Influence of the Gas Mixture Ratio on the Correlations Between the Excimer XeCl* Emission and the Se

logy Vol 4, No 4(2002)Influence of the gas Mixture Ratio on theCorrelations Between the Excimer XeCI*Emission and the Sealed Gas Temperaturen Dielectric Barrier Discharge LampsO AXU Jing-zhou(徐金洲), LIANG Rong-qing(梁荣庆), REN Zhao-xing(任兆杏)Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, ChinaAbstract For dielectric barrier discharge lamps filled with various gas mixture ratios, the cor-relations between the excimer XecI"emission and the sealed gas temperature have been founded,and a qualitative explication is presented. For gas mixture with chlorine larger than 3%, theemission intensity increases with the sealed gas temperature, while with chlorine about 2%, theemission intensity decreases with the increase in the gas temperature, and could be improved bycooling water. However, if chlorine is less than 1.5%, the discharge appears to be a mixture modeth filaments distributedhegKeywords: dielectric barrier discharge lamps, gas mixture ratio, XeCl emissionPACS:52.80.-s1 IntroductionThe gas temperature inside filamentary dischargcan be determined by emission spectrum (6, 7, 8)There are quite a number of factors involved with the influence of the gas mixture ratio on the corre-improvements in efficiency of excimer lamps, such as lation between the excimer XeCl" emission and thethe gas ratio [1, 2 1, power supply (3.4, discharge char- sealed gas temperature is rarely reported. In this pa-acteristics and the structure of lamp [5 etc. For the per, the correlations are presented, and aoperat ion at high gas pressure, dielectric barrier dis-charge or filament discharge generates more heat owing to the frequent collisions among the particles suchas energetic electron, excited atoms or active species2 Experimental set-upinside discharge channels and atoms/ molecules inground state inside/ near these discharge channelsAs a result, the coaxial tubes are heating up by theheated gas till achieving a thermal equilibrium be-In our experiments, the dielectric barrier dischargetween the heat input and the loss through the tubes. lam中国煤化工 quartz tubes,This work is supported by Jiangsu Suzhou Purification Group Co.YHaCNMHG1411xU Jing-zhou et al: Influence of the Gas Mixture Ratio in Diclectric barrier Discharge L ampsgss gap: 4.0mmERsE88丐Seria numberFig 1 Discharge lamps display according to their relative UV intensity.which are served as two dielectric barrier layers. The qualitatively determined by this waylamp structure and gas-filling procedure was fully de-scribed in [2, 4). When the total pressure reached apreset level, the gas was sealed immediately. The 3 Results and discussionslamp samples prepared are shown in Fig. 1 according to their relative UV emission intensities(mainlyat 308 nm). The operation and gas parameters(theFrom Fig. 1, it is clear that the emission intenformer indicates the initial gas ratio, the latter showssity depends on the gas mixture ratio and gas pres-the total gas pressure)are also given respectively. sure. In our experiment, the optimum gas pressureThe power supply and instruments for electrical and is located at about 60 kPa with a gas mixture ratioptical measurement was also described elsewhere(chlorine) in the rang of 1.8%- 2.2%. Because of151. The surface temperature of lamp is monitoredthe non-uniformity of electric field in the dischargethrough Raytek 3i(LRL2, 8 N 14 um). Becausegap, the gas temperature rises rapidly on the surfacethere is noof inner tube during the early stage of discharge, ande/Cl2), it is then reach a constant afler a period of timdifficult to measure accurately the gas temperaturepressure, it is interesting that the gas mixture ratioFrom the surface temperature, we can approximatelyhas different influences on the correlations betweenspeculate the temperature on the surface of the inthe emission intensity and the gas temperature. Forner tube. As the Joule heat is proportional to the 1-1# and 2-2#, the emission intensity of excimernergy density jEXecI'(mainly at 308 nm) shows a decrease from atof the inner tube surface is determined roughly byr/r? To. The varying trend