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J. Phys. Chem. 1982, 86, 2282-2284
H2 Production in the 'Li3+ Ion Radiolysis of Benzene' Jay A. Laverne and Robert H. Schuler' Radiation Laboratory and Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 (Received: February 22, 1982; In Final Form: April 15, 1982)
The 'Li3+ ion radiolysis of liquid benzene results in a substantial yield of H2 (43000 molecules/particle) above that produced in fast electron radiolysis. This excess hydrogen is produced predominantly in processes which occur very near the end of the track but while the lithium ion is still predominantly in the 3+ charge state. The differential yield decreases rapidly with increasing particle energy from a maximum of -0.5 molecules/100 eV in the region of a few MeV where the LET is -40 eV/A and approaches the yield observed in electron radiolysis (0.04 molecules/100 eV) at energies above 25 MeV. The observed dependence suggests that the excess H2results primarily from intratrack reactions involving intermediates which decay in very rapid first-order processes.
The radiolysis of liquid benzene by heavy particles results in a substantially higher yield of H2 than is observed in y-ray and fast electron radiolysis.2-8 Since this latter yield is very low (0.038)2the additional hydrogen produced by heavy particles is an appreciable fraction of the total so that its dependence on particle energy can be examined in detail. Studies with protons, deuterons, and helium ions have shown that this additional hydrogen results predominantly from processes which occur very near the end of the track.6 The differential yield (Gi = d(G$o)/dEo) observed for irradiation with helium ions, for example, does not increase appreciably above that for fast electrons until the helium ion energy drops below -10 MeV6 but then rapidly increases by an order of m a g n i t ~ d e .While ~ one expects the radiation chemistry of low-energy heavy ions to be somewhat different than that of less densely ionizing radiations as a result of the increased importance of intratrack processes, it is not obvious how the differences can be of a type to increase the efficiency for production of H2 to the extent observed. Burns3 has suggested that, as in the case of water radiolysis, intratrack combination of H atoms becomes very important at high values of linear energy transfer (LET = a E / d x ) . If such is the case then the high differential yields observed with helium ions at low energies require that the radiolytic dissociation of benzene results in a primary yield of H atoms considerably greater than 1. The parallel production of acetylene and the even higher H2yield of 2.1 reported for fission fragment recoils8 would seem to implicate molecular processes as a major source. Alternate sources of H2 production involving ions or excited states must be explored. It has been suggested6 that the charge separation that effectively results from the reversible charge exchange between the ionizing particles and the medium being irradiated may be an important source of hydrogen. While this may possibly be the case for protons and deuterons, where approximately 1 molecule of H2 is produced for each cycle of charge exchange,6calculations for helium ions from available cross (1) The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This ig Document No. NDRL-2315 from the Notre Dame Radiation Laboratory. (2) T. Gaumann and R. H. Schuler, J.Phys. Chen., 65, 703 (1961). (3) W. G. Burns, Trans. Faraday SOC.,58, 961 (1962). (4) W. G. Burns and C. R. V. Reed, Trans. Faraday SOC.,59, 101 (1963). (5) A. W. Boyd, J . Nucl. Mater., 9, 1 (1963). (6) R. H. Schuler, Trans. Faraday SOC.,61, 100 (1965). (7) W. G. Burns and W. R. Marsh, Trans. Faraday SOC.,64, 2375 (1968). (8) A. W. Boyd and H. W. J. Connor, Can. J. Chem.,42,1418 (1964). 0022-3654/82/2086-2282$01.25/0
section datag indicate that the number of such cycles is insufficient to explain the large amount of excess H2 observed. The data of Burns and Marsh' also show that for 4He2+ions an appreciable fraction of the excess H2results from processes occurring at energies above a few MeV, where the predominant charge of the helium ion is 2+ and charge exchange cannot be an important source. The energy dependence observed indicates that intratrack processes must be responsible. Detailed information obtained at higher LETs and with more highly charged ions is needed to provide further insight but appropriate experiments are difficult because they must be carried out at relatively low energies where particle ranges are small. We have been able to examine the H2 yields produced by irradiation of benzene with beams of 7Li3+ions of welldefined energy from a Van de Graaff accelerator and briefly present the results here. Lithium ion radiolysis with particles from this instrument has the advantage that LETs above 5 eV/A are readily accessible so that experimental studies of the LET dependence of the differential yields in the range of 5-40 eV/A can be carried out relatively easily.
