10984
J. Phys. Chem. 1993,97, 10984-10988
Radio Frequency Plasma Decomposition of Ammonia: A Comparison with Radiation Chemistry Using the GValue George P. Milled and James K. Baird'a Department of Chemistry, University of Alabama in Huntsville, Huntsville, Alabama 35899 Received: March 31, 1993; In Final Form: July 23, 1993' We have measured G(-NH3) for ammonia vapor passing through a 13.56 MHz, inductively coupled discharge operating within a wide range of pressures (3.5-40 Torr), flow rates (1-3 L/min (STP)), and absorbed power (0.2-1.4 kW). Under these conditions, we found values of G(-NH3) lying between 4.2 and 30 molecules/ 100 eV. By comparison, under similar conditions, the efficiency of ammonia conversion by a radio frequency, capacitatively coupled discharge is 6.0-20 molecules/lOO eV (Baird, J. K.;Miller, G. P.; Li, N. J. Appl. Phys. 1990, 68, 3361), while for ionizing radiation it is 2.7-10 molecules/lOO eV (Peterson, D. B. The Radiation Chemistry of Gaseous Ammoniu, Report No. NSRDS-NBS 44, U S . Department of Commerce, Washington, DC, 1974). The differences are probably due to the specific form assumed by the electron velocity distribution in each case. The similarities with respect to order of magnitude, however, may have their origin in a common reaction mechanism. Exploiting this hypothesis, we have derived a formula for G(-NH3) in terms of scattering cross sections, the electron velocity distribution, and appropriate photochemical quantum yields.
1. Introduction
Taking ammonia as an example, the purpose of this article is to explore to what extent common principles are shared by the plasma chemistry and the radiation chemistry of gases. This theme has already had a distinguished history in the hands of Lind and Burton, who have summarized their results in extensive reviews.lJ There exists also some literature linking plasma chemistry and radiation chemistry to photochemistry.3-5 Ammonia provides a rich source of evidence for these connections. For example, one finds it reported in the literature that the production of hydrazine increases with increasing flow rate when ammonia gas is stimulated by photons,6 1-MeV electrons? or a radio frequency discharge.8 These observations collectively imply that hydrazine is destroyed by back reactions associated with the same excitation that produces it. The radiation chemistry of ammonia vapor has been shown to be independent of linear energy transfer (LET), which suggests that electron-ion pairs cause most of the observed chemical ~ h a n g e .The ~ average energy expended by an ionizing particle in producing one electron-ion pair in ammonia is FV = 28 eV.l0 The largest part of the 28 eV shared by the average electron-ion pair is likely dissipated by collisionsof the electron with ammonia molecules. Platzman stressed that electric dipole selection rules governing the absorption of light could be applied to an electron-molecule ineastic collision, if the de Broglie wavelength of the momentum transferred was greater than the size of the m~lecule.~ For low energy electrons, this implied that the energy transferred in a collision was less than AE = h2/2maz,where m is the electron mass, a is the radial extent of the molecular electronic wave function, and h is Planck's constant divided by 27r.11 Although no definite value can be assigned to the size of a molecular wave function, we can take in the case of a small moledule a = 1.06 A, which is twice the radius of the first Bohr orbit of the hydrogen atom. With this estimate, we compute AE = 5 eV. Electrons having energies, E, less than 5 eV perforce satisfy this criterion. For high-energy electrons, the maximum AE consistent with the PlatzmancriteriondependsuponEandisgiven by A E = 3.7(E)V, where both E and AE are expressed in electronvolts. In the case of low-pressure discharges, current probes have shown that the electron energy distribution peaks at about 5 eV Also University of Alabama System Ph.D.Program in Materials Science. *Abstract published in Aduance ACS Absrracrs, October 1, 1993.