of gas temperature isTYH中国煤化工the radiationCNMHGPlasma Science Technology, Vol 4, No 4 (2002)55# 5, not calibrated by monochromator fac10tors)in the total gas pressure range of 21.3- 76 kPaSo it can be concluded that the steady-state XeCl品史876(B, C)vibration distribution mainly locates at lowerlevels at high pressure of bufer gas(here xenon). InNo coolingLamp2-2#DBD. the excimer xecl" is formed when a micro-20Wdischarge is extinguished (10) and so the quenchingof excimer due to electrons collision may be ignoredbecause of enhanced electron attachment to chlorineunder a lower electric field, contributing to the negative ions Cl. for the same reason that the concentration[] decreases in the discharge chFig.2 The variations of UV emission intensity withoutthe UV loss due to the absorption [11] of the radiaand with inner coolingtion parallel to the axis of channel is smaller, also thecollision quenching process of chlorine with excimercharacteristicsd obin this region may be also ignored. A. N. Panchenkoig. 2, 4 respectivelyet al (12 pointed out that the UV output was lim-Under a lower gas pressure, the excimer XeCI. ited due to overheating of the gas mixture causedcould be generated through the reactive quenching by the discharge constriction. This limitation can beof resonance states or metastable states of Xe witlgnored, as there is not any noticeable varietion inCl2 by a resonance radiation technique or a flowingdischarge channel size. Because the vibration relaxafterglow method (91. The spectrum shows obviously ation of high-lying B-states is considered as a possithe vibrational relaxations with the increase in theble mechanism for the repopulation of the low-lyingressure of buffer gas, especially in the shorter wave- levels [13 the excimers(Xeci)contributed to UVlength region of B-X band. The intensity ratio of emission mainly locate at the low-lying levels. SoIB/Ic also increases with the pressure of buffer gas. the influence of the gas temperature on UV emissionFrom the numerical simulation, the vibration-levelntensity may be focused mainly on the correlationdistribution of the excimers moves from higher to between the gas temperature and the number of ex-lower levels because of collision process. In our ex- cimers XeCI(B, C) at the low-lying states. Thisperiments, the ratio of IB-x/Ic-A remains constant could be interpreted qualitatively as followsXeCI(x)+hv;←XeC'(B,v;)←→XeC"(C,v;)→XeCl(A)+hv+M(K R,KSXeC(x)+hv;-1←XeC(B,v-1)←→XeC'(C,v;-1)一+XeC(A)+hv;-14+M(k21K21)↓+M(KB1;1)Xe((x)+hv(308mm)←XeC(B,v=0)←→Xec1XmC】A)+hv(340nm)中国煤化工↓+M(Q)CNMHGXU Jing-zhou et al: Influence of the Gas Mixture Ratio in Dielectric barrier Discharge Lamps24#.9N二A09024明)1605140No cooling No cooling No cooing上Hx1m号070000065Time/ minDischarge time/minFig. The variations of UV emission intensity and the surface temperature with and without water-coolingkr, ki is the vibrational relaxation rate constant (T), ki here is the rate constant of V-T transfer. Thisand quenching rate constant of XeCI(B, C) in theequation is different from that of Ref. 16 in which theith vibration level respectively. Provided that theseup-transfer term of excimers caused by V-T transferexcimers locate at ith levels or under ith level, the is included. As the discharge lampdissociation quenching processes could not take place the temperature of the sealed gas is rising. As aunless the kinetic energy of collision partners(Xe)is result, the number of excimers under the ith levelin the magnitude of the dissociation energy of ex- that are available to emission become less becausecimer XeC"B=0 (4.53 ev (4)or there are xenon the distribution of excimers shifts to the states cor-atoms in the resonance and or metstables sates in responding to a higher gas temperature. The numberthe previous discharge channel. There is no correla- of excimers in the states above the ith level is risingtion between the temperature and the excited xenoncorrespondingly, and these excimers are mostly dis-atoms, and the xenon atoms have so high a kinetic sociated through the collision quenching processenergy in this kind of non-equilibrium dischargefor the lamps with the ratio of xenon to chlowe