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Transition Yields in Irradiated Gases
Radiation Chemistry Downloaded from pubs.acs.org by COLUMBIA UNIV on 12/06/18. For personal use only.
C O R N E L I U S E . KLOTS, Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. Yields are computed for a number of electronic transitions i n gases under alpha-particle irradiation. The results are found to be usefully insensitive to approximations i n the computational procedure. Compari son with experiment permits commentary on the latter. Deviations from the predictions of the optical approximation are interpreted as departures from high-energy equivalence. Research sponsored by the U . S. Atomic Energy Commission under contract with Union Carbide Corp.
A Microwave Cavity Method of Studying Recombination Following Pulse Radiolysis M . S. Z U C K E R and L . A . F E R R A R I , Brookhaven National Laboratory, Upton, Long Island, Ν. Y. Microwave transmission through a cylindrical resonant cavity is used to study high density ( . — 1 0 e cm." ) low temperature (0.025 e.v. 0.050 e.v.) plasmas formed at high neutral pressures (.—1 atmosphere) of various gases. A plasma column is formed by passing a pulsed beam of high energy electrons from a V a n de Graaff accelerator along the axis of the cavity. The plasma starts to decay immediately after the pulse passes causing the resonant frequency of the cavity to shift with time. Since the resonant frequency can be correlated with plasma density, the latter can thus be followed as a function of time. Slater's perturbation theory can be relied upon to supply the relation between frequency shift and plasma density only when ( ω / ω ) < < 1 , where ω is the plasma, and / = ( ω / 2 π ) the microwave frequency. F o r ( ω / ω ) ^> 1, and par ticularly when ( ν / ω ) « 1 (where ν is the collision frequency) it is neces sary to solve exactly the field equations for a cavity with ionization present in it. The conditions ( ω / ω ) > 1 and ( ν / ω ) > 1 correspond precisely to that situation where a method utilizing an accelerator beam to produce uniform ionization is most appropriate, namely, for "dense" media with a high degree of ionization. The exact solution is known, and though complicated has been reduced to a computer code. The method is dem onstrated for a situation where Slater's theory is valid. Choices of cavity dimension, microwave modes, etc., are discussed. 3
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G a s - P h a s e P u l s e R a d i o l y s i s of O x y g e n a n d the B e n z e n e A r g o n - H y d r o g e n Systems J. F. R I L E Y and R. W . C A H I L L , Lockheed Palo Alto Research Laboratory, Palo Alto, Calif. Lockheed Missiles & Space Co. ( L M S C ) has a gas-phase pulse radiolysis program funded by L M S C as an independent research project. The program began in 1967 although L M S C has operated a 2 Mev. Febetron since 1965. Our first pulse radiolysis experiments were to measure the rate of formation of ozone from oxygen at 200-800 torr. Ozone formation was pseudo-first order with a half-time of about 10 microseconds. The reaction was complete in 60 microseconds; ozone formation was followed by monitoring the optical absorption i n the ultra violet region on a microsecond time scale. A 9558Q E M I photomultiplier, 0.5 meter Ebert spectrometer, and H g - X e arc were used, together with a Tektronix 555 scope. The same apparatus was used to study two ternary systems: benzene-argon-hydrogen and benzene-argon-helium. In the re gion from 265 to 325 nanometers transient absorption spectra were re corded in both systems. The system containing hydrogen produced a spectrum characteristic of the cyclohexadienyl radical. W e are making a detailed comparison of spectral differences and relative yields i n the two systems. SOLIDS
G r o w t h a n d D e c a y K i n e t i c s of R e a c t i o n I n t e r m e d i a t e s F o r m e d i n the R a d i o l y s i s of O r g a n i c S o l i d s J O H N E. W I L L A R D , University of Wisconsin, Madison, Wis. This paper w i l l discuss initial growth rates, steady-state plateau concentrations, and decay rates of trapped free radicals and other reaction intermediates produced by radiolysis of: (a) glassy and polycrystalline alkyl halides at 77°K.; (b) glassy hydrocarbon matrices containing dis solved alkyl halides at 77°K.; and (c) polycrystalline C B r . Evidence w i l l be presented: (a) that most radicals combine with other radicals within the parent spur; (b) that in some systems combination is exclu sively with the sibling partner; and (c) that the rate of decay of trapped radicals is "radiation catalyzed"—i.e., is faster during exposure to ionizing radiation than i n its absence. Effects of temperature, type of matrix, viscosity of the matrix, and isotopic substitution i n the matrix and in the decaying radicals have been investigated. Methods of producing radicals have included irradiation with C o γ-rays, with tritium beta rays and with ultraviolet light. Photoionization of tetramethyl-p-phenylenediamine 2
e o
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541 to produce electrons which cause dissociative electron capture of alkyl halides has also been used.