0022-3654/93/2097- 10984$04.00/0
and extends negligibly beyond 15 eV.12J3 It is plausible, although not certain, that the Platzman criterion applies to a large fraction of the electron-molecule collisions taking place in the discharge. The four main excitation events produced by photon absorption by ammonia are14
-
NH,
+ hv
NH,
+ hv
NH,
+ hv NH,
NH2(wzBl)
+ -
+H
+ H, NH(X3Z) + H + H NH(a'A)
hv
NH,'
+e
(1.1) (1.2) (1.3) (1.4)
Within the dipole approximation, eqs 1.1-1.4 should also be the primary events in the radiation chemistry of ammonia. This argument may apparently be extended to apply also to the plasma chemistry of ammonia where both NH(alA) and NH(X3Z), as well as NH2, have been reported to occur.15.16 Given the existence of primary species, such as those on the right hand sides of eqs 1.1-1 -4,final products are formed through secondary reactions (termed dark reactions in photochemistry) that proceed in the absence of the excitation. As a quantitative measure of the final products, the standard in photochemistry is the quantum yield, which is the number of molecules converted per photon absorbed. In radiation chemistry, the practice is to employ the G value, which is the number of molecules converted per 100 eV of energy deposited by the ionizing radiation. By contrast, in plasma chemistry there is no accepted standard measure, although in previous work we have adopted the G value.17J8 This practice was originated by Burton2 and finds support in the work of both Eremin, who correlated the yield of products with the volume energy density in the plasma,19 and Kline, who has linked yields to the energy absorbed per molecule of reactant.20 We report herein experimental measurements of the yield of destroyed ammonia, G(-NH3), obtaing using a low pressure, inductively coupled radio frequency discharge. We make a quantitative comparison of our values of G(-NH3) with those tabulated by Peterson9 for ionizing radiation and also with those which we have calculated previous1yl7using experimental results21 obtained with a capacitativelycoupled radio frequency discharge. Assuming a reaction mechanism, we derive a formula which links 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 42, I993 10985
Plasma Decomposition of Ammonia ITD Mass Spcctometer
To vacuum
T G%l?p’e
Pump
Coaxial Cable
L rf Generator
Figure 1. Schematic diagram of experimental apparatus.
G(-NH3) to the quantum yields for various processes involved in the photochemical destruction of ammonia.
2. Experimental Apparatus A schematic drawing to the experimental apparatus is shown in Figure 1. Power Supply. The radio frequency power source was a Model HFS 2500-D supply with a Model AMN 2501E impedance matching network, both obtained from RF Plasma Products, Inc. The supply operated at 13.56 MHz and was capable of delivering up to 2.5 kW. The power supply was connected through the matching unit to a four-turn inductance coil wound from 5-mm 0.d. by 3-mm i.d. copper tubing. The radius of the coil was 32 mm, while its length was 28 mm. The coil was cooled by passing deionized water through it. Gas Handling. The plasma was contained in a 45-mm 0.d. by 42-mm i.d. fused quartz tube. With machined Teflon spacers, the plasma tube was held inside and concentric with an outer quartz tube, which had the dimensions 58-mm 0.d. by 54-mm i.d. Both tubes were 1 m in length. Deionized water flowed through the annulus between the tubes. The inductance coil was wound around the outer tube. Ammonia with a stated the purity as 99.99% was admitted to the inner tube at a rate of 0-5 L/min (STP) through an MKS Model 1159 B flow controller. To start the discharge in the plasma tube, one of the tube supports was touched with an energized Tesla coil. The pressure in the system was determined using a Granville Phillips Convectron vacuum gauge, Series 275, and the system was pumped by a 310 L/min Precision Vacuum Pump, Model DD 310. The products of the discharge were condensed by passing them through a liquidnitrogencooledtrap. Upon warming, the contents of the trap were analyzed using a Perkin-Elmer Model 8240 GC/ MS. The chromatograph contained a Chrompack PLOT, fused silica 25 m X 0.32 mm ID column packed with 5A molecular sieve. The output of the chromatograph fed a Perkin-Elmer ITD ion trap mass spectrometer. The GC/MS was connected
