/ J ¥4~r~~~:
'o j~ ~7 ~,~U;~I u t= ~]f~LL*~ ~~~~~~; 4. Isotope Separation in Discharge Plasmas of Molecular Gases
EZOUBTCHENKO, Alexandre N.t, AKATSUKA Hiroshi and SUZUKI Masaaki Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, Tokyo 152-8550, Japan
(Received 25 December 1997)
Abstract We review the theoretical principles and the experimental methods of isotope separation achieved through the use of discharge plasmas of molecular gases. Isotope separation has been accomplished in various plasma chemical reactions. It is experimentally and theoretically shown that a state of non- equilibrium in the plasmas, especially in the vibrational distribution functions, is essential for the isotope redistribution in the reagents and products. Examples of the reactions, together with the isotope separ- ation factors known up to the present time, are shown to separate isotopic species of carbon, nitrogen and oxygen molecules in the plasma phase, generated by glow discharge and microwave discharge.
Keywords: isotope separation, vibrational nonequilibrium, glow discharge, microwave discharge, plasma chemistry
4.1 Introduction isotopic species will be redistributed between the rea- Plasmas generated by electric discharge of molecu- gents and the products. We can elaborate a new lar gases under moderate pressures (10 Torr < p < method of the plasma isotope separation for light ele- 200 Torr) are usually in a state of nonequilibrium. We ments . can define various temperatures according to the kine- There were a few experimental studies of isotope tics of various particles in such plasmas, for example, separation phenomena in the nonequilibrium electric electron temperature ( T*), gas translational temperature discharge. Semiokhin et al. measured C02 enrichment ( To), gas rotational temperature ( TR) and vibrational in i3C02 and 12C160180 with a separation factor a ~ temperature ( Tv), where the relationships T* > Tv ;~ 1.01 in the C02 Silent (barrier) discharge in the pres- TR- To usually hold. Especially pronounced non- sure range of 100 - 760 Torr [2]. They measured 02 equilibrium in found in the plasmas of N2, C02 and CO enrichment in 180 with a = 1.09 through dissociation discharges. The high vibrational temperature. can acti- of C02, and nondissociated C02 enrichment in 13C vate some endothermic reactions of the molecules listed with (x = 1.024 in the glow discharge plasma of pure above, and at the same time, Iower To Prevents reversal C02 (30 - 100 Torr). They also observed relatively high exothermic reactions, which can not be activated by (oe - 1.10) ozone enrichment in 180 during ozone syn- vibrations. Since isotopic species of these gases have thesis in the 02 Silent discharge under 760 Torr [3]- different vibrational quanta, they can have different The latter seems to be profitable for oxygen isotope en- vibrational distribution functions (VDF). This phe- richment because of relatively small energy cost of nomenon is similar to the Treanor effect in the VDF of ozone molecule ( - 15 - 20 eV) in the industrial anharmonic oscillators [ I J . It can result in the different ozonizers. There were, however, no further investiga- rate constants of endothermic reactions, therefore the tions in this field. The silent discharge in the molecular
tPresent Address: Laboratory of New Methods of Isotope Separation, Molecular Physics Institute. Russian Research Center Kurchatov Institute, Kurchatov Sq. 1, 123182, Moscow, Russia corresponding authorb e-mail: hakatsuk@ nr. titech. ac.jp
233 ~~ ;~7 ･ ~~~~~~A~i'~~~~~-~"~* 1998~~ 3 ~ gases provides a nonequilibrium condition ( T. >> To), but the role of vibrations was not elucidated. 1 .O The important role of vibrations in the gas dis- 0.9 charge became clear when the CO and C02 plasmas were discovered to be active media for infrared lasers. 0.8 :~ It was established both theoretically and experimentally O, (DL 0.7 :: that CO, C02 and N2 can be easily vibrationally excited O = 0.6 by an electron impact in the glow discharge with a high O 1'L rate constant k*v * 10-8 cm3/s at T* -- I - 2 eV. A G,O 0.5 a, resonance exchange of vibrational quanta between the ~- O 0.4 same molecular species usually has the rate constant E ~:O kvv * 10-11 _ 10-9 cm3/s, which is much larger than C:IO 0.3 L that of vibration-to-translation energy transfer kvT * LL 0.2 10-19 _ 10-14 cm3/s. It is the fundamentals for the vi- brationally nonequilibrium state in the gas discharge 0.1 with an appropriate pressure and an ionization degree. 0.0
