22P040-P
Absorption Spectra Measurement of Helium Atoms in Micro-Hollow Cathode
Atmospheric-Pressure Plasmas by a Diode Laser
マイクロホローカソード大気圧プラズマ中ヘリウム原子の半導体レーザー吸収スペクトル計測
Shota Yamawaki, Daisuke Morita, Koichi Katayama, Taiichi Shikama and Masahiro Hasuo
山脇翔太，森田大輔，片山光一，四竈泰一，蓮尾昌裕
Graduate School of Engineering, Kyoto University
Yoshida-Honmachi, 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
We have measured absorption spectra of the He I 21P-31D (667.8 nm) transition in a micro-hollow cathodehelium plasma at gas pressures from 0.04 to 1 atm with a vertical-cavity surface-emitting laser diode. Froma fit with a Voigt function we estimated Lorentz and Doppler widths. The Lorentz width increases as anincrease in the gas pressure, while the Doppler width gradually decreases below 0.2 atm. It is found that theLorentz width at the gas pressure lower than 0.2 atm is mainly due to the pressure broadening.

1. Introduction
Recently, micro-plasmas attract much attention
due to their possibility of many applications such asa UV radiation source, pollutant removal andsurface cleaning [1,2]. Micro-plasmas can begenerated even at an atmospheric pressure [3].
Line profile analysis with atomic emission
spectroscopy is one of the characterization methods
for such a small plasma [4]. Laser absorption
spectroscopy has advantage in the spectral
resolution because its instrumental function can beusually neglected. However, since the line profile
broadens largely due to atom, ion and electron
collisions in a high pressure plasma, a large rangeof light frequency scan is necessary.
It is known that a vertical-cavity surface-emittinglaser (VCSEL) diode offers this feature [5]. Here,we apply it to high-resolution absorption
spectroscopy for a micro-hollow cathode helium
plasma.
2. Experiment
Figure 1 shows a schematic diagram of theexperimental set-up. The light source is a VCSEL(vixar, V670S-002-0001). We scan a lightfrequency over the He I 21P-31D transition bysweeping an injection current to the laser diodeusing a function generator (IWATSU ELECTRIC,FG-330) at a repetition rate of 5 Hz. The swept lightfrequency range is about 230 GHz.
A collimated laser beam passes through an
optical isolator (ISOWAVE, I-6070-CM), and thenis split into two beams by a half mirror (SURUGASEIKI, F56-30). One of the beam enters a confocal
Fabry-Perot interferometer (TecOptics, SA-300,

free spectral range=1.89 GHz) used as a frequency
marker. The other beam passes through a dischargechamber. In the chamber, a hollow cathode DC
plasma is generated between brass cathode andanode separated by a 1 mm-thick ceramic insulator
plate, all of which have an 1 mm-diameter
discharge hole in the center [4]. The dischargechamber is filled with a pure helium gas at pressure
from 0.04 to 1.0 atm. The light intensity transmitted
through the plasma is detected by a photo-detector
placed behind the chamber. The signals from thefunction generator, the photo-detector and theFabry-Perot interferometer are simultaneously
recorded by a personal computer via an AD
convertor (KEYENCE, NR-2000).

Fig. 1. The experimental set-up.

3. Result and Discussion
Figure 2 shows an example of raw data at the gaspressure of 0.04 atm. Fig. 2(a) shows the injection
current and the frequency marker signal. Fig. 2(b)shows the photo-detector signals recorded for thefollowing three cases; (1) both the laser anddischarge on, I(1), (2) only the laser on, I(2), and (3)only the discharge on, I(3). The absorption spectrum

can be calculated as -ln (I(1) - I(3))/ I(2).

Fig. 2. An example of raw data. (a) The injection
current to the laser diode and the frequency marker
signal. (b) The photo-detector signals.

Figure 3(a) shows thus calculated absorption
spectra at gas pressures of 0.04, 0.13, 0.39 and 1
atm as a function of the light frequency detuningfrom the absorption peak. The light frequency iscalibrated by the frequency marker signal. In Fig.
3(b), the absorption is normalized by the peak value
for the purpose of comparing the line shapes. The
pressure dependent broadenings are clearly seen.

estimated Doppler and Lorentz widths as a functionof the gas pressure. The Lorentz width increases as
an increase in the gas pressure. Since the Lorentz
width becomes very large at the gas pressureshigher than 0.2 atm, it becomes hard to estimate theDoppler width. Therefore, we plot the Doppler
width at the pressures below 0.2 atm.

Fig. 4. Pressure dependence of the Doppler, Lorentz
and pressure widths.

We calculate the gas temperature from theDoppler width to be 470.4 ± 0.7, 414.4 ± 0.6 and402.6 ± 1.5 K at 0.04, 0.06 and 0.13 atm,
respectively. From these gas temperatures andpressures, we estimate the pressure width, which is
caused by atom collisions, using its broadening
coefficient [6]. The result is shown in Fig. 4. The
difference between the observed Lorentz width andthe pressure width may give the Stark width, whichis caused by ion and electron collisions. However,the difference is not clearly detected.
Acknowledgments
Prof. Namba of Hiroshima University is greatly
acknowledged 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).

Fig. 3. (a) Absorption spectra at gas pressures at 0.04,0.13, 0.40, 1 atm. (b)Normalized absorption spectra.

It is known that there are two kinds ofbroadenings; the Doppler and Lorentz broadenings.
The former is caused by the atomic motions
reflecting the gas temperature and the latter iscaused by atom, ion and electron collisions.
We fit the absorption spectra with a Voigt
function having five adjustable parameters of theDoppler and Lorentz widths (FWHM), the verticaland horizontal offsets and the area. Fig. 4 shows theReferences[1] K. H. Becker, K. H. Schoenbach, and J. G. Eden: J.
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