to the gas handling system through a gas supply valve obtained from the Valco Instruments Co. Calorimetry. Fiber glass insulation was used to insulate the quartz tubes from the laboratory air. The temperature of the water circulating between them was measured using two K-type thermocouples connected through a Fluke 80 TK thermocouple module to a Fluke Model 45 digital multimeter. To obtain the electrical energy absorbed by the plasma, the heat registered by the calorimeter was corrected for the heat of formation,22AHf0 = -46.15 kJ/mol, released by the decomposed ammonia. The calorimeter system was calibrated by measuring the heat deposited in the water by a known voltage applied to a standard resistor. The heat absorbed by the calorimeter apparently accounted for all the energy produced by the plasma. Various thermocouples, which were inserted into the exit gas stream as well as placed against the outside of the outer quartz tube, all registered room temperature. The radiant heat loss was less than 1%,which was shown by observingthe change in the rate of rise of the calorimeter temperature after making the coolant water opaque by adding India ink. Our observation concerning the minor role of radiant heat confirms that of Hanes and BairaZ3 Determination of G(-NHs). Determination of G(-NHp) depended upon evaluation of the formula17
G(-NHJ = 7.17@A@’/ W (2.1) where j 3 is ~ the fraction of the ammonia molecules decomposed by the discharge, 9’ is the gas feed rate in L/min, and Wis the power absorbed by the gas in kilowatts. In eq 2.1, C(-NH3) is reckoned in molecules/ 100 eV. By multiplying the reading on the mass flow controller by the elapsed time, the number of moles, no, of ammonia fed to the discharge was determined. The gas exiting the discharge, which was caught in the liquid nitrogen cooled trap, was shown by GC/ MS analysis to consist entirely of ammonia. [In a separate experiment, both nitrogen and hydrogen were pumped through the chilled trap, and neither was found to be condensable with our system.] We thus concluded that the overall reaction in our plasma was After the discharge when the contents of the trap were warmed to room temperature, the number of moles, n, of unreacted ammonia was computed from the measured pressure and the known volume of the vacuum system. The value of /?A was then computed using (2.3) 3. Experimental Results
It was possible to establish a steady-state plasma discharge over a wide range of pressures, flow rates, and values of input power. The plasma appeared within the coil as a pinkish-red doughnut, whose axis of symmetry was coincident with that of the coil. Depending upon pressure, flow-rate, and input power, a flowing after-glow extended up to 60 cm beyond the end of the coil. Figure 2 summarizes our results for G(-NH3) and b.t, as functions of power for a number of pressures and a fixed flow rate of 1 L/min. In general, the power required to consume the ammonia with 100%efficiency (BA = 1) increased with increasing pressure. As BA approached unity, however, the energy of the plasma was increasingly deposited in the products, and the efficiency, as measured by G(-NH3), diminished. Above a pressure of 20 Torr, we did not achieve 100% conversion of ammonia to products at 9’ = 1 L/min. Figure 3 summarizes the results obtained at a flow rate of 2 L/min. Above a pressure of 30 Torr, it was impossible to decompose all the ammonia available.
Miller and Baird
10986 The Journal of Physical Chemistry, Vol. 97, No. 42, I993 4'= 3L /MIN
4'8IL /MIN 151,
, , , , , , , , , , , , , , , , JI 100
- 50
7 30
:I
TORR
0
.O
I
- 50
-I 25
1;'
5 t
l'i[
-0
- 50
25 TORR
0
7 q -
50
5
- 75 - 50 - 25
25
",
-
I -
I
(3
5
0
7.5 TORR
25
3.6 TORR
- 75 - 50 - 25
0
01
20
0 200 400 800 800 1000 1200 1400 1600 1800
"0
200
400
600
W(WATTS)
Figure 2. G(-NH3) (vertical dashes) and , ! 3 ~(circles) at a' = 1 L/min as functions of pressure and absorbed power. @'a2L/MIN
100 50
25
(3
15 TORR
Figure 4. G(-NH3) (vertical dashes) and , ! 3 ~(circles) at a' as functions of pressure and absorbed power.
