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for coloration and thermoluminescence is remark- chemistry, he is now, as he has been for more than ably sensible in present-day terms, and the discus- fifty years, one of the foremost physical chemists of sions of nuclear physics, photochemistry and other his time, topics in some of his reviews reveal a most lucid It remains only to wish Lind a happy eightieth grasp of modern theoretical developments. The birthday and continued happiness and success. quick, penetrating intelligence which shows The papers in this Jubilee Issue are but an insignifithroughout his writing and appears daily in his cant token of the esteem and affection in which he is conversation about science, world affairs, or peo- held by his colleagues. ple, could have brought him success in ‘any field. OAKRIDQENATIONAL LABORATORY Since he elected to apply his talents to physical OAK RIDQE,TENNESSEE ELLISON H. TAYLOR
CHEMICAL EFFECTS OF LOW ENERGY ELECTRONS‘ BY RUSSELLR. WILLIAMS, JR. Haverford College, Haverford, Pa. h’eceiued October 16,1068
A new technique has been developed for the study of chemical decomposit,ion of gases by low energy electrons. Ultraviolet light falling on a silver surface generates photoelectrons which are accelerated in the gas by application of an electric field. The electron yields of products formed by irradiation of methane and ethane have been examined as a function of applied potential and the behavior of the systems suggests that an important primary process is non-ionizing excitation by electron impaet.
It has been presumed for some time that radiolysis by high energy electrons produces chemical reaction by excitation2 as well as by ionization although most of the mechanisms proposed have emphasized the importance of ionization as the primary process, relegating excitation to an undetermined role. This report describes the first results of a new attempt t o investigate the chemical effects of electrons of such low energies that ionization will be impossible or inefficient. The prior literature reveals several instances in which chemical decomposition by electron excitation appears to occur. Essex and his collaboratorsa found that the ion-pair yield in the alpha radiolysis of gaseous ethane was increased by the application of an electric field in a manner which led them to conclude that electron excitation was responsible for the increase. I n like manner Meisels, Hamill and Williams4found that the ion pair yield in the X-radiolysis of methane in argon was increased by application of an electric field. Kiser and Johnston6 have analyzed the products and determined electron yields in Geiger-Muller discharge in ethanol-argon and 2-propanol-argon mixtures. They find, respectively, 285 and 56 molecules decomposed per electron collected. They conclude that the magnitude of these yields indicates a primary process of excitation rather than ionization. Mechanisms of decomposition in electric discharge have been proposed6which depend heavily on excitation as a primary process. (1) Work supported by a special grant from the Board of Managers of Haverford College. (2) In the present context the use of the term excitation will be confined to processes which do not result in positive ion formation in the initial act or in any subsequent unimolecular process. (3) E.o., N. T. Williams and H . Essex, J . Chem. Phy;., 17, 995 (1949). (4) G. G. Meisels, W. H. Hamill and R. R. Williams, Jr., THIB JOURNAL, 61, 1456 (1957). (5) R. W. Kiser and W. H. Johnston, J . A m . Chem. Soc., 78, 707 (1956): 79, 811 (1957). (6) E.@.,M.Burton and J. L. Magee, J . Chem. Phys., a8, 2194,2196 (19651.
I n the present investigation low energy electrons are introduced into a gas via the photoelectric effect from a metal surface. The electrons then are caused t o drift through the gas by application of an adjustable electric field. Chemical decomposition is determined as a function of applied potential per unit gas pressure.