can write the rate equation for the population N;rine near to or less than 97 /3, such as 2-4# lamp(B or C)in the ith critical level without consideringcontrary to the above, it is observed that the dethe coupllng between B and C states [15]crease in the temperature in the sealed gas causeddt= ri(p, o E)-As N-ki i-IIMJNi-by water-cooling leads to an immediate decrease inUV emission, as shown in Fig. 3. For these lamps[M]M+k+1[M]N;+1+k;-1A[Mwe should seek other reasons that are responsible forR is the generation rate of excimer Xecl due to the degradation of UV emission in the formation ofa three-body recombination, A i is the rate constant excimers. For a higher partial pressure of chlorine inof spontaneous emission, M is the number densitye gas mixture, the maximum UV emission intenof heated xenon atoms. The number of the heatedsity always appears at a higher pressure [2]fromxenon atoms is so much that the excimers are sur.he surface temperature of the lamp, we can approxounded by them and the energy transfer bctweenmately speculate that the temperature of the innerxcimers and the heated xenon atoms do not changehe equilibrium distribution of heated xenon atomsTH中国煤化工han the surface tem-CNMHGontaining a higher14l4Plasma Science Technology, Vol 4, No 4(2002)bno-cooing〓m二」〓0100300350400450500550600650Time/ minFig 4 Variation of the UV emission of lamps with various gas ratios of chlorine to xenon(a)relative intensity,(b)normalized emission spectra.concentration of chlorine, the rise in the gas temper- different appearances such as filament or the mixature leads to the increase in excimer numbers be- ture of filamentary and diffused discharge, and Uvcause of the increase in the discharge channels size emission shows a different trend of variation withdue to a less attachment of electrons to chlorine atand without inner water-cooling. For chlorine largerhigh gas temperature.than 3% in a gas mixture the emission intensity in-In addition to this. there are some cases in the creases with the sealed gas temperature, while forgas mixture with less than 1. 8% of chlorine in which chlorine being about 2%, the emission intensity decreases with the increase in gas temperature, andlations with the sealed gas temperature(see lampl- could be improved by water-cooling. However, if1-4#). From the appearance of discharge, theschlorine being less than 1. 5%, the discharge appearslamps operate in a mode that the bright current fil- to be a mixture mode with filaments distributed inaments scatter in the background of diffused dis-a diffused glow-like discharge and, the UV emissioncharge, which is similar to the case when addingindependent on the gas temperaturebuffer gas Hle into the mixture (51. Its emission in-tensity is lower than the two cases mentioned aboveand the emission spectra are also different, especiallyin the range of 400-550 nm, as seen in Fig 4. But, Referencesthere have been no convincing explications on theseI ZHANG Y, Boyd I w.J. Appl. Phys, 1996. 806332 XU J Z, LIANG R Q, REN ZX. Plasma Science4 Conclusion3 Mildren R P, Carman R J J. Pyhs. D: ApplWe have observed that when sealing the lamparious ratios of chlorine/ xenon, the discharge灬YH中国煤化工 X. Vacuum ScienceCNMHG415XU Jing-zhou et al: Influence of the Gas Mixture Ratio in Dielectric barrier Discharge. Lamps5 XU JZ, LIANG R Q, REN Z X Plasma Science12 Panchenko A N, Sosnin F. A, Tarasenko V F Op-&z Technology, 2001, 3: 102tics Communications, 1999, 161: 2496 Motret O, Hibert C, Pellerin S, et al. J Phys. D13 Bourne O L, Alcock A J. Appl. Phys., B 1983Appl.Phys.2000,33:14937 Bibinov NK, Fateev AA, wiesemann K.J. Phys.14 Popovic MM, Krstic P. The Physics of ionizedD:Appl.Phys,2001,34:1819Gases, World Scientific Publishing Co Pte Lid8 Nozaki T, Unno Y, Miyazaki Y, et al. J Phys. DAppl.Phys2001,34:250415 MA X, KONG F. Laser Chemistry, Published9 Dreiling T D, Seter D w.J. Chem. Phys., 1981by The University of Science and Technology ofChina, 1990, 15510 XU X Ph D. Thesis, University of Illinois(200016 Kvaran A. Appl. Phys. B, 1988, 46: 9511 McCown A W, Eden J G J. Chem. Phys. 1984(Manuscript received 13 June 2002)81:2933E-mail address of XU Jing-zhou:中国煤化工CNMHG1416