+ NH, - N H , + H + e'
+ NH, NH, + NH3 H + NzH4 NH, + N2H3 H
50
IOL ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' 1 I 0 0 200 400 600 800 1000 1200 1400 1600 1800 W(WATTS)
Figure 3, G(-NHs) (vertical dashes) and , ! 3 ~(circles) at a' = 2 L/min as functions of pressure and absorbed power. Figure 4 shows results for a flow rate of 3 Llmin. Above a pressure of 10Torr, it was impossibleto approach 100%conversion of the ammonia. Using mixtures of nitrogen and hydrogen, we carried out experiments to see if ammonia could be regenerated in our discharge. We were unable to detect, however, more than a 1% conversion. Hence, the decrease in G(-NH3) with increasing power and pressure, which is generally observed in Figures 2-4, is not due to back reactions re-forming NH3. 4.
Theory
The energy thresholds for the photodissociation reactions given by eqs 1.1-1.4 are4.5,5.6,8.9, and 10.2eV, respectively.14Taking
3 L/min
the minimum energy (4.5 eV) required for reactive excitation, therecan be at most 10014.5= 22 dissociations/lOO eV absorbed by the plasma. By contrast, G(-NH3) reaches a maximum value of 30 in our experiments. See Figure 4. To account for the larger than expected value of G(-NH3), we propose a chain mechanism based upon the species, H and NH2, acting as chain carriers. Since we have not carried out the experiments required to detect each of the intermediate species which we shall introduce, our mechanism has the status of an hypothesis rather than a theoretical proof of our observations. Moreover, for the sake of simplicity, we shall concentrate on only eqs 1.1 and 1.4 as the primary steps in the mechanism. 4.1. Neutral Species. We first consider the following mechanism involving only neutral intermediates: e
0
1000 1200 1400 I600
800
W (WATTS)
+ H,
-- ++ -- + + ---*
N,H,
k, = W , ( u ) ) (4.1)
NH,
k1.1
N2H4 H N2H3 H2 N,H,
N,
H,
k1.z k1,3
NH3
k1,4
k1.5
(4.2)
(4.3) (4*4)
(4.5) (4.6)
In eq 4.1, e is an electron with velocity D, while e' is an inelastically scattered electron with lower velocity. The cross section for collisional dissociation of NH3 into NH2 and H is q(v), while kl is the corresponding rate constant. The angle brackets indicate an average over the prevailing electron velocity distribution. The rate coefficient of the jth dark reaction is klJ wherej = 1, ,..,5. Equations4.2and4.3 arethechainpropagating steps in the mechanism. By assuming that the production rates of H, NH2, N2H4, N2H3, and NzH2 are at steady state, we find for the rate of disappearance of NHj -d(NH,)/dt = 2klNe(NH,) (4.7) where (NH3) and Ne are the particle densities of ammonia molecules and electrons, respectively. If we replace eq 4.1 by eq
The Journal of Physical Chemistry, Vol. 97, No. 42, 1993 10987
Plasma Decomposition of Ammonia 1.1 andapply thesteady-stateapproximation,we find thequantum yield for photodestruction of ammonia by this mechanism to be cp1 = 2. 4.2. Ionic Species. Positive ions and electrons are necessary to maintain the discharge. Collisions between NH3+ and NH3 rapidly convert the former to NH4+.24925 The principal ionization and neutralization steps in the plasma are probably e
+ NH3
-
NH3++ 2e'
-
k2 = (vg,(v))
+ NH, NH: + NH2 e + NH4+-+ NH3 + H
NH,'
kz,, k2,2
(4.8) (4.9) (4.10)
In eq 4.8, u2(u) is the cross section for the collisional ionization of NH3 by impact with an electron having velocity u. If eqs 4.8-4.10 are combined with eqs 4.2-4.6, we obtain a chain mechanism leading to products, N2 and Hz. Eqautions 4.8-4.10 serve as chain initiation steps, while eqs 4.2 and 4.3 again serve as chain propagation steps. By assuming that the rates of production of NH3+, NH4+, NHz, N2H4, H, N2H3, and N2H2 are each at steady state, we find that therateofdisappearanceofNH3according tothismechanism is
-d(NH3)/dt = 2k2N,(NH3)
(4.1 1)