Experimental Apparatus.-The reaction chamber consists of a cylindrical silver cathode 12 mm. in diameter X 120 mm. in length centered in a cylindrical anode 29 mm. in diameter X 120 mm. in length constructed from 18 mesh bronze screen. Thus the electrode sprtcing is 8.5 mm. with a maximum variation of ca. i 1 mm. This electrode assembly is encased in a Vycor 7910 jacket with appro riate gas and electrical connections as shown in Fig. 1. iome care must be exercised to avoid discharge points on the electrodes and leads. The reaction chamber is inserted within the coil of a Hanovia SC2537 mercury resonance lamp formed from 10 mm. quartz tubing shaped in the form of a four turn helix with inside diameter 50 mm. and length 100 mm. The radiation from the lamp passes through the Vycor jacket, through the screen anode and falls on the surface of the silver cathode as shown in Fig. 1. Power for the mercury resonance lamp is furnished by a 5000 volt transformer operated from a variable transformer in the primary. The electrode potentials for the reaction chamber are furnished by the d.c. supply of a GeigerMuller counting circuit and a d.c. microammeter is placed between the cathode and ground to permit measurement of the photoelectron currents, which ranged from 2 to 100 microamp. Preliminary tests on several metals resulted in the selection of silver for its relatively high photoelectron efficiency. After pretreatment of the silver by high voltage, high frequency discharge in hydrogen at ca. 1 mm. pressure the vacuum photoelectron current, collected with an applied potential 99.6 mole %; ethane, Phillips research grade, >99.75 mole %; argon, Matheson, >99.9 mole %. Procedure.-A typical experiment with methane or methane and argon proceeded as follows. The reagents were cooled in their reservoirs to liquid nitrogen temperatures and then admitted to the reaction chamber, the desired pressure being obtained by allowing the reservoir to warm slowly from liquid nitrogen temperature. This procedure was used t o reduce the possibility of introducing small amounts of contaminants which might be condensable on liquid nitrogen. A Dry Ice-acetone bath was placed around the trap next to the reaction chamber during sample introduction and the ultraviolet lamp was turned on for warm-up. After sample introduction the electric field was applied for a measured period of time and the photoelectron current observed during bhe irradiation. The lamp primary, and therefore its intensity, was adjusted during the run to maintain a constant photocurrent. It was shown that the rate of reactions is simply proportional to the light intensity. After completion of the exposure liquid nitrogen was placed around the trap next to the reaction chamber and the sample was passed slowly through the trap. Noncondensables were discarded and the condensable products (ethane, etc.) were warmed to room temperature and collected in the modified McLeod where their P V product was measured. The amount of product collected was typically 10-40 cc. X mm. at room temperature, and the reaction chamber volume was cu. 250 cc. Therefore the extent of decomposition was usually less than one per cent., except where larger samples were collected for mass spectrometric analysis. Comparison of the amount of product collected with the total charge passed through the system permitted calculation of the electron yield, Y = moles of product/ faradays of charge. The procedure for ethane was similar except that ethane was condensed on liquid nitrogen and the non-condensable products (hydrogen and methane) were collected.
Results Ethane.-Blank runs (ultraviolet irradiation with no field applied) demonstrated a small but significant rate of reaction which might be either a mercury photosensitized reaction or a photolyzable impurity in the sample. Several attempts to purify the ethane by distillation failed t o reduce the blank. However, the rate of this reaction was very consistent even after complete disassembly of the chamber for replacement of electrodes, and the rate was approximately proportional t o the pressure of ethane. These observations favor the impurity hypothesis. The correction for the blank was as high as 20% in one or two experiments a t very low fields, but otherwise always less than 10%. The electron yield, Y , in the decomposition of ethane was examined principally at two pressures as a function of applied voltage. The products collected in these experiments were non-condensable on liquid nitrogen. Mass spectrometric analysis7 of several samples showed that the hydrogen-methane ratio was ca. 5: 1. The observed yields are plotted versus V I P in volts/mm. gas pressure in Fig. 2. (The field strength is approximately uniform a t 1.18 volts/cm. a t 1 volt applied potential.) The upper limit of the measurements was determined by the onset of intermittent discharges in the (7) Courtesy of Atlantia Refining Co., Philadelphia, Pa.
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chamber a t high potentials and no results are reported in which this occurred. The lines drawn through the data are merely to show the apparent trends of the points and have no immediate theoretical origin. It is seen that, within the precision of the data, the lower points fall on straight lines having a common intercept and with slopes approximately proportional to the gas pressure.
RUSSELL R. WILLIAMS, JR.
