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A time resolved spectroscopic study is performed on He plasma in a special orthogonal
double pulse experiment employing an ns laser for the He plasma generation and a ps laser for target ablation. The study is focused on the most dominant He I 587.6 nm (triplet) and He I 667.8 nm (singlet) emission lines responsible for the previously proposed He assisted
excitation mechanism. The result reveals a transient enhancement of He I 587.6 nm emission in concomitance with the delayed target ablation, signifying the effect of He atom
bombardment by electrons ejected from the target. The maximum enhancement is achieved with the optimal 5 s delay of target ablation following the He plasma generation in the
double pulse LIBS experiment.
Results and Discussion
Conclusions
Direct Proof of the Role of He Triplet over He Singlet Metastable State in Excitation Process of
Laser-Induced He Gas Plasma
Hery Suyanto
1, Rinda Hedwig
2, Zener Sukra Lie
2,3, Koo Hendrik Kurniawan
3, Kiichiro Kagawa
41
Department of Physics, Faculty of Mathematics and Natural Sciences, Udayana University, Kampus Bukit Jimbaran, Denpasar 80361, Bali, Indonesia
2
Department of Computer Engineering, Bina Nusantara University, 9 K.H. Syahdan, Jakarta 14810, Indonesia
3
Research Centre of Maju Makmur Mandiri Foundation, Jakarta, Indonesia 11630
4
Fukui Science Education Academy, Takagi Chuo 2 chome, Fukui 910-0804, Japan
References
The first experiment was focused on the He plasma spectrum in order to sort out the
metastable excited states most relevant to the He assisted excitation process. For this study, only the ns Nd-YAG laser in Fig.1 was operated to generate He gas plasma and emission
spectrum was measured with OMA system resulted as shown Fig. 2.
The experiment investigate the excitation process of He emission through He I 587.6 nm and He I
667.8 nm emission lines. For this study, both laser in figure1 operated and the result as shown in figure 3.
The advantages of using helium ambient gas in LIBS has been widely reported in the published literatures (1-7), that are for emission enhancement, to suppress the signal
background and to decrease the limit of detection. These results can be explained to be closely related to the crucial role of helium metastable excited state which serves as a temporary energy reservoir for exciting of the ablated atom through penning effect (8-12).
Unfortunately , the identification of the specific He metastable excited state has rarely been reported. This study focused to investigate the excitation process of He emission through He I 587.6 nm (triplet) and He I 667.8 nm (singlet) emission lines.
Background
Experimental Set up and Methods
nano second Nd:YAG laser
motor pico second
Nd:YAG laser
OMA
Spectrograph
PMT
oscilloscope mask
fiber
He gas
ps laser ns laser
He gas plasma
target plasma Z-axis
X-axis
vacuum pump
computer
He* (metastable) + e
-
He
++ e
-He+ + e- He** (triplet) He + h (587.6 nm)
He** (singlet) He + h (667.8 nm)
Emission
recombination
We observed the induced
emission of 587.6 nm and 667.8
nm using oscilloscope trace
ps laser as a source of fast electron ns laser
Figure 1. Experimental Set up : The Orthogonal double pulse LIBS
587.6 nm (Triplet)
667.8 nm (Singlet)
Singlet and triplet lines are comparable
Time integrated spectrum, low resolution
Figure 2. Emission spectrum oh He from Laser-induced He gas plasma and He discharge Lamp
Induced emission area
ns laser ns + ps laser ps laser
He emission by electron bombardment
Starting of ps Laser
Starting of ns Laser
Starting of ps Laser
Oscilloscope trace of He I 587.6 nm (μs) Oscilloscope trace of He I 587.6 nm (μs) Oscilloscope trace of He I 587.6 nm (μs)
1 div = 1 μs
He gas plasma He gas plasma
Target plasma due to ps laser
Target plasma due to ps laser
Actually, put mask
Fig. 3(a) shows the oscilloscope recorded time profile of He I 587.6 nm emission intensity associated with transition from the 3d triplet states of the metastable excited He atoms when only the ns laser is switched on.
Fig. 3(b) is the same He I 587.6 nm time profile recorded with the ps laser switched on 3 μs after the generation of the He gas plasma.
Fig. 3(c) shows the same emission intensity time profile when only the picosecond laser is sent onto the target.
the signal in Fig. 3(b) signal in Fig. 3(a) = signal in Fig. 3(c).
gain of the He emission due to bombardment of the He atoms by the fast electrons
He＊
Figure 4. The resulted time profiles obtained from the two modes of measurements are substracted (Fig 3b – fig 3.a) for each delay time and then integrated to give the amount of the associated intensity enhancement effect.
0 100 200 300 400 500 600 700
0 5 10 15 20
In
d
u
ce
d
e
m
is
si
on
in
te
n
si
ty (c
ou
n
ts)
Time delay between ns and ps laser (µs)
Induced emission of He I 667.8 nm (Singlet)
Induced emission of of He I 587.6 nm (Triplet)
Abstract
Figure 4. Induced emission intensities versus time delay of ps laser with respect to the ns laser irradiation.
1. By the ns laser irradiation, hot He plasma is produced resulting He ions and electrons.
With time, they recombine and mostly following to triplet path emitting 587.6 nm emission, then finally triplet metastable state is produced.
2. Thus, we conclude He triplet metastable state is the major in the He gas plasma
1
2
Fig.3.
1. K.H. Kurniawan, T. Kobayashi, K. Kagawa, Effect of different atmospheres on the excitation process of TEA-CO2 laser-induced shock wave plasma, Appl. Spectrosc. 46, 581 (1992), and references therein.
2. M. Kuzuya, H. Matsumoto, H. Takechi, O. Mikami, Effect of laser energy and atmosphere on the emission characteristics of laser-induced plasma, Appl. Spectrosc. 47, 1659 (1993), and references therein.
3. M.R. Joseph, N. Xu, V. Majidi, Time-resolved emission characteristics and temperature profiles of laser-induced plasmas in helium, Spectrochim. Acta B 49, 89 (1994), and
references therein.
4. M. Tran, Q. Sun, B.W. Smith, J.D. Winefordner, Determination of F, Cl, and Br in solid