Experimental Section Irradiations were carried out with 'Li3+ ions of 15-36MeV initial energy (ranges of 12-32 mg/cm2 in aluminum) from the heavy ion FN Tandem Van de Graaff operated by the Notre Dame Physics Department. Energies were controllable to better than 0.01 MeV. Irradiations of outgassed samples were carried out in -35-cm3 Pyrex cells with -6 mg/cm2 mica windows. The accelerator exit window was 0.013-mm dural (-4 mg/cm2). In some cases additional aluminum absorbers were used to attenuate the beam energy. The particle energy expended in the sample was determined by correcting the initial particle energy for the energy loss in the window system as calculated from energy loss compilations. Error limits, as indicated by similar measurements on the Fricke system, correspond to an estimated uncertainty of h0.3 mg/cm2 in the residual range. Absolute doses were determined from the particle energy and integrated beam current. The latter is known to 1% so that accuracy is primarily limited by uncertainty in the energy and errors in the gas analysis. Beam currents were - 5 x A and total doses -1019-1020 eV/g but because the volume subject to irradiation was very small (CO.01 cm3) the local dose rates were as high as 3 x 1019eV g-l s-l. Benzene was Aldrich Spectropho-
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(9) S.K. Allison, Reu. Mod. Phys., 30, 1137 (1958)
0 1982 American Chemical Society
The Journal of Physical Chemistry, Vol. 86, No. 13, 1982 2283
Letters 50,000
ENERGY (MeV1
Figure 1. Excess molecular hydrogen yield as a function of energy: 0lithium ions, this w ~ r k ;(0) helium ions, ( 0 )deuterons, @jprotons, ref 6; (A)helium ions, ref 7; (0) protons, ref 3 (theyields with protons are doubled and plotted as twice the energy). The dotted line is an estimate of the upper limit to the excess hydrogen yield due to the 1+ and 2+ charge states of lithium. The dashed line is an approximate limit of the dlfferentlal yield (0.5) for lithium ions at low energy. The error limits given for the lithium ion data are (horizontally) for an uncertainty in the particle energy corresponding to f0.3 mglcm' in range and (vertically) f3% In the gas analysis.
tometric grade. Product Hz was toeppled from the sample and measured gas chromatographically with argon as the carrier gas.
Results and Discussion The yield of Hz produced by the highest energy 7Li3+ ions available (30 MeV after passing the window system) is 0.180 f 0.004 molecules per 100 eV. This yield corresponds to 42 600 molecules of H2above that expected from the yield observed in electron radiolysis (0.038). The energy dependence of the excess H2,given in Figure 1,shows that about half is produced at 7Li3+ion energies below 6 MeV and that there is relatively little increase above 20 MeV. Data for 4He2+ions from ref 6 and 7 are given in Figure 1 for comparison. Neglecting effects of differences in ionization potential, the energy expended in a given velocity interval is directly proportional to mass for different ions in the same charge state. An upper limit to contributions produced by the 1+ and 2+ charge states of 7Li can be estimated by scaling the helium ion data at the same mass-energy product by a factor of 714 (dotted curve in Figure 1). However, because the maximum population of the lithium 2+ state is only -60%,1° the actual contribution from these lower charge states should be considerably lower. Teplova et al.'O have found that after passing through a celluloid film the predominant charge (10)Ya. A. Teplova, I. S. Dmitriev, V. S. Nikolaev, and I. N. Fateeva, J.Exptl. Theor. Phys. (USSR),32, 974 (1957),as referenced by S. K. Allison, J. Cuevas, and M. Garcia-Munoz,Phys. Rev., 120, 1266 (1960).