0.1 4.2 Basic qualitative concepts of possible sep- Reduced electric1 .O field I E/N0.0 [ 10-16 v cm2] aration phenomena in the vibrationally 0.35 1.0 1.5 2.5 3.5 nonequilibrium gas discharge plasmas Average electron energy T* [ eV J In discharging plasmas of diatomic or triatomic ga- Fig. I Channels of electron energy loss in C02 electric dis- seous molecules, it is well-known that it is possible to charge: I - C02 rotations, 2 - C02 vibrations, 3 - ex- direct most of the electron energy into the molecular vi- citation of electron levels and ionization. brational energy, by adjusting the reduced electric field E/N into the range of (2 - 6) X 10-16 V ･ cm2 [4]･ For example, the energy balance of the electrons in the C02 plasma is shown in Fig. I as a function of E/N, which 10+16 controls T*. A rate constant of the excitation of the co -'~ lower lying vibrational levels of C02 by an electron im- ~o I0+15 pact is estimated to be k*v = 10-8 cm3/s at T* = I eV. ~~, We can estimate kvv to be 10-9 cm3/s at To = 300 K F~ OH I0+14 12C160 for lower lying vibrational levels of C02 and kvT ~ 11:; c~ 10-14 cm3/s for the fastest process of C02 v2 bending ,~Ff P~o l0+13 mode. Therefore some kinds of a vibrational population p~ 13C160 distribution will be formed with Tlo = (hv)/(kln(No/ F: L~3Ho I n+'^2 N1)) >> To, where h is the Planck constant, k is the c~; Iv :H Boltzmann constant, N1 and No are the number den- 12C180 ~*H> sities of the population of the first excited vibrational l0+11 level and of the ground state, respectively. For a dia- o 5 Io 15 20 25 30 35 V tomic gas CO, such a VDF was experimentally vibration level measured up to the 40th vibrationally excited level. As Fig. 2 The CO vibrational distribution function (VDF) in the an example, the VDF's for the components of 12C160, CO - N2 - He giow discharge. E/N * 10-16 V･cm2, To 13C160 and 12C180 are shown in Fig. 2 [5]. We can see * 100 K, T~o = 2,000 - 3,000 K. that they are not Boltzmann distributions, but that each of them has a plateau for (5 - 7) < v < (30 - 40), We have to consider more complicated systems of where v is the vibrational quantum number. Such char- the vibrational excitations in triatomic gaseous mole- acteristics of the VDF are determined by the anhar- cules, since they have plural vibrational modes. Bailly et monicity of molecular vibrations. The drop with the al. measured a VDF for the C02 v3 asymmetric mode angle - 1/To in the populations of the highly excited in the glow discharge of the mixture of C02 - N2 - He levels appears when kvT' which is proportional to v, for a C02 Iaser up to v3 ~ 10 [6]. They showed that an becomes larger than kvv' observed VDF slightly deviated from the Boltzmann
234 /i'4~~i~ 4. Isotope Separation in Discharge Plasmas of Molecular Gases Ezoubtchenko, ~i~~~~ ~^.~~
N 13 16 Nv mA > mB 1 E-1 V( C O) NVIll2r L 16nvl l No NOA << N.^_~ KB << KVT ,: Ea E'a :SO '~' A :, l S ,l ll2Cl60 I11) t ~'10 O Cl SO a [ nat'Jrat= C:
:~!O 1 E-2 af B ,:L N 12 18 > V( C O) :-O ~5 N 12 16 ,S V( C O) 15 O: nClsO 2 1 10 '2CldO ) -3 [ '1'~ral= vibratiOn level V 1 E-3 10 Fig.4 Vibrational distribution functions in the isotopic vibration level mixture of diatomic molecules for a not rapid O 20 30 40 chemica] reaction. E~ denotes the activation energy. Fig. 3 Relative vibrational populations of heavier isotopic components in a glow discharge in the mixture CO - N2 - He. transferred through a quasiresonance energy exchange, which is called a V - V' quasiresonance exchange, simi- distribution in the region of v3 * 7 - 8. lar to the exchange among anharmonic oscillators, but Belenov et al. first suggested the possibility of ap- with a larger resonance defect. A qualitative diagram of plication of the vibrational nonequilibrium for isotope a possible VDF for the not rapid chemical reaction is separation [7]. Subsequently, the difference in a VDF shown in Fig. 4, where Ea and Ea' are the possible acti- for isotopic species of diatomic gaseous molecules, such vation energy of the reaction. The activation energy Ea as 12C160 - 13C160 or 14Nl4N - 15Nl4N, was calculated in Fig. 4 makes the reaction products enriched in the A [8, 9]. It is shown that there will be a strong difference component. Meanwhile, the activation energy Ea' re- in the population of highly excited vibrational levels for sults in the opposite sign. isotopic components when we keep Tro >> To' Figure Concerning a rapid chemical reaction with the 3 shows the strong excess of the populations of the vi- rate constant kR, that is, when the drop of the VDF is brationally excited states for 13C160 and 12C180 in the CO - N2 - He glow discharge. The ratio amounts to about 5 times as much as that of their natural abun- Nv dance [5]. Such excess will give a difference in the rate No constants for some endothermic reactions activated by mA > mB vibrational energy of isotopic components. Ea NOA <