Change of Electric Conductivity of Organics by Gamma Irradiation A. B R E C C I A / S. D E L L O N T E , F. F U O C H I , and J. N U C I F O R A , Centro di Chimice delle radiasioni del C.N.R., Bologna, Italy
Studies have been carried out on the conductivity in d.c. on the following dried or melted compounds: (1) Polyamides at M = 2,000; 10,000; 16,000; 40,000; 60,000. (2) Urea, thiourea, selenium urea; xanthene, thioxanthene, Se-xanthene; and selenium-chromone-4-olo. (3) Linear hydrocarbons and their keto and acid derivatives with C numbers in the range of 24 to 32. The gamma doses varied in the range of 0.2 to 4 Mrads with a 4 Χ 10 rads/min. dose rate. A n increase in conductivity in all the compounds and other findings were noted: (1) Before the irradiation polyamides showed a decrease of con ductivity at the increasing M except at M 60,000. After radiolysis the conductivity was higher but with the same value at different M , except at M 2000. (2) For selenium derivatives and homologues the increasing con ductivity was correlated with the distribution of the new traps and with the induced stable radicals. (3) For hydrocarbons and derivatives, the increase of conductivity varied widely as a function of physical phase (melted or powdered) in a range 10 to 10° higher. A decrease of energy activation with i n creasing molecular weight (2.2-1.2 e.v.) was noted. n
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Present address: University of Manchester, England.
ORGANIC LIQUIDS T h e Lifetime of Charged Species in Irradiated Dielectric Liquids ANDRIES H U M M E L , Reactor Institute, Delft, Netherlands
The distribution of lifetimes of the charged species, formed i n cyclo hexane by high energy radiation, is calculated, using the experimental results on the number of these species reacting with solute molecules as a function of the concentration of the solute. The time i n which half of the ions recombine, when no reaction with a solute takes place, is 6 X
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10" seconds. Because of the fact that the diffusion coefficient of the negative entity in cyclohexane at room temperature is very large (2.5 X 10" sq. cm. sec." ), scavenging of this entity increases the lifetime of the ions considerably. The effect of scavenging of the negative entity on the lifetime distribution is calculated. Absolute rate constants for reactions of the charged species with solutes are also calculated. 10
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Luminescence Studies as a Device for Investigation of Early Processes in Radiation Chemistry P E T E R K. L U D W I G and M I L T O N B U R T O N , Radiation Laboratory, University of Notre Dame, Notre Dame, Ind. 46556 Details of the neutralization process following radiation-induced primary charge separation may be examined via the medium of ultrafast techniques now employed in studies of luminescence decay processes. As an example, the form of luminescence decay curves of dilute organic scintillator in aliphatic hydrocarbon solution excited by x-ray pulses of about 0.5-1.0 nsec. duration is attributed (in previous papers) to neu tralization processes involving ions. The relation, ί α ή for the time required for neutralization of an ion pair of initial separation r, when applied to such curves, leads to a distribution function of ion-pair sepa rations. A more appropriate and desirable approach involves solution of a diffusion equation (which includes a Coulomb interaction term) for various initial conditions. Such solutions are obtained by computer tech niques employed in analogy to corresponding electrical networks. The results indicate that the t oc r law affords a fair description of the decay if the initial distribution can be assumed to be broad. The Radiation Laboratory is operated by the University of Notre Dame under contract with the U.S. Atomic Energy Commission. This is A E C document number COO-38-606. 3