state of a 7Li ion is 1+below 0.7 MeV, 2+ between 0.7 and 2.0 MeV, and 3+ at higher energies. Although they find that the fraction of the ion beam in the 2+ charge state is about 10% at 10 MeV and presists to -20 MeV it is extremely unlikely that chemical effects directly associated with the charge changing collisions can contribute appreciably above -10 MeV and certainly cannot be totally responsible for the very large yields of excess H2observed here. A fairly detailed description of the dynamics of the charge exchange processes that occur with highly charged ions is needed in order to estimate a limit to the possible contribution from this source. At this point it is clear that most of the excess Hz originates from processes other than charge exchange that occur while the 7Li ion is still in the 3+ charge state and with a velocity appreciably above the electrons in the medium being irradiated. Energy is lost primarily by Coulombic interactions so that the primary processes should be basically the same as for fast electrons. This similarity in the physics implies that second-order intratrack chemical processes must be responsible for most of the excess Hz produced. The data presented here show that in the region where the energy loss parameter of the 7Li ion is at a maximum (-40 eV/A at 2 MeV) the differential yield of Hz is -0.5 (dashed line in Figure 1) or about the same as reported by Burns and Marsh for 4He2+ions at their LET maximum (-20 eV/A). In the comparison between these low mass particles &ray effects appear to compensate for increased LET so that the limiting differential yield is rather insensitive to the particle. The considerably higher yield of 2.1 reported for fission fragments8indicates, however, that this limit may slowly increase with mass and charge. Figure 1shows that the differential yield drops dramatically with increase in energy. At 25 MeV the LET is still very high (8 eV/A) but the yield is only 50% above that observed in electron radiolysis. In water radiolysis an equivalent effect is observed at LETS well below 1eV/A." We conclude that the processes responsible for the intratrack production of Hz must be very efficient but also involve species with a very short intrinsic lifetime. This effect suggests that bimolecular reactions of excited species are involved, as has been previously p r ~ p o s e d . ~ Burns and Marsh7have pointed out that at a given LET the differential yield for H, production is less for helium ions than for protons and have attributed this difference to the contribution of energy loss processes resulting in high energy secondaries. We find, similarly, that the yields for lithium ions are still lower. If the dependence on LET was common to all ions one would, neglecting ionization potential differences, predict that for the fully stripped ions the integrated yield from the track processes, i.e., the limiting excess Hz observed at high energies, should be proportional to Mz2 where M and 2 are the mass and charge of the irradiating particle. The experimental data are, however, less strongly dependent than indicated by such a relationship. The lower dependence results from the fact that as mass increases there is an appreciable increase in the fraction of energy which is expended in 6 rays that have effective LETS considerably lower than for the track core.12 The data for lH+, ,D+, 4He2+,and 7Li3+ ions (1000, 2000, 13000,and 43 000 molecules excess Hz, respectively) can, in fact, be correlated quite well by the relationship limiting excess molecules Hz = 1000M22(0.83)z-1
(1)
(11)N. F. Barr and R. H. Schuler, J. Phys. Chem., 63, 808 (1959). (12)A. Mozumder, A. Chatterjee, and J. L. Magee, Adu. Chem. Ser., NO.81,27-48 (1968).
J. Phys. Chem. 1982, 86, 2284-2286
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suggesting that, at least for low masses, the &ray effect is of the magnitude of (0.83)z-1. Strict application of eq I to charged particles of higher atomic number predicts that a maximum excess of H2 of -400 000 molecules/particle should be reached for irradiating particles in the region of 2 = 10. The yield of 2.14 reported for fission recoilss corresponds to at least an order of magnitude greater excess than estimated by eq I so that the effect of 6 rays would appear to be limited by their coalescence into the track.
We are currently extending this study to irradiations with "B5+ and 12C6+ions to explore these effects further and to obtain quantitative data for more detailed modeling.
Acknowledgment. The authors are indebted to Dr. C. P. Browne for making the facilities of the Physics Department Nuclear Structure Laboratory available to them. The latter is supported by the National Science Foundation. They especially thank Dr. E. D. Berners for his assistance in the development of these experiments.