235 ~~ ;~7 ･ ~~~~Ar~'~~~~~-~O~ ~~74~~~~ 3 ~~- 1998~j~ 3 ~ determined not by kvT but by kR, the qualitative Fig. 3). The isotopic effect in Eq. (5) in the glow dis- scheme for the VDF is shown in Fig. 5. At this time the charge can be masked by another channel of CO de- component B will react faster than the component A. composition (by a direct electron impact, which seems to be nonselective), and by subsequent nonselective re- 4.3 Experiments on vibrationally nonequili- actions of atomic carbon. Gorse et al. showed the con- brium plasmas ditions of the glow discharge for the predominance of Basov et al. found the isotope separation in a vi- pure vibrational CO dissociation mechanisms [ 1 5 J . brationally nonequilibrium plasma for the first time in With respect to the possible isotope separation in a the world [11]. They measured the enrichment in 15N nonequilibrium plasma, a group of P.N. Lebedev In- of nitrogen oxide after the impulse glow discharge in stitute studied the reaction of the carbon dioxide de- the mixture of N2 - 02 cooled by LN2 through the composition in the glow and microwave discharges [10, process 16]. N2 + 02 = 2NO. (1) C02 = CO + 1/2 02' (6) The main elementary processes were considered to be The concept of their study was the same as in the the followings: previous investigation of Eqs. (3) and (5). Isotopically variant molecules 12C02 and 13C02 can have different 02 + e~ = 20 + e~, (2) populations in highly excited vibrational levels, there- O + N2(v) = NO + N fore their rates of dissociation into CO molecules are with the activation energy E* = 3.3 eV, (3) different from each other. There can be two possible channels of dissociation: ( 1) through the excitation of N + 02 (v) = NO + O with E. = 0.3 eV. (4) vibrational levels up to the dissociation energy Equation (3) seemed to be isotopically selective C02(v) - CO + O with E* = 5.5 eV, (7) due to a different population of the appropriate levels near E*, namely at v ~ 12, for 14Nl4N and 15Nl4N or (2) the direct excitation by an electron impact components, where 15Nl4N was considered to have larger populations. Equation (4) was not considered to CO + e - C02* - CO + O + e~. (8) be isotopically selective with respect to oxygen isotopes, It depends on the discharge conditions which channel is because of the small influence of vibrations on the reac- dominant. The most probable final stage of C02 de- tion due to the small activation energy. composition is as follows: Further investigations of Eq. (1) did not confirm C02(v) + O = CO + 02 With E. = 1.5 eV. (9) large separation factors. The separation factors up to ce = 1.2 were observed [12, 13]. They were smaller than This reaction seems to be isotopically nonselective or the theoretically possible ones for a pure vibrational ex- can have an opposite effect to Eq. (7) concerning the citation in Eq. (1). It can be explained by the existence sign of selectivity. of nonselective channels of NO production in the N2 - By spectroscopic measurements of the ratio 13CO/ 02 glow discharge, such as the production of nitrogen 12CO in the gas mixture of C02, CO and 02 after a atoms by an electron impact with a subsequent non- glow discharge in the initially pure C02, it was shown selective stage (4) of NO production. that CO was enriched in 13C with a * 1.3, whereas Another possible candidate for carbon and oxygen carbon monoxide produced after a glow discharge in isotope enrichment was the reaction of CO dispropor- the C02 Iaser mixture C02 - N2 - He was depleted in tionation 13C with a ~ 2 [10]. They explained the results by as- suming the different C02 dissociation channels. In the CO(v) + CO(v) = C + C02 With E. = 6 eV. (5) pure C02 discharge, the electron temperature was T* * In a LN2 cooled CO impulse glow discharge, Abzia- 2 - 3 eV. C02 dissociates mostly by a direct electron nidze et al. observed small C02 enrichment in 13C and impact from the lower lying C02 vibrational levels. (x * 5 in the ion C20+, which was supposedly pro- Under this condition, 13C02 dissociates faster (see Fig. duced under a fragmental ionization in the mass-spec- 4). On the other hand, in the mixture C02 - N2 * He, trometer from carbon suboxide C302 [14] . The possible the electron temperature T* ~ I eV, which is the best selectivity of Eq. (5) is determined by the strong dif- conditions for C02 vibrational excitation (see Fig. 1). ference in VDF's of 12C160, 13C160 and 12C180 (see Consequently, these conditions establish the relation-