Negative Ion Formation in the Radiolysis of Perfluorocarbon Solutions in a-Hexane L . A. R A J B E N B A C H , Soreq Nuclear Research Centre, Yavne, Israel The radiolytic yields of hydrogen, hexenes, dedecanes, and hydrogen fluoride in solutions of perfluorocarbons in n-hexane were determined. Perfluoroalkanes from C F i upward were found to depress the radio lytic hydrogen yield. The reduction in hydrogen was accompanied by formation of hydrogen fluoride. Perfluorocyclocarbons were found to 4
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543 have the most pronounced effect on the hydrogen yield. The ability of perfluoroalkanes to reduce the hydrogen yield is correlated with the capacity of these solutes to attach thermal electrons in a resonant process, as measured in the gas phase by mass spectrometric and microwave cavity resonance techniques. The fact that the decrease in radiolytic hydrogen yield in solutions of perfluoro unsaturates is accompanied by the formation of hydrogen fluoride is attributed to the, as yet otherwise unestablished, ability of perfluoro olefins to undergo resonant electron attachment. The constancy of the radiolytic yields of hexane in solutions of perfluoroalkanes in n-hexane is interpreted as confirming our previous assumptions that the neutralization reaction between solvent molecular cation and an electron does not lead to the formation of molecular hydrogen and that the decomposition paths of the excited states formed by direct action of energy and those resulting from charge recombination reaction are not identical. Our results suggest also that the autoionization lifetime of the solute anions may be a more important factor affecting the radiolytic decomposition of solvent than the solute's cross section for electron capture.
Gas-Phase Photolysis of Cyclohexane in the Photoionization Region P. AUSLOOS, R. E . R E B B E R T , and S. G. LIAS, National Bureau of Standards, Washington, D . C. 20234 Cyclohexane (ionization energy, 9.88 e.v.) has been photolyzed at 1236 A . (10.0 e.v.) and 1048-1067 A . (11.6-11.8 e.v.). A l l major products have been determined in the absence of free radical scavengers and i n the presence of added N O and 0 . In addition, c y c l o - C D i 2 - H S mixtures were irradiated to determine the free radicals formed in the decomposition of the superexcited cyclohexane molecule. Accurate quantum yield determinations have been made, both by use of saturation current measurements during photolysis and by chemical actinometry. It is seen that an increase in photon energy results in an increase in the relative importance of processes producing H atoms or alkyl radicals while the yields of products attributed to "molecular elimination" processes, such as the formation of molecular hydrogen, diminish. Similar trends are seen in the solid-phase photolysis. The relative importances of the various primary processes are derived. The application of this information to the interpretation of the radiolysis of cyclohexane is discussed. 2
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Kinetics of the Radiation Induced Isotopic Exchange between Iodobenzene and Iodine H . ELIAS and R. RIESS, Lehrstuhl fur Kernchemie, Technische Hochschule Darmstadt, Germany In literature the radiation induced isotopic exchange reaction be tween iodobenzene and labelled iodine Ph-I + i*l
M M ^ ph.*! + £
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is discussed on the basis of the following steps: P h - I — M M _
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In this paper the kinetics of Reaction 1 were studied in benzene as solvent at 30 °C. in the presence and absence of oxygen ( Co-radiation). For R, the rate of exchange, the following relationship was obtained: eo
^ 1 (4) l + b[I ],/[Ph-I] > This rate law cannot be explained on the basis of Reactions 2 and 3. It is instead compatible with the following steps: H =
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*Ι· + Ph-I->Ph-*I + 1-
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Phenyl radicals seem to be involved only in the formation of radiolysis products, like diphenyliodonium iodide and tri-iodide, respectively. The detailed mechanism of Reaction 1, the results of scavenger experiments, and the influence of radiolysis products on the isotopic exchange are discussed.