If we replace eq 4.8 by eq 1.4 and apply the steady-state approximation, we find that the quantum yield for photodestruction of ammonia by the above mechanism is = 2. 4.3. Theory of G(-NH3). By way of the mechanism of section 4.1, which has quantum yield 01 = 2, ('/2)(4.5 eV) is absorbed from the plasma per ammonia molecule destroyed. To convert ammonia to products with an efficiency of G(-NH3) = 30 molecules/ 100 eV, only 67.5 eV are required to pass through eq 4.1, leaving 32.5 eV to be absorbed through nonchemical energy loss channels such as elasticscattering and rovibrational excitation. Because 10.2 eV must be expended per ionization via eq 4.8, the mechanism described in section 4.2 is by itself insufficient to account for G(-NH3) = 30. Nevertheless, the ionization mechanism does serve as an additional sourceof product. Without specific knowledge of the electron velocity distribution, however, it is impossible to know what fraction of electrons pass through eq 4.1 as compared with eq 4.8. By combining the results of sections 4.1 and 4.2, the overall disappearance of ammonia is given by
As required, the sum in eq 4.12 can be augmented to take into account the destruction of ammonia by mechanisms based upon the electron impact equivalents of eqs 1.2 and 1.3, respectively. On the basis of eq 4.12, the G value may be expressed as follows: 17.18
(4.13) The numerator of eq 4.13 is proportional to the overall rate of conversion of ammonia to products, while the denominator is proportional to the power absorbed by the gas through electronmolecule scattering events, including those such as eqs 4.1,4.8, and 4.10, which are associated with chemicalchange. In eq 4.13, m is the electron mass, M is the mass of the ammonia molecule, ae(v) is the electron-molecule elastic collision cross section, ah( u ) and are the cross sections and energies, respectively, of all rovibronic transitions of an ammonia molecule, CI is the ionization
potential, and a&) and k, = k2,z = ( U U , ( U ) ) are the cross section and rate constant, respectively, for capture of an electron by NH4+. The factor of 100 converts the denominator, which we assumeis computed in electronvoltsto 100's of electronvolts (heV) required by the definition of G(-NH3). Equation 4.13 demonstrates how data on photochemical quantum yileds and electron-molecule scattering cross sections can be combined to compute a G value. If the electron velocity distribution were known, the integrals indicated by the angle brackets could be evaluated and eq4.13 could be compared directly with experiment. Lacking that, eq 4.13, nevertheless, allows one to draw some qualitative conclusions concerning the origin of the density dependenceof G(-NH3). Since the electronvelocity distribution will depend upon the ammonia density, G(-NH3) will depend upon (NH3) implicitly through the integrals represented by the angle brackets. Moreover, given a sufficient lifetime of ammonia electronic excited states, NH3*, preceding formation of the primary products summarized by eqs 1.1-1.4, we may need to add to our proposed mechanisms quenching collisions, NH3* NH3 2NH3. In the case of a mechanism to which a quenching reaction has been added, the corresponding quantum yield will depend upon (NH3),I7
-
+
5. Discussion
The measured values of G(-NH3) encountered in our experiments ranged from a low of 4.2 molecules/ 100eV (a' = 1 L/min, W = 1420W, pressure = 25 Torr) to a high of 30 molecules/ 100 eV (V = 3 L/min at W = 455 W, pressure = 10 Torr and at W = 303 W, pressure = 20 Torr). By comparison, calculating on the basis of data obtained by DAgostino et al. from a capacitatively coupled discharge operating at 35 MHz, we found values of G(-NH3) lying between 6 and 20 molecules/l00 In Peterson's compilation of radiolysis results for NH3, G(-NH3) ranged from 2.7 to 10 molecules/ 100eve9 According to eq 4.13, some disparity among these sets of values is not unexpected, however, since the electron velocity distributions prevailing in the three cases are almost certainly different. The