05aE04P
飽和吸収分光法によるアルゴン準安定状態の緩和周波数測定：電子密度計測への応用
Relaxation frequency measurement of metastable argon by saturation spectroscopy:
application to electron density measurement
王 恵民, 西山 修輔, 佐々 木浩一
Huimin Wang, Shusuke Nishiyama, Koichi Sasaki
北大院工Hokkaido Univ.
Introduction We developed a system of saturation spectroscopy with the intention of applying it to the measurement ofelectron density in an argon-containing plasma.The principle of the electron density measurement is that the amplitude of thesaturation spectrum in the 4 s[3 / 2]o2  4 p[3 / 2] 2 line of argon is a function of the electron density.The broadband spectrum
which was originated from the velocity changing collision was separated from the observed saturation spectrum to obtain thetrue amplitude of the 4s[3 / 2]o2  4 p[3 / 2]2 line. The electron densities in argon-helium mixture plasmas were evaluated by the
proposed method, and the results were in reasonable agreements with the measurements using a Langmuir probe.
Experiment We used an inductively-coupled plasma source with an internal antenna in this experiment.The antenna wasconnected to an rf power supply at 13.56 MHz via a matching network.The discharge gas was the mixture of argon andhelium.The partial pressure of argon was 5 mTorr.The light source of absorption spectroscopy was a diode laser.The diodelaser beam was divided into two beams, and the diode laser beams with higher and lower intensities were used as the pumpand probe beams, respectively.The pump and probe laser beams were injected into the plasma from the contrary directions.
The intensity of the probe laser beam passing through the plasma was detected using a photodiode.
Results and discussion Fig. 1 shows the difference between absorption spectra observed with and without pump laser beamby saturation spectroscopy. The shape of the saturation spectrum is Lorentzian in the simple theory of saturation spectroscopy.
However,the saturation spectrum shown in Fig. 1 has Gaussian components which are attributed to “velocity changingcollisions” of the argon metastable state at the bottom of the spectrum.The saturation spectrum with the components due tovelocity changing collisions was analyzed on the basis of a theory[1].The sharp spectrum(b)(the red curve) at the line center inFig. 1 corresponds to the Lorentzian component separated from the measuredspectrum, from which we evaluated / in.Thisequation shows the relationship between the amplitude of saturation spectrum andthe saturation parameter.The saturation parameter is given by(a),where B12 is the Einstein’s B coefficient,I is the spectral intensity of the*pump laser beam,c is the speed of light,and R is the effective relaxation frequencyof the relevant energy system.
For 4s[3 / 2]o2  4 p[3 / 2] 2 absorption which we employ in this work,thelower energy state of this absorption line is metastable. In the case of theabsorption line originated from an argon metastable state, the relaxation frequency(b)is dominantly determined by the relaxation frequency of the metastable state. The
lifetime of the metastable state in a plasma is determined by collisional
quenching.There are two kinds of quenchers, neutral species and electrons.The
former has much smaller rate coefficients of collisional quenching than the later’s Fig.1. Spectrum (a) shows the difference in thewhich is known as a large value of 2x10-7 cm3/s [2]. The relaxation frequency in two absorption spectra observed with andwithout pump beam.Sharp spectrum (b)showssaturation spectroscopy has an additional factor, which is the transit time of the Lorentzian component separated frommetastable argon through the diameter of the pump laser beam. Hence, the total velocity chaning collision.
relaxation frequency is given by,where kqe is the rate
coefficient of electron impact quenching, ne is the electron density, a is thediameter of the pump laser beam, and v is the mean velocity of metastable argon.
Therefore, by measuring the amplitude of saturation spectrum /, we canevaluate the relaxation frequency R*, and then the electron density ne according to
the theory mentioned above.
Fig.2shows the comparison between 1/S0 and the electron density.The electrondensity was measured using a Langmuir probe, and 1/S0 was evaluated from/using. As shown in Fig. 2, linear relationships
were observed between 1/S0 and ne. The linear relationships represent, and the intercepts correspond to a/v. Therefore, in conclusion, theexperimental result shown in Fig. 2 indicates that the proposed method can work
as a method for the electron density measurement in argon-containing plasmas.
[1] C. Brechignac and R. Vetter, Phys. Rev. A 17, 1609 (1978).
[2] K. Sasaki and R. Asaoka, Jpn. J. Appl. Phys. 50, 08JB02 (2011).

Fig.2. Comparison between the reciprocal of
the saturation parameter and the electrondensity.