Excitation and Ionization Yields in Cyclohexane F. S. D A I N T O N , E. C. F R A N K E V I C H , T. M O R R O W , G . A. S A L M O N , and G. F. T H O M P S O N , The University of Leeds, Cookridge High Energy Radiation Research Centre 1
For solutions of naphthalene in cyclohexane the yield of triplet states of naphthalene, the intensity of monomer and excimer fluorescence, and the yield of singlet state naphthalene have been measured as a function of naphthalene concentration by pulse radiolysis methods. The influence of a number of electron scavengers on these yields has been determined
545 and also the influence of xenon which is known to quench singlet states by enhancing the rate of intersystem crossing. The electron scavenger nitromethane was found to be of particular interest since in addition to scavenging electrons it was found to be an efficient singlet state quencher, and these effects were observable in distinct concentration regions. The participation of charge neutralization processes in the production of excited states has been clearly demonstrated by observations of the influence of electrical fields on the radiation induced fluorescence from naphthalene solutions. 1
Present address: Institute of Chemical Physics, Moscow.
Olefin Formation in the Radiolysis of w-Hexadecane L. G. W A L K E R , W . A. SUNDER, and W . E. F A L C O N E R , Bell Telephone Laboratories, Inc., Murray Hill, N . J. Prior studies of the condensed phase radiolysis of n-hexadecane ( I , 2) have not included data on olefin formation. Consequently, the products could not be correlated with a complete material balance. W e have now developed a method of coulometric titration to an amperometric endpoint which gives total olefin content at low concentrations i n high molecular weight paraffins. This technique, together with gas chromatography on polar columns stable at high temperatures, provides a quantitative measure of the yields of both congruent olefins (hexadecenes) and of the scission or fragmentation olefins (ethylene through the pentadecenes). The radiolysis of n-hexadecane close to its melting point i n both solid and liquid phases has been examined using these techniques with the objectives of extending the material balance data, of understanding specifically the mechanisms whereby olefins are formed, and of distinguishing generally between ionic and radical precursors to the radiolytic end products. The fragmentation olefins are, within the experimental error, all linear and terminal. A t least those from 1-hexene to 1-tetradecene have similar yields. A striking feature both in liquid and solid state radiolyses is that for the low molecular weight products, G (olefin) is only 0.1-0.2 G (paraffin). This low yield ratio of olefins to paraffins is incompatible with radical processes' being the sole mode of formation of the low molecular weight products. Although the congruent olefins were not resolved individually, nevertheless it could be shown by gas chromatography that 1-hexadecene is only a small contributor to the C i olefin fraction. The mechanisms 6
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whereby congruent olefins are formed both in the liquid and solid phases are considered, and the yields of olefins are correlated with those of other major radiolytic products. (1) Falconer, W. E., Salovey, R., J. Chem. Phys. 44, 3151 (1966). (2) Salovey, R., Falconer, W . E., J. Phys. Chem. 69, 2345 (1965).
Excitation in Some Irradiated Organic Liquid Systems G A B R I E L STEIN, Hebrew University, Jerusalem, Israel
The yield of excited states in organic liquids, and i n particular i n benzene solutions, is critically compared using results from light emission measurements, light absorption i n pulse radiolysis studies, and total chemical yields, i n particular as determined by isomerization methods. It is shown that the total yield of excited states originating from solvent excited states and from solute excited states formed without previous solvent excitation is not less than G = 5 . Quantitative data are obtained for the yields of the different precursors including singlet and triplet states.