observed overlap in G(-NH3) values may be sufficient to suggest a common reaction mechanism, however, such as that analyzed in section 4. We may summarize the theoretical results of section 4 as follows: (1) Equations 4.1-4.13 indicate how in principle data on electron-molecule scattering cross sections, rate constants, and photochemical quantum yields can be combined to compute the value of G. This has been a long-time goal in radiationchemistry. (2) The effects of the various electron-molecule scattering cross sections are segregated in eq 4.13. Only thosecross sections which lead to chemical change appear in the numerator, while the cross sections for all energy transfer events appear in the denominator. This segregationof cross sectionsis implicit in the difinition of G(-NH3) as the efficiency with which electron kinetic energy is converted to chemical change. (3) A pressure dependenceof the G value is implied in eq 4.13. The effect of pressure may enter not only through the quantum yields but also through the electron velocity distribution upon which the electron velocity averaged quantities in eq 4.13 necessarily depend. The electron velocity distribution and its pressure dependence are initimately linked to the electron-molecule scattering cross sections through the Boltzmann transport equation. Some advances in solving this equation have recently been reported.2629 (4) Platzman stressed that within the dipole approximation electrons populate the same excited states of molecules as do photons.3 Not all electron-molecule scattering events have photochemical analogs, however, even within the dipole approximation. Electron-attaching molecules provide a case in point.
10988 The Journal of Physical Chemistry, Vol. 97, No. 42, 1993
+
For example, the dissociative attachment reactions,30 e NHJ -NH2+ H-ande+NH3-NH2-+ H,whichwehaveneglected in our analysis, would seem to have no photochemical equivalents. (5) In deriving17J*eq 4.13, we have ignored both wall reactions and the ambipolar diffusion of ions, which may be important at the low pressures employed in our experiments. In the former instance, the chemistryinvolves a second phase while in the latter it involves concentrationgradients. In both cases the kinetics are inhomogeneousand could be included in the theory by adding the pfoper surface reactions, transport equations, and boundary conditions. By integrating over the reaction volume and its bounding surfaces, the global rates of reaction and energy loss could be calculated. Since the C value is a global quantity, the ratio of the former to the latter would yield a theoretical expression for G(-NH3). In recognition of the many physical and chemical processes shared by photochemistry, radiation chemistry, and rf plasma chemistry, the use of terminology having a common etymology may be warranted. Already well established are the words, “photolysis”and “radiolysis”,based upon the Greeksuffix, “lysis”, meaning to split. We suggest here adding the term, “plasmolysis”, meaning chemical decomposition by plasma discharge. Although “plasmolysis” is a recognized biological term referring to the bursting of a cell wall by osmotic pressure effects,31its use in the context of electric discharge reaction kinetics would probably not lead to confusion.
Acknowledgment. J.K.B. is grateful for the hospitality of the Condensed Matter and Radiation SciencesDivision at the Naval Research Laboratory, where part of this article was prepared. The authors would like to thank one of the reviewers for pointing out the importance of eq 4.2. This research was supported by the National Aeronautics and Space Administration under Contract NAS8-37195 with the Marshall Space Flight Center. Acknowledgment is also made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
References and Notes (1) Lind, S. C. Radiation Chemistry of Gases; ACS Monograph No. 151; Reinhold: New York, 1961.