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here does not correspond quantitatively to the decomposition experiments and such curves are given only to show their shapes. Methane.-Blank runs with methane showed 30 that the sample contained a very small amount of condensable impurity but, in contrast to the h; 20 ethane, there was no evidence of reaction upon ultraviolet irradiation. The collected impurity was shown t o be a constant proportion of the meth10 ane and therefore accurate correction was possible. Again the correction was as large as 20% only for a few experiments a t very low rates and was ordinarily much less than 10%. The electron yield in the decomposition of meth8 ane was examined principally at two pressures as a function of applied voltage and the results are shown in Fig. 3. The products collected in these 6 experiments are condensable on liquid nitrogen and mass spectrometric analysis’ of two runs showed L the following principal products: ethane, 72%; 4 propane, 19%; n-butane, 2%; isopentane, 4%. Exceptionally long runs were required to obtain sufficient product for analysis and it may be that 2 this analysis does not accurately represent the initial product distribution. As with ethane the lower yields appear t o be a linear function of V I P with a I I common intercept and a slope proportional t o the pressure. Curves of photocurrent us. V I P are 10 20 30 40 given for both pressures of methane. VIP. Methane in Argon.-Blank runs with methane in Fig. 3.-Decomposition of methane: 0 , 3 0 mm.; 0,66 mm., A, 82 mm. argon showed that no condensable impurity was obtained from the argon and only that attributable to the methane was observed. The addition of small amounts of methane to argon has a strong effect. on the current versus V / P curves. The current in the flat portion of the curve ( V / P < 3) is less by 25-50 than in pure argon and the “plateau” is longer than in pure argon. The yields of condensable products as a function of applied voltage were determined for three mixtures as shown in Fig. 4 and the corresponding photocurrent curves were obtained. The yield curves are of the same general form as those obtained for I I I 1 I I pure ethane and pure methane, but the yields are higher and the reaction occurs a t lower values of V / P , The data a t 10 mm. CH4, 190 mm. Ar include two points which do not fit the presumed curve. The low yield at V I P = 6.8 is a result of an unusually long exposure which was subsequently 2o shown to result in diminished yield. There 15 is no evident reason for the high yield a t V / P = 5. Mass spectrometric analysis’ of two runs * showed a product composition essentially the same 10 as that observed in irradiation of pure methane. Discussion 5 The chief point of interest in the present study is the assessment of the significance of excitation as opposed t o ionization in the primary process. I I I I I I I While there are practically no positive ions formed 4 5 6 7 8 2 3 a t low V / P , the sharp rise of the current a t higher VlP. V / P , e.g., V I P > 20 for ethane at 38 mm., indiPig. 4.-Decomposition of methane in argon: 0,10 cates significant ionization by electron impact t o min. CHI, 190 mm. Ar; 0, 10 mm. C&,400 mm. Ar; A, form positive ions and additional electrons. The 20 mm. CH4, 390 mm. Ar. positive ions thus formed could lead t o product Figure 2 also shows the current-voltage curve a t formation through ion-molecule reactions and/or the lowest ethane pressure. The scale of current charge neutralization at the cathode. The onset of 40
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CHEMICAL EFFECTS OF Low ENERGY ELECTRONS
June, 1959
auch ionization cannot be clearly defined since the electron swarm acquires a broad distiibution of energies as it drifts through the gas under the influence of an applied field.8 I n the cases of methane and ethane the onset of chemical reaction with increasing V I P occurs a t values of V I P comparable with those a t which ionization sets in. I n the argon-methane system there appears to be a significant difference between these subjective “thresholds” but because of the breadth of the electron energy distribution this is a nearly worthless test. Two characteristics of the yield curves strongly suggest that excitation is an important primary process in these systems. First, the shapes of the chemical yield curves are clearly different from those for electron multiplication, i.e., positive ion formation, which are approximately exponential. Secondly, the values of the chemical yield are so large as t o defy explanation by any ordinary ionic mechanism. I n radiolysis the ion-pair yield for production of non-condensable gas from ethane is l.!jaand for decomposition of methane it is 2.2.9 The behavior of these systems may be analyzed in further detail as follows. The dependence of the number of electrons collected at the anode, n, on the number leaving the cathode, no, the distance between electrodes, l, and the gas pressure, P is well known to be8