Radiolysis of Perfluorobicyclohexyl at Elevated Temperatures F. W . B L O C H , D . R. M A C K E N Z I E , and V. H . W I L S O N , Brookhaven National Laboratory, Upton, Ν. Y. 1 1 9 7 3
Perfluorobicyclohexyl is conspicuously less stable thermally than most other fluorocarbons. A t temperatures up to 3 5 0 ° C . we have found that its radiolytic decomposition rate is moderate and similar to that of other alicyclic fluorocarbons. However, in the temperature region above 3 8 5 ° C . and at pressures above 1 0 0 atm. G-values for its radiolytic decom position increase drastically. A t 3 8 5 ° C , G . F 2 t y 5 0 , indicating that a chain mechanism is just becoming effective. At 4 5 0 ° C , G . F 2 * approximately 1 0 , so that a chain of great length is operating. The apparent activation energy for the radiolytic decomposition at high pres sure is 1 1 0 ± 1 0 kcal./mole. A t pressures of 1 - 2 atm., thermal decompo sition rates are much higher than at 1 0 0 atm., but radiolytic decomposition rates are low. In fact at 4 1 5 ° C . and 1.6 atm. radiolysis does not appear to contribute to the over-all decomposition rate. Reasons for the behavior are discussed in the light of the reactions known to occur in the complex system. C I 2
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Radiolysis of Liquid Formamide: Gaseous Products and Pulse Data A. M A T S U M O T O and N . N . L I C H T I N , Boston University, Boston, Mass. 02215 Steady Co-gamma radiolysis at a dose rate of approximately 10 e.v. m l . " min." of dried, extensively purified, deaerated formamide gives four principal gaseous products with the following dose-independent yields per 100 e.v.: G ( H ) = 0.9, G ( C O ) = 0.6, G(COo) = 0.2, G ( N H ) = 0.3. N 0 , equilibrated at pressures of 5-500 torr, has little if any effect on G ( H ) or G ( C O ) but gives yields of N per 100 e.v. which increase from 0.2 to 1.9 over this range. Added water, 10"- to 1 0 M , has little if any effect on G ( H ) , G ( C O ) , or G ( C 0 ) but increases G ( N H ) by an amount which decreases with increasing dose. (Correction of radiolytic yields for concurrent hydrolysis is necessary only for N H . ) Pulsing with 1.15 Mev. electrons (work done at Brookhaven National Laboratory) results in a transient absorption spectrum characterized by an intense maximum at 400 n.m. and a shoulder centering around 500 n.m. Decay of absorption at 400 n.m. follows second-order kinetics (k/el = 1.6 X 10* sec." ) and is slower than the apparently first-order decay observed at 500 n.m., for which the specific rate is 3 X 10 sec." . Mechanistic speculations w i l l be offered. G0
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The Gamma Irradiation of Isoprene in the Presence of Vermiculite CYRIL P O N N A M P E R U M A and C O L I N S. M U N D A Y , ' Exobiology Division, National Aeronautics and Space Administration, Ames Research Center, Moffett Field, Calif. The irradiation of isoprene, either by ultraviolet photolysis ( L i u and Hammond, J. Am. Chem. Soc. 85, 477, 1963) or by cobalt-60 rays, normally leads to the production of cyclic polymerization products. H o w ever, γ-irradiation of isoprene absorbed in clay minerals of known cata lytic action, produced substantial yields of acyclic polymers containing discrete multiples of the basic 5-carbon skeleton. Analysis of these hydrogenated acyclic polymeric products, by gas chromatography and mass spectrometry, indicated that the C^ isoprenoid farnesane and the C compound phytane were present. Since importance is attached to the presence of pristane C i and phytane in distinguishing hydrocarbons of biogenic origin from those of abiogenic origin in ancient sediments, the results may lead to a reappraisal of the established criteria on such ques tions as the origin of hydrocarbons in sedimentary rocks. 2 0
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Initiation Mechanism of Radiation Induced Polymerization H . YOSHIDA, Η. ΚΑΜΙΥΑΜΑ,' KA. H A Y A S H I , ' Η. Y A M A O K A , K. HAYASHI, and S. O K A M U R A , Kyoto University, Kyoto, Japan The rigid glass matrix method is useful to study intermediate species such as trapped electrons and ions formed by ionizing radiations because different species are selectively formed in proper matrices and trapped stably. In this investigation, ion radicals of solute monomers (nitroethylene, isobutylene, and styrene) in the irradiated glass matrices (2methyltetrahydrofuran, η-butyl chloride, and 3-methylpentane) were stud ied mainly by E S R and electronic: spectrum measurements. The formation mechanism of the ion radicals and their contribution to the initiation process of polymerizations (anionic polymerization for nitroethylene, and cationic one for isobutylene and styrene) are discussed. In any case where the polymerization occurred in the glass, the corresponding anion or cation radicals were observed from the glass containing monomer irradiated at 77 °K. This suggests the ion radicals play an important role in the initiation process of radiation induced ionic polymerization. 1
Address: Osaka Lab. JAERI., Osaka, Japan.
