further experiment with hydrogen-doped zircaloy samples and the further technical development of a three-dimensional quantitative micro-analysis of detrimental hydrogen impurity in zircaloy vessels used in nuclear power plants.
[Show abstract][Hide abstract] ABSTRACT: An experiment was performed for the observation of H emission induced in a cooled laser-induced atmospheric pressure gas plasma of He atoms in their metastable excited state. The strong H emission detected clearly established, to the exclusion of other well known major excitation processes, the exclusive contribution of the He-induced excitation (HIE) mechanism. The process is suggested to take place by means of energy transfer from the excited He atoms to the H atoms via Penning collision induced ionization involving electron exchange. The result further shows that this mechanism may also work for elements other than H and thereby strongly suggests the use of ambient He gas to broaden and complement the applications of standard laser-induced breakdown spectroscopy.
[Show abstract][Hide abstract] ABSTRACT: An experimental study of ultraviolet (UV) laser-induced plasma spectroscopy (LIPS) on Ti samples with low-pressure surrounding He gas has been carried out to demonstrate its applicability to quantitative micro-analysis of deuterium impurities in titanium without the spectral interference from the ubiquitous surface water. This was achieved by adopting the optimal experimental condition ascertained in this study, which is specified by 5 mJ laser energy, 10 Torr helium pressure, and 1-50 mus measurement window, which resulted in consistent D emission enhancement and effective elimination of spectral interference from surface water. As a result, a linear calibration line exhibiting a zero intercept was obtained from Ti samples doped with various D impurity concentrations. An additional measurement also yielded a detection limit of about 40 ppm for D impurity, well below the acceptable threshold of damaging H concentration in Ti and its alloys. Each of these measurements was found to produce a crater size of only 25 mum in diameter, and they may therefore qualify as nondestructive measurements. The result of this study has therefore paved the way for conducting further experiments with hydrogen-doped Ti samples and the technical implementation of quantitative micro-analysis of detrimental hydrogen impurity in Ti metal and its alloys, which is the ultimate goal of this study.
[Show abstract][Hide abstract] ABSTRACT: An experimental study has been performed on the effects of crater depth on the hydrogen and deuterium emission intensities measured from laser plasmas generated in low-pressure helium ambient gas from zircaloy-4 samples doped with different H and D impurity concentrations as well as a standard brass sample for comparison. The results show that aside from emission of the host atom, the emission intensities of other ablated atoms of significantly smaller masses as well as that of the He atom generally exhibit relatively rapid initial decline with increasing crater depth. This trend was found to have its origin in the decreasing laser power density arriving at the crater bottom and thereby weakened the shock wave generated in the crater. As the crater deepened, the declining trend of the intensity appeared to level off as a result of compensation of the decreasing laser power density by the enhanced plasma confinement at increasing crater depth. Meanwhile, the result also reveals the significant contribution of the He-assisted excitation process to the doped hydrogen and deuterium emission intensities, leading to similar crater-depth dependent variation patterns in contrast to that associated with the surface water, with growing dominance of this common feature at the later stage of the plasma expansion. Therefore, a carefully chosen set of gate delay and gate width which are properly adapted to the crater-depth dependent behavior of the emission intensity may produce the desired intrinsic emission data for quantitative depth profiling of H impurity trapped inside the zircaloy wall.
[Show abstract][Hide abstract] ABSTRACT: An experimental study was conducted on the spatial distributions of hydrogen emission intensities from low-pressure plasmas generated by laser ablation of zircaloy-4 and black stone targets in nitrogen and helium ambient gases. In addition to confirming the previously observed intensity enhancement effect in ambient helium gas, the hydrogen and helium emission intensities measured along the plasma expansion direction revealed remarkable extended spatial distributions featuring unexpected maxima near the far end of the plasma where the available shock-wave generated thermal excitation energy should have been significantly reduced. This “anomalous” feature necessarily implied the presence of an additional excitation process beside the well known shock-wave excitation process which is responsible for the plasma emission of heavy atoms in low-pressure ambient gas. Further analysis of the data led to a suggested physical mechanism explaining the possible contribution of a helium metastable excited state to the unusual phenomenon observed in this experiment.
[Show abstract][Hide abstract] ABSTRACT: A series of measurements have been performed on the time dependences of the intensities of helium, hydrogen, and deuterium emission lines from the corresponding laser-induced helium plasma at atmospheric pressure for two different He flow rates. The prolonged H <sub>α</sub> and H <sub>β</sub> emissions along with their constant intensity ratio over a relatively extended period indicate the need to provide an alternative excitation mechanism other than the well-known thermal excitation process in a hot plasma. This additional excitation mechanism is also related to the metastable excited state of a He atom as indicated by the similar characteristics of the observed time dependence of the emission intensities. The enhanced intensity and lifetime of He emission at a high He flow rate was explained in terms of the collision-induced increase in the number of He atoms excited to above the 2 <sup>1</sup>S<sub>0</sub> metastable state, which was also responsible for the delayed excitation of H and D atoms via an energy transfer mechanism involving a Penning-like chemi-ionization process. Finally, the benefits of He-assisted delayed excitation of H and D atoms and the aforementioned enhanced intensity and lifetime at a high He flow rate were demonstrated by the achievement of clearly resolved H <sub>α</sub> and D <sub>α</sub> emission lines.