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As suggested by this equation N/(n - no) has been plotted versus V I P in Fig. 5 for the three systems. (The quantity N / ( n - no)is also the yield of prodn = no exp(aiP1) (1) uct molecules per positive ion, which is to be conin a parallel plate system a t constant X / P (field trasted with the ion pair yields in radiolysis.) The strength per unit gas pressure). The coefficient ai lack of any true “plateau” in the ionization curves represents the number of new electrons created per makes it difficult to make an objective determinaunit distance per unit gas pressure. It is found tion of no (from lo) and this introduces a large unthat ai is a smooth function of X / P alone, to be certainty into values of ap/aiat low values of n discussed below. no. In spite of this difficulty it is clear that ap/ai In a manner analogous to that used in deduction decreases with increasing V / P . Therefore the of equation 1, we now write an expression for the formation of products cannot be a simple conserate of formation of product molecules as a function quence of a single ionization process. Since dissociaof P and I, assuming that a fixed number of product tive excitation may be expected to occur a t enermolecules is formed as an ultimate consequence of gies below that required for ionization the evidence each productive electron impact. Let N be the indicates that excitation is an important process in number of product molecules and ap the number of the formation of products. product molecules created per unit distance per The dependence of ai on X / P has been experiunit pressure. The latter property may be expected mentally determined for a few simple gases, but no to be a function of X / P . manageable theoretical expression is available for description of this behavior, largely due to difficuldN = apn d(P2) Substituting for n by equation 1 and integrating ties in describing the electron energy distribution. There is, however, a simple equation derived by yields Townsend but now known to be inadequatelawhich N (aP/ai)no [exp(aiPI) - 1 J (2) approximates the observed behavior. This is of This equation indicates that the electron yield, Y = the form N/n, at constant X / P should depend on P and 1 as ai = A exp [ -BEi/(X/P)] N / n = (ap/ai)[l - exp( -aiPZ)] (3) where Ei is the ionization energy. If the dependAt small values of the exponent, N/n = a,P2 and ence of a, on X / P were of the same form, but with the yield is approximately proportional to the gas Ei replaced by E, < E i , then the ratio a , / a i would pressure. vary with V I P ( X / P ) as shown in Fig. 5. I n It is more interesting to examine the behavior of fact a plot of log ( a p / a i ) us. l / ( V / P )is nearly linap/ai with changing V I P (proportional to X / P ) . ear, but unfortunately the theory is inadequate to For this purpose it is appropriate to replace exp(ai justify such a quantitative interpretation. PI) by n/noand rearrange equation 2 to the form I n the argon-methane system reaction occurs at (8) See L. B. Loeb, “Fundamental Processes in Electrical Dismuch lower V / P than in the case of pure methane. charge in Gases,” ea,. Ch. VIII, John Wiles and Sons, Inc., New This can be understood from examination of curves York. N. Y.. 1939. of mean electron energy, E , vs. X / P for various (9) 8. C. Lind and D. C . Bardwell, J . Am. Chsm. Soc., 48, 2335 gases.1° These show that for argon e rises rapidly (1926,. Only about 25% of the product is condensable.
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ROBERTC. BRASTED AND CHIKARA HIRAYAMA
with increasing X / P , reaching ca. 12 e.v. a t 2 volts/ cm./mm. This corresponds t o the energy of the first excited state of argon. Evidently the loss of energy by elastic collision is very inefficient and the energy of the electrons increases readily until inelastic collision is possible. On the other hand, a polyatomic molecule can undergo inelastic collisions a t much lower energies. The data show that e increases much more slowly with X / P and the increase continues to much higher values of X / P than in the case of a monoatomic gas. Consequently much higher values of X / P will be required to produce an appreciable number of electrons of the high energy required for chemical decomposition. In the argon-methane mixtures the possibility exists that either component could be the seat of the primary excitation. If it were argon it then would be necessary t o postulate a process such as A* CH, = CH3 H A to initiate reaction. The higher yields of methane decomposition in the presence of argon and the increase in yield with increas-
Vol. 63
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ing proportion of methane tempt one t o adopt this view. However, the decision should not be made on this basis, for the electron energy distribution is certainly a function of gas composition and is undoubtedly a strong factor in determining the observed yields. Little can be said a t present about the nature of the products of these reactions other than t o note that they correspond, qualitatively, to those obtained in radiolysis reactions. It is difficult t o conceive of a radical reaction which could produce a product such as isopentane from methane a t room temperature. The possibility remains that ionic reactions may contribute some small part of the total product yield and may be primarily responsible for such a product. Further investigation to examine this and other aspects of the problem has been initiated.