236 .J'4~~{~ 4. Isotope Separation in Discharge Plasmas of Molecular Gases Ezoubtchenko, ~~~~~, ~^v7l~
ship kD >> kvT' therefore 12C02 dissociates faster (see phenomenon gives us a profitable method of carbon Fig. 5). The carbon monoxide produced by microwave and oxygen isotope enrichment. discharge in pure C02 under moderate pressure ( = The possibility of isotope separation by Eqs. (3) 100 Torr) was also depleted in 13C with a * 2.5, and (5) was also demonstrated in the nonequilibrium where T* ~ I eV and the dissociation is mainly at- conditions created by another method being different tributed to the vibrational one. The dissociation degree from electron impact. For example, Akulintsev et al. of C02 Was about 25 "/*, which meant that the nondis- studied the carbon isotope separation in the expanding sociated C02 Was enriched in 13C with (x * 1.3. hot gaseous mixture of CO and Ar passing through a As an example, an experimental scheme for the Laval nozzle, where the relationship Tv >> To also C02 dissociation in a microwave discharge is shown in holds [ 1 8]. They reported that separation factors of the Fig. 6 [ 1 7]. The microwave generator was operated at carbon isotopes were ce * 2 - 6. the frequency 2.45 GHZ (~o = 12.2 cm). Its output In addition, Eq. (5) was investigated in a CO gas power was up to 5 kW. The microwave was fed through pumped optically by a CO Iaser [19]. Spectroscopic rectangular waveguides with its dimension of 1 10 X 55 measurements of a C2 visible emission from the gas vol- mm2, 90 X 45 mm2 and 72 x 34 mm2, which were ter- ume revealed that the diatomic carbon molecule was minated by a closed-end short circuit or by a water enriched in 13C12C, whose molar fraction became up to cooled load. The plasma chemical reactor was made of 8 "/* of that of 12C2' It means that the significant selec- a quartz tube of its inner diameter 26 - 28 mm, cross- tivity was accomplished mainly through Eq. (5). The ing the waveguide perpendicularly to the wide side. The subsequent reactions of carbon with CO, however, C02 flow was regulated by a flow-meter within the made the carbon into a stable compound C302- range of 0.1 - O.6 nl/s. The power absorbed by the polymer, where the final separation factor was only GL plasma was I - 2 kW, and the discharging pressure was ~ 1.2. about 100 Torr, which meant E/N - (2 - 5) X 10-16 When we study the carbon isotope separation Vcm2. The gas temperature measured in the after-dis- under the conditions of a large ( > 25 "/.) C02 dissoci- charge flow was relatively small ( * 700 K), whereas ation degree, we have to take account of possible C02 - the vibrational temperature in the discharging region CO exchange reactions, such as exchange by an oxygen was about 3,000 K. The difference in these two tem- atom, peratures explains the observed isotopic effects. 12C02 + 13CO - 12CO + 13C02' (10) When we take account of the small specific energy consumed in the microwave discharge ( Ew = I - 5 eV/ We can see that this reaction leads to the carbon molecule C02) and a large throughput of the device isotope redistribution between C02 and CO. An equili- (0.5 g C02/s), it is evident that the further study of this brium constant for Eq. (10) at T = 300 K was calcu- lated to be 1.055 by Urey [20], shifted to the right. Of course, rate constants for forward and reversal reac- l CO gas tions are small at room temperature. Under conditions Gas holder of microwave discharge, however, the rate constant of flow meter this exchange reaction may be as large as that of the gas distributor C02 dissociation. It is not clear whether the equilibrium Quartztube QMS-200 constant of Eq. (10) is larger than I or not, because Magnetron both reactions are activated by C02 and CO vibrations. generator Pc Exchange reactions P=4.9kW Waveguide [) f~2.45GHz ¥ / C160160 + 180160 - C160180 + 160160 (11) ~ pump Receiver can lead to C02 enrichment in 180. This reaction was studied in a plasma generated by a silent discharge [2] . ･~, J They measured C02 enrichment in 180 with (x = I .024, whereas the equilibrium constant for Eq. (11) at T = 600 K was calculated to be 1.01 1. Fig. 6 A schematic diagram of the experiments on a car- Jaffe and Klein studied the rate constants of the bon isotope enrichment by microwave discharge in C02 flow. following exchange reaction [21]
237 フ。ラズマ・核融合学会誌 第74巻第3号 1998年3月
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