[Show abstract][Hide abstract] ABSTRACT: An experimental study was performed on the N <sub>2</sub> -induced quenching of He-induced intensity enhancement effect in reduced-pressure plasma emission produced by Nd-YAG irradiation on solid zircaloy and porous fossil samples. The spatial distributions and temporal variations in the emission intensities show pronounced intensity quenching effects on the He I 667.9 nm, H I 656.2 nm, and D I 656.1 nm emission lines in both samples when a tiny amount (5% by volume) of nitrogen was added to helium gas, while leaving the spatial and temporal intensity profiles of the heavier Zr and Ca atoms virtually unaffected. In both cases of different ambient gases, the spatial and temporal variations in the He, H, and D emission intensities exhibit distinct features and changes, which are clearly distinguishable from those observed on the Zr and Ca emission lines, which were mainly produced by the shock-wave induced thermal excitation process. The analysis of these data unambiguously revealed the presence of an additional and distinct “He-assisted” excitation mechanism in the He plasma, which was further suggested to be related to the He metastable excited state. The quenching effect was therefore explained as a consequence of energy depletion of the He metastable excited state triggered by the Penning ionization process induced by the presence of nitrogen. This also explains the relatively insensitive response of the Zr and Ca emission intensity profiles to nitrogen addition despite the increased plasma electron density resulting from the ionization process.
[Show abstract][Hide abstract] ABSTRACT: The applicability of spectrochemical analysis of minute amounts of powder samples was investigated using an ultraviolet Nd-YAG laser (355 nm) and low-pressure ambient air. A large variety of chemical powder samples of different composition were employed in the experiment. These included a mixture of copper(II) sulfate pentahydrate, zinc sulfide, and chromium(III) sulfate n-hydrate powders, baby powder, cosmetic powders, gold films, zinc supplement tablet, and muds and soils from different areas. The powder samples were prepared by pulverizing the original samples to an average size of around 30 microm in order to trap them in the tiny micro holes created on the surface of the quartz subtarget. It was demonstrated that in all cases studied, good quality spectra were obtained with low background, free from undesirable contamination by the subtarget elements and featuring ppm sensitivity. A further measurement revealed a linear calibration curve with zero intercept. These results clearly show the potential application of this technique for practical qualitative and quantitative spectrochemical analysis of powder samples in various fields of study and investigation.
[Show abstract][Hide abstract] ABSTRACT: An experimental study was conducted in search of the experimental condition required for the much needed suppression of spectral interference caused by surface water in hydrogen analysis using laser-induced low-pressure helium plasma spectroscopy. The problem arising from the difficulty in distinguishing hydrogen emission from hydrogen impurity inside the sample and that coming from the water molecules was overcome by taking advantage of similar emission characteristics shared by hydrogen and deuterium demonstrated in this experiment by the distinct time-dependent and pressure-dependent variations of the D and H emission intensities from the D-doped zircaloy-4 samples. This similarity allows the study of H impurity emission in terms of D emission from the D-doped samples and thereby separating it from the H emission originating from the water molecules. Employing this strategy has allowed us to achieve the large suppression of water induced spectral interference from the previous minimum of 400 microg/g to the current value of 30 microg/g when a laser beam of 34 mJ under tight focusing condition was employed. Along with this favorable result, this experimental condition has also provided a much better (about 6-fold higher) spatial resolution, although these results were achieved at the expense of reducing the linear calibration range from the previous 4 300 microg/g to the present 200 microg/g.
[Show abstract][Hide abstract] ABSTRACT: This experiment was carried out to address the need for overcoming the difficulties encountered in hydrogen analysis by means of plasma emission spectroscopy in atmospheric ambient gas. The result of this study on zircaloy-4 samples from a nuclear power plant demonstrates the possibility of attaining a very sharp emission line from impure hydrogen with a very low background and practical elimination of spectral contamination of hydrogen emission arising from surface water and water vapor in atmospheric ambient gas. This was achieved by employing ultrapure ambient helium gas as well as the proper defocusing of the laser irradiation and a large number of repeated precleaning laser shots at the same spot of the sample surface. Further adjustment of the gating time has led to significant reduction of spectral width and improvement of detection sensitivity to ~50 ppm. Finally, a linear calibration curve was also obtained for the zircaloy-4 samples with zero intercept. These results demonstrate the feasibility of this technique for practical in situ and quantitative analysis of hydrogen impurity in zircaloy-4 tubes used in a light water nuclear power plant.
[Show abstract][Hide abstract] ABSTRACT: An experimental study has been performed to demonstrate the advantage of employing ambient helium gas in the spectral quality
improvement of hydrogen emission in laser-induced plasma from zircaloy-2 samples at both atmospheric and low gas pressure.
It was further shown that the optimal results achieved in the two pressure regimes require the adoption of different sets
of experimental parameters consisting of the laser energy, the focusing lens position and the detection gate delay. A strictly
linear calibration line with extrapolated zero intercept was nevertheless exhibited in the case of atmospheric gas pressure
only. Additional time-evolution measurement of the emission intensities of hydrogen, helium and zirconium clearly suggests
a distinctly different excitation mechanism for hydrogen atoms associated with the presence of ambient helium atoms and their
meta-stable excited state.
Applied Physics B 10/2007; 89(2):291-298. DOI:10.1007/s00340-007-2780-x · 1.63 Impact Factor