(10) B. Rossi and H. Staub, "Ioniaation Chambers and Counters," Ch. 1, N. N. E. S., Div. 5, Vol. 2, MoGraw-Hill Book Co.,New York, N. Y.. 1949.
Acknowledgment.-The author wishes to acknowledge the several valuable suggestions and comments received from Prof. W. H. Hamill of the Universit,y of Notre Dame during the conduct of this work
+ +
AN EXAMINATION OF THE ABSORPTION SPECTRA OF SOME COBALT(II1)AMINE COMPLEXES. EFFECT OF LIGAND AND SOLVENTS I N ABSORPTION BY ROBERTC. BRASTED AND CHIKARA HIRAYAMA Contribution from the School of Chemistry, University of Minnesota, Minneapolis, Minn. Received October 24, 1968
There are numerous reports in the literature with regard to the absorption spectra of coordination compounds of cobalt(II1). Practically all of these reports are concerned with the absorption spectra in aqueous solutions. There are some inconsistencies in the data in regard to the absorption characteristics of some of the cobalt compounds. I n some cases the positions of maxima are not clearly defined in aqueous solutions, whereas some of these maxima become clearly defined in non-aqueous solvents. A study was initiated to coordinate some of the absorption spectra of some of the cobalt(II1) amine complexes, compare a.nd contrast reported spectra from other sources, and to make a study of the absorption spectra in alcohols as well as in aqueous solutions. Certain of the spectra are interprete,d in terms of crystal field theory. Experimental Preparation of Compounds.-The compounds were prepared by well-established methods.'-* Most of these com(1) H. F. Walton, "Inorganic Preparatione," Prentice-Hall, Inc., New York, N. Y., 1948, p. 91. (2) A. Werner, Ber., 40,4821 (1907). (3) S. M.Jorgensen, 2. a n o ~ uChem., . 14,416 (1897). (4) S. M.Jorgensen, J . prakt. Chem., 42, 211 (1890). (5) W. C. Fernelius, "Inorganic Syntheses," Vol. 11, John Wiley and Sons, Inc., New York, N. Y., 1946,p. 222. (6) W. C.Fernelius, ibid., Vol. 11, p. 222. (7) A. Werner and A. Frohlich, Ller., 40, 2228 (1907). (8)J. C.Bailar, Jr., "Inorganic Syntheses," Vol. IV, John Wiley and Sons. Inc., New York, N. Y., 1953,p. 176.
pounds were assayed by analyzing for cobalt, and the analyses in each case showed satisfactory agreement with the theoretical percentage of cobalt. The diaquo complexes were prepared only in solution, by hydrolysis of the corresponding dichloro complexes. Method of Analysis for Cobalt.-The analyses for cobalt were done by the electrolytic method.9 I n the analysis of the ethylenediamine and pro ylenediamine complexes, it was first necessary to treat t f e sample with concentrated sulfuric acid and ignite to remove t,he amine. The residue was then dissolved by treatment with nitric acid and a 30% HzOgsolution. The nitric acid subsequently was removed by evaporation to fumes after adding sulfuric acid. Determination of Absorption Spectra.-The absorption spectra in the various solvents were determined at room temperature by use of a Cary recording spectrophotometer. Fused quartz cells of two- and ten-cm. lengths were used. Certain of the com lexes isomerize as soon as they are dissolved, e.g., cis-dichyoro complexes of the tetramine, bis(ethylenediamine) and bis-( propylenediamine). Some are unstable in aqueous solutions in which they undergo rapid aquation, such as the chloro complexes. Still others are unstable to heat, such as the nitritopentammine cobalt( 111) chloride. Therefore, the solids of the more readily soluble compounds were weighed into volumetric flasks and the solutions were made immediately before determining the spectrum. In this way, it was possible to determine the spectra of these compounds within 15 to 20 minutes after the solutions were made. In those instances in which the compounds were only slightly soluble (as the nitritopentamminecobalt(II1) chloride in methanol) a solution was made by shaking a large excess of Lhe solid with the solvent followed by rapid filtration and then obtaining the spectrum. The (9) I. M.Kolthoff and E. B. Sandell, "Textbook of Quantitative Inorganic Analysis," 3rd Ed., The Macmillan Co., New York. N. Y., 1953, p. 410.
