22P042-P
Laser-induced Fluorescence Spectroscopy with Sub-nanosecond Laser Pulse
toward Observation of Excitation Transfer Processes of Helium Atoms
in an Atmospheric-pressure Plasma
大気圧プラズマ中でのヘリウム原子励起移行過程の観測に向けた
サブナノ秒レーザー誘起蛍光分光
Kohei Akai, Yukihiro Takaie, Hiraku Matsukuma, Taiichi Shikama and Masahiro Hasuo
赤井浩平, 高家幸弘，松隈啓，四竈泰一，蓮尾昌裕
Graduate School of Enginerring, Kyoto University
Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan
京都大学大学院工学研究科 〒606-8501 京都市左京区吉田本町
In a glow discharge helium plasma at pressures from 0.001 to 0.1 atm and in a micro-hollow cathodeplasma from 0.05 to 1.0 atm, a laser light pulse of 620 ps width excites helium atoms from the 21S level to31P level. Temporal developments of the intensities of the subsequent fluorescence from n=3 levels areobserved. From the coupled rate equation for the populations in the 31P and 31D levels with theexcitation-transfer and quenching rates of these levels, the temporal developments are examined.

1. Introduction
It is known that excitation transfer processes
between energetically close excited levels takeplace in a plasma due to atom collisions together
with collisional quenching. Such processes may
become substantial in high pressure plasmas like anatmospheric-pressure plasma, which gathers muchnotice recently for many kinds of applications
because of their freedom from vacuum enclosureand high particle density [1]. For the plasma
diagnostics, quantitative evaluations of the collision
rates and of the limit of binary collision
approximation are necessary.
For a helium plasma at a gas pressure less than
0.01 atm, excitation from an n=2 level to an n=3
level with a several ns laser pulse has been done
and subsequent fluorescence from several n=3levels have been observed, where n is a principle
quantum number [2-5]. From the analysis of thetemporal development of the fluorescenceintensities, the excitation-transfer and quenchingrate coefficients have been determined [2-5].
At the gas pressures larger than 0.01 atm, thetime constant evaluated from the rates becomecomparable to the laser pulse-width, and then thequantitative analysis of the temporal development
becomes difficult. Furthermore, a conventional
glow discharge cannot generate a stable plasma at
the gas pressures larger than 0.1 atm.
In this work, we use a laser light source having a
sub-nanosecond pulse-width and observe laser
induced fluorescence (LIF) for a pure helium
plasma generated in a glow discharge at pressuresfrom 0.001 to 0.1 atm or in a micro-hollow cathodedischarge from 0.05 to 1.0 atm.
2. Experiment
Figure 1 shows a schematic diagram of theexperimental set-up. A 620 ps laser pulse from adye laser excited by a sub-nanosecond N2 laser(Usho, KEN-X) excites helium atoms from the 21Slevel to 31P level. We observed LIF from n=3 levels
with a monochromator (Nikon, G-250), a
photo-multiplier (Hamamatsu, H6780-2, 780 ps risetime) and a digital oscilloscope (Agilent,DSO5054A, 800 ps rise time) for a glow discharge
plasma (5 mm diameter and 100 mm length, 1 mA
discharge current) or for a micro-hollow cathodeplasma (1 mm diameter and 1 mm anode-cathodedistance, 15 mA discharge current) [6].

Fig. 1. A schematic illustration of the experimental
set-up with a glow discharge or a micro-hollow cathodedischarge

3. Results and discussion
Figure 2 shows an example of the temporaldevelopments of the observed relative fluorescenceintensities at 0.05 atm for (a) 21S-31P transition andfor (b) 21P-31D, 21P-31S and 23P-33D transitions,divided by respective Einstein’s A coefficients. The
intensities of the 21S-31P and 21P-31D transitions
are dominant, and then populations of respective
upper levels are dominant.

Fig. 2. Temporal developments of the fluorescenceintensities of the (a) 21S-31P transition and (b)21P-31D, 21P-31S and 23P-33D transitions.

Figures 3 (a-e) show temporal developments ofthe fluorescence intensities observed for the glow
discharge plasma. Figs. 3 (f-h) show the results forthe micro-hollow cathode plasma. In the case of thegas pressures beyond 0.1 atm we can show only the21P-31D intensity because the 21S-31P intensity
becomes too weak to examine the temporaldevelopment. In Fig. 3, each of the temporaldevelopments is normalized to its peak intensity for
all the cases. It is noted that the temporaldevelopments of the 21P-31D intensity at 0.1 atm inthe glow discharge and in the micro-hollow cathodedischarge are similar to each other.
Considered with the excitation transfer andquenching of the 31P and 31D levels, the coupled
rate equation for the populations in these level,
N(31P) and N(31D), are given as
d1
1
1111
 dt N (3 P)  K (3 D  3 P)N (3 D)  K (3 P)N (3 P) d N (31 D)  K (31 P  31 D)N (31 P)  K (31 D)N (31 D) dtwhere K(31P→31D) and K(31D→31P) are theexcitation transfer rates from 31D to 31P and from
31P to 31D, respectively, and K(31P) and K(31D) are
the quenching rates of the 31P and 31D levels,respectively.
The coupled rate equation can be solved
analytically. With the solution and the rate
coefficients due to atom collisions reported in ref.
[7], we try to reproduce the temporal developmentsof the fluorescence intensities. The result is shownby the solid line in Fig. 3, where convolution of theinstrumental time response is done. It is found thatthe temporal development of the 21S-31P intensity iswell reproduced while that of the 21P-31D intensity
is not for the gas pressures between 0.005 and 0.1
atm. The reason is not clear at present.

Fig. 3. Temporal developments of the 21S-31P and
21P-31D fluorescence intensities and the results of thereproduction by the solution of the coupled rate equation.

Acknowledgments
The authors thank Prof. Namba of Hiroshima
University for his initiation of the micro-hollow
cathode discharge system. This work was partly
supported by Grant-in-Aid for Scientific Research(B) (No.21340170).
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