Miller and Baird (2) Burton, M.; Funabashi, K. Adu. Chem. Ser. 1%9,80, 140. (3) Platzman, R. Radiation Res. 1962, 17, 419. (4) Auslm, P. Ed. Fundamental Processes in Radiation Chemistry; Wiley-Interscience: New York, 1968. (5) Nicholas, J. E.; Spiers, A. I.; Martin, N. A. Plasma Chem. Plasma Proc. 1986,6, 39. (6) Groth, W.; Romel, H. J. Z . Phys. Chem. (Munich) 1965, 45, 96. (7) Jones, F. T.; Sworski, T. J. Trans. Faraday Soc. 1%7,63, 241 1. (8) Anderson, W. H.; Zwolinski, B. J.; Parlin, R. B. Ind. Eng. Chem. 1959, 51, 527. (9) Peterson, D. B. The Radiation Chemistry of Gaseous Ammonia; Report No. NSRDS-NBS 44, US.Department of Commerce, Washington, DC, 1974. (10) Spinks, J. W. T.; Woods, R. J. Radiation Chemistry, 3rd ed.; Wiley-Interscience: New York, 1990; p 520. (1 1) Bethe, H. A.; Jackiw, R. W. Intermediate Quantum Mechanics; W. A. Benjamin, Inc.: New York, 1968, p 299-300. (12) Semiokhin, I. A.; Andreev, Yu.P.; Salimova, K. M.; Gorvat, F. Yu. Russ. J. Phys. Chem. 1968, 42,473. (13) Abramov, V. L.; Svettsov, V. I. Sw.J. Plasma Phys. 1978, 4, 638. (14) Okabe, H. Photochemistry of Small Molecules; Wiley-Interscience: New York, 1978; pp 270-271. (15) Foner, S.; Hudson, R. L. J . Chem. Phys. 1966, 45,40. (16) Robinson, G. W.; McCarty, M. J. Chem. Phys. 1959, 30,999. (17) Baird, J. K.; Miller, G. P.; Li, N. J. Appl. Phys. 1990, 68, 3661. (18) Baird, J. K.; Miller, G. P. Trends Chem. Phys. 1991, 1, 119. (19) For a history of this concept as well as Eremin’s other work on plasma
chemistry, see: Gerasimov, Ya. I.; Topchieva, K. V.;Mal”,
A. N.; Pentin,
Ya.A.;Kitaev,L.E.;Viliev,L.V.;Tatevskii,V.M.;StraLhov,B.V.;NeLrasov, L. I.; Rubtsova, E. A. Russ. J. Phys. Chem. 1978,52, 1566. (20) Kline,L.E.;Partlow,W.D.;Youn&R.M.;Mitchell,R.R.;Congendo, T.V. IEEE Trans. Plasma Sci. 1991, 19,718. (21) D’Agostino, R.; Cramarossa, F.; DeBenedictis,S.; Ferraro,G. Plasma Chem. Plasma Proc. 1981,1, 19. (22) Hammil, W. H.; Williams, R. W., Jr. PrinciplesofPhysicalChemisrry; Prentice-Hall: Englewood Cliffs, NJ, 1959; p 66. (23) Hanea, M. H.; Bair, E. J. J. Chem. Phys. 1%3,38,672. (24) Dorfman, L. M.; Noble, P. C. J. Chem. Phys. 1959,63, 980. (25) Dawson, P. H.; Tickner, W. J. Chem. Phys. 1964, 40, 3745. (26) Mozumder, A. J. Chem. Phys. 1982, 76, 3277. (27) Koura, K. J. Chem. Phys. 1984,81, 303. (28) Shizgal, B. Chem. Phys. Lett. 1987, 138, 65. (29) Kushner, M. J. Appl. Phys. 1989, 66, 2297. (30) Compton, R. N.; Stockdale, J. A,; Reinhardt, P. W.Phys. Reu. 1969, 180, 111. (31) Glasstone, S. Textbook of Physical Chemistry, D. Van Nostrand: New York, 2nd 4.1947; ; p 661.