Thermodynamics of Formation of a Crystalline Salt of the Sodium Anion U. Schlndewolf, Institul fur Physikaiische Chemie und Elektrochemie der Universitat, Karlsruhe, West Germany
Long Dinh Le, and James L. Dye* Department of Chemistry, Michigan State University, East Lansing, Michigan 48824 (Receive& February 23, 1982; In Final Form: April 20, 1982)
Measurement of the emf of the cell PtlNa(s)lNa+ @-aluminalNa'C222-Na-(sat),C222(sat)IPt as a function of temperature from -75 to +28 O C for several pure and mixed amine solvents permitted calculation of the thermodynamics of the reaction 2Na(s) + C222(s) Na+C222*Na-(s)(1) in which C222 is [2.2.2] cryptand. The results are AH"'= -34 f 2 kJ mol-', AS", = -90 f 8 J mol-' K-l, and = -7.1 f 0.6 kJ mol-l in agreement with the observed thermodynamic stability of Na+C222*Na-(s)at 0 "C and its temperature of decomposition (83 "C). The results are compared with those obtained by using a modified Born-Haber cycle. -+
Introduction Since the preparation and characterization of the first crystalline salt which contains an alkali metal anion, Na+C222-Na-,'-3 a number of other salts of alkali metal anions (alkalides) have been prepared."" Because the parent compound can be formed isothermally at 0 "C from solid sodium and C222 in solution,12the Gibbs free energy of formation of the solid salt according to 2Na(s) + C222(s)
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Na+C222*Na-(s)
(1)
(1) J. L. Dye, J. M. Cereso, M. T. Lok, B. L. Barnett, and F. J. Tehan, J . Am. Chem. SOC.,96, 608-9 (1974). 96, (2) F. J. Tehan, B. L. Barnett, and J. L. Dye, J. Am. Chem. SOC., 7203-8 (1974). (3) The IUPAC name for the cryptand [2.2.2] is 4,7,13,16,21,24-hexaoxa-l,l0-diazabicyclo[8.8.8]hexacosane; the abbreviation is C222. (4) J. L. Dye, C. W. Andrews, and S. E. Mathews, J. Phys. Chem., 79, 3065-70 (1975). (5) J. L. Dye, M. R. Yemen, M. G. DaGue, and J.-M. Lehn, J . Chem. Phys., 68, 1665-70 (1978). (6) J. L. Dye, J . Chem. Educ., 64, 332-9 (1977). (7) J. L. Dye, Angew. Chem., Znt. Ed. Engl., 18,587-98 (1979). (8) J. L. Dye in 'Progress in Macrocyclic Chemistry", Vol. 1, R. M. Izatt and J. J. Christensen, Ed., Wiley-Interscience, New York, 1979, pp 63-113. (9) J. L. Dye, J.Phys. Chem., 84, 1084-90 (1980). (10) B. Van Eck, L. D. Le, D. Issa, and J. L. Dye, Inorg. Chem., in press. (11) J. L. Dye, M. G. DaGue, M. R. Yemen, J. S. Landers, and H. L. Lewis, J. Phys. Chem., 84, 1096-1103 (1980). (12) M. G. DaGue, Ph.D. Dissertation, Michigan State University, 1979. 0022-365418212086-2284$01.25/0
must be negative. Estimates of the enthalpy, free energy, and entropy of reaction 1 were made based upon a modified Born-Haber cycle?!' These original estimates led to AGOzg8 = +28 kJ mol-', AH" = -10 kJ mol-', and ASo = -128 J mol-' K-l. Since then, the enthalpy of solution of C222 in water,13J4 the distribution coefficient between water and cy~lohexane,'~ and its solubility in cy~lohexane'~ have been measured, which permit correction of the thermodynamic estimates. Although more detailed calculations and comparisons can be made,15 straightforward inclusion of the new data for the process C222(s) C222(aq) (2) yields the following thermodynamic estimates for reaction 1: AGOzse = +22 kJ mol-', AH" = -35 kJ mol-', AS" = -190 J mol-' K-'. The present work follows from the development of a high-precision sodium electrode by Schindewolf and coworkers,1618based upon the selective transport of sodium ions through Na+ /3-alumina.lS2' Since platinum or
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(13) M. H. Abraham, E. C. Viguria, and A. F. Danil de Namor, J. Chem. Soc., Chem. Commun., 374-5 (1979). (14) M. H. Abraham, A. F. Danil de Namor, and R. A. Schulz, J. Chem. SOC.,Faraday Trans. I, 76,869-84 (1980). (15) B. Van Eck, Ph.D. Diesertation, Michigan State University, 1982. (16) U. Schindewolf and M. Werner, J . Phys. Chem., 84, 1123-7 (1980). (17) M. Werner and U. Schindewolf, Ber. Bunsenges. Phys. Chem., 84, