30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK

Collisional Quenching Rate Coefficient of 3 +2 uN (A Σ ) by o-xylene and

Influence of Xylene By-products Attached to Electrode Surface

S. Suzuki P

1P, H. Itoh P

1P

P

1 Department of Electrical, Electronic and Computer Engineering, Chiba Institute of Technology, Tsudanuma, Narashino, Chiba 275-0016, Japan

Determination of the effective lifetime of )A(N 3

2u in N2/o-xylene (C8H10) mixtures is carried out

by the waveform analysis of the transient current after turning off the UV light in the Townsend discharge region. The collisional quenching rate coefficient of )A(N 3

2u by o-xylene is

determined together with the diffusion coefficient of )A(N 32

u , and the reflection coefficient on electrode surface of )A(N 3

2u . The obtained collisional quenching rate coefficient of )A(N 3

2u

by o-xylene is (2.3±0.3)×10-9 cm3/s. The value is a high than that of m-xylene measured previously．In the repeated measurements of the current-voltage curves, it is found that curves shift to the high E/p0 side gradually with time as well as the case of m-xylene, and the effect of current prevention phenomena in o-xylene is larger than the case of m-xylene. In addition, the phenomenon that it is impossible to measure the transient current experienced in N2/m-xylene mixtures is not observed.

1. Introduction

The metastable nitrogen molecule )A(N 32

u

plays an important role as active nitrogen in air plasma [1]. During the past decade, the study on the purification of the air utilizing discharge plasma has been actively carried out in the world. For that reason, we have determined the collisional quenching rate coefficient of )A(N 3

2u by air pollutant gases, such

as CO, CF4, CH4, NO, CCl2F2, and CH2FCF3 [2-5]. Furthermore, we also have determined the same coefficients of )A(N 3

2u by gases that cause sick

building syndrome, such as benzene (C6H6), acetone ((CH3)2CO), toluene (C6H5CH3), and formaldehyde (CH2O) [3,6].

Recently, we determined the collisional quenching rate coefficient of )A(N 3

2u by m-xylene

(C8H10) together with the current prevention effect [7]. The phenomenon is found as follows: when measurements of the current-voltage curves in a N2/m-C8H10 mixture are repeated, the curves gradually move to high-E/p0 side over the time. Therefore, an higher applied voltage is required to maintain a certain constant ionizing current. Therefore, the voltage increases gradually and reaches a saturated condition. This effect is reproduced well independently of gas exchange in the chamber before each experiment, but it strongly depends on the surface condition of the cathode.

In this paper, it reports on the result of a measurement of o-xylene (o-C8H10), and the results of o-xylene are compared with that of m-xylene.

2. Experimental apparatus and method

The experimental apparatus and procedure were already reported in detail [2-7], but they will be described in the following briefly. A vacuum chamber was evacuated down to pressures of approximately 10-8 Torr by a turbomolecular pump. A pair of gold-plated plane parallel electrodes were mounted on the central part of the chamber and a stabilized DC high voltage was applied to the anode. The UV light driven by a pulse lighting of about 0.4 Hz period was irradiated onto the cathode through a quartz window and holes on the central part of the anode. A transient ionization current produced by a fast current amplifier was observed after turning off the UV light. A typical transient current waveform is shown in Fig. 1. This is drawn on a semilogarithmic graph as an average of 64 measurements. If turning off the UV light, a stepwise decrease in the current Ip is first observed from the total current I. This current

Topic number: A 1

Cur

rent

I (

arb.

uni

ts)

UV light off

I

Ip

Im

im(t)

Time t (s)

Time constant τ1

0 0.01 0.02 0.03 0.04 0.05 0.060.1

1

10

Fig. 1. Typical transient current waveform.

30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK

component is multiplied by the collisional ionization of electrons emitted by UV light and the secondary electron emission caused by positive ions and photons, so called the first γ-action. Then, it was followed by a gradual decrease in the current im(t) with the initial amplitude Im and the time constant τm due to the decay of the electrons released by metastables striking the surface, so called the γm process [8]-[12]. By using both the observed currents I and Im, the ratio Im/I and τm, the effective lifetime τ1 of 3

2N A( )u is given [2] by

I

Im

m11 . (1)

The effective lifetime τ1 is calculated by analysis [2,4,13] that considers the reflection of )A(N 3

2u on

the surface of electrodes. It consists of the diffusion term and the term for the collisional quenching by

)X(N g1

2 and o- xylene as follows:

010

2

21

1 '11

1

pNbkakpd

Dm

. (2)

Here, Dm1 (=Dm ･ p0/p1) cm2s-1 is the diffusion coefficient of )A(N 3

2u at 0℃ under p1 Torr, k =

1.79×10-18 cm3s-1 [4] is the collisional quenching rate

coefficient of )A(N 32

u by )X(N g1

2 , k' cm3s-1 is

the collisional quenching rate coefficient of )A(N 3

2u by o-xylene, N1 cm-3 is the density of gas

molecules at 0℃ under p1 Torr, d cm is the gap length, and a and b (=1 - a) are the numbers of gas molecules of the fractions of N2 and o- xylene against the total number of gas molecules in the chamber. p0 is the reduced gas pressure at 0℃. p1 is taken to be 1 Torr. μ1 is the first order constant that describes the density function of the metastables, that is, the first root of the Fourier radiation series given by the reflection coefficient R of metastable nitrogen molecules at the surface of the electrode. 3. Experimental results and discussion 3.1 Effective lifetime in o-xylene

Figure 2 shows the gas pressure dependence of the observed effective lifetime τ1 of )A(N 3

2u in

N2/o-xylene(1.00ppm) mixtures. The effective lifetime τ1 is given by the inverse of the slope of the decreasing transient current waveforms shown in Fig. 1, according to equation (1). The obtained effective lifetime is plotted with the error bar of the standard deviation on a log-log scale. The solid lines are given by the curve fitting procedure using equation (2) based on our theory [7]. The curves are proportional to gas pressure up to 5 Torr, beyond which they reach

their maxima and become inversely proportional to gas pressure. From these curves, the diffusion coefficient Dm1, the collisional quenching rate coefficient k' of )A(N 3

2u by o-xylene and the

reflection coefficient R are determined as 155±2 cm2/s, (2.3±0.3)×10-9 cm3/s and 0.14±0.01, respectively. The result of the effective lifetime τ1 of

)A(N 32

u in N2/m-xylene(1.00ppm) mixtures is

shown in same figure in the broken line for the comparison. The diffusion coefficient Dm1 and the reflection coefficient R are determined as 151.2±0.7 cm2/s and 0.10±0.02. These values are almost the same values. However, the collisional quenching rate coefficient k' of )A(N 3

2u by

m-xylene is (4.4±0.6)×10-9 cm3/s, and it is the twice larger than o-xylene.

3.2 Repeated measurements of current-voltage characteristic

Figure 3 shows the temporal variation of the current-voltage curves that is measured at a suitable interval time. During the measurement interval, a constant ionization current of about 40 μA flows, which varies applied voltage, and some surface reactions are induced between the electrodes. The current-voltage curves shift to the left side until 10 minutes, after that, these curves shift to the right side gradually with time. It is also found that there is no variation in the current-voltage curves from 240 minutes. It is considered that certain decomposition products such as hydrocarbons would attach to the cathode surface [7]. In this case, the measurement of the transient current waveform in o-xylene mixtures is possible though it is impossible to observe that in m-xylene mixtures.

N2/m-C8H10(1.00ppm)

Gas pressure p0 (Torr)

Eff

ectiv

e lif

etim

e

(m

s)

d=1.5cm d=1.2cm d=1.0cm d=0.7cm

N2/o-C8H10(1.00ppm)

0.1 1 100.1

1

10

Fig. 2. Effective lifetime of )A(N 3

2u .

30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK

Figure 4 shows the temporal variation of the applied voltage between the gap necessary to obtain the currents of 0.1 nA, 10 nA and 1μA in N2/o-xylene(1.00ppm) mixtures as shown in Fig. 3. It is observed that the voltage increases gradually with time; in other words, the electric resistance between the gap is enhanced in all the results. However, it decreases gradually around in passing 200 minutes. This figure has shown the values of the same current of N2/m-xylene(1.00ppm) mixtures previously measured for the comparison. Though it is almost the same voltage-time change, it is understood that the voltage decreases slightly in 180 minutes.

Figure 5 shows the temporal variation of the current at 300 V/cm･Torr as an example of the region where the current increases with an increase in the voltage in Fig. 3. With increasing E/p0, the current in N2/o-xylene(1.00ppm) mixtures reaches its maximum then decreases gradually with the increase in E/p0. A similar variation of current is seen in N2/m-xylene(1.00ppm) mixtures. However, the absolute value of the current in o-xylene mixtures is larger than that in m-xylene mixtures.

From Figures 3, 4, and 5, it is understood that the electric resistance between the gap is enhanced by exposing the electrode surface to the discharge, that is, the effect of current prevention phenomena in o-xylene is larger than the case of m-xylene.

Reduced electric field E/p0 (V/cmTorr)

Cur

rent

I (

A)

N2/o-C8H10(1.00ppm)

po=1.0Torrd=1.0cm

0 min. 10 min. 60 min. 120 min. 180 min. 240 min. 360 min. 420 min. 480 min.

0 100 200 300 400 500

10-12

10-10

10-8

10-6

10-4

Fig. 3. Temporal variations of current-voltage characteristic in N2/o-xylene(1.00 ppm).

Vol

tage

V (

V)

1　10-10 A

o-C8H10 m-C8H10

Time t (min)

×

0 100 200 300 400 500

200

300

400

500

Vol

tage

V (

V)

1　10-8 A

o-C8H10 m-C8H10

×

200

300

400

500

Vol

tage

V (V

)

1 10-6 A o-C8H10 m-C8H10

×

200

300

400

500

Fig. 4. Comparison between N2/o-xylene (1.00ppm) and N2/m-xylene (1.00ppm) of temporal variation of applied voltage to maintain each current at 1 Torr, 1 cm.

Cur

rent

I (

A)

Time t (min.)

m-C8H10 o-C8H10

E/p0 = 300 V/cm Torr・

0 100 200 30010-12

10-11

10-10

10-9

Fig. 5. Comparison between N2/o-xylene (1.00ppm) and N2/m-xylene (1.00ppm) of temporal variation of current at 300 V/cm･Torr.

30th ICPIG, August 28th – September 2nd 2011, Belfast, Northern Ireland, UK

3.3 Observation of the cathode surface by Auger electron spectroscopy

Surface analyses of the cathode by the Auger electron spectroscopy (AES) are carried out to investigate the cause of the variation in the current-voltage curves as shown in Fig. 3. Figure 6 shown the results of the relative intensities of O, Cu, C, and N atoms against spattering time, and it is the result of the copper disc sample exposed to the discharge for 480 minutes. C accounts for about 60 % of the relative density at 0 s, after that, it is understood that C decreases gradually, and crosses with Cu at 30 s. However, the relative intensities of O, N, and Cu atoms are almost constant regardless of the sputtering time. This result suggests that some carbon atoms are deposited on the cathode surface after exposing the discharge to N2/o-xylene mixtures. It corresponds to the depth of 4 nm in 30 s when thinking from the sputtering rate to the direction of depth of this AES. Therefore, it is thought that thickness of carbon atoms deposited on the cathode surface is about 4 nm.

It is confirmed to show intensity that Cu is the

largest from t = 0 s in the sample not exposed to the electrical discharge as shown in Fig. 7. Therefore, it can be considered that the deposited material caused the current prevention phenomenon as shown in Fig. 3, 4, and 5. However, we think that we should investigate how the molecule including c deposits on the surface. 4. Conclusions

The diffusion coefficient Dm1 of )A(N 32

u in

N2/o-xylene(1.00ppm) mixtures and the collisional quenching rate coefficient k' of )A(N 3

2u by

o-xylene are determined as 155±2 cm2/s and (2.3±0.3)×10-9 cm3/s, respectively. The collisional quenching rate coefficient k' of )A(N 3

2u by

o-xylene was found that it is the same order as m-xylene.

The current-voltage curves in N2/o-xylene (1.00ppm) mixtures shifted to a high-E/p0 region because of certain decomposition product detected by AES as C atom that attached to the electrode surface. 5. References [1] U.Kogelschatz: Plasma Chem. Plasma Process., 23 (2003) 1-46. [2] S. Suzuki, H. Itoh, H. Sekizawa and N. Ikuta: J. Phys. Soc. Jpn., 62 (1993) 2692. [3] T. Suzuki, S. Suzuki and H. Itoh: Proc. 13th Asian Conference on Electrical Discharge (2006) P-2-40. [4] S. Suzuki, H. Itoh, H. Sekizawa and N. Ikuta: Jpn. J. Appl. Phys., 36 (1997) 4744. [5] S. Suzuki, H. Itoh, H. Sekizawa and N. Ikuta: The Papers of Technical Meeting on Electrical Discharges, IEE Japan (1998) ED-98-117 [in Japanese] [6] S. Suzuki, D. Shibuya, M. Aoyagi and H. Itoh: Proc. 29th Inter. confer. on Phenomena in Ionized gases, Cancun, Mexico (2009) PA1-8. [7] S. Suzuki and H. Itoh: IEEJ Trans. FM, 131 (2011) 82-88. [in Japanese] [8] J. P Molnar: Phys. Rev., 83 (1951) 933. [9] J. P Molnar: Phys. Rev. , 83 (1951) 940. [10] H. Krisch: Z. Phys., 208 (1968) 322. [11] S. C. Haydon and O. M. Williams: J. Phys. B: Atom. Mol. Phys. , 6 (1973) 227. [12] S. Suzuki, H. Itoh, N. Ikuta and H. Sekizawa: J.Phys.D: Appl. Phys., 25 (1992) 1568. [13] H. Itoh and N. Ikuta: Trans.IEE of Japan, 105-A, (1985) 189 [in Japanese].

(％)

Sputtering time (s)

Rel

ativ

e de

nsity

N

Cu

O

C

Exposed time: 480 min. O Cu C N

0 100 200 300

20

40

60

80

100

Fig. 6. Result of Auger electron spectroscopy of sample of copper exposed to discharge.

(％)

Sputtering time (s)

Rel

ativ

e de

nsity

N

Cu

O

C

Blank test O Cu C N

0 100 200 300

20

40

60

80

100

Fig. 7. Result of Auger electron spectroscopy of sample of copper exposed to discharge (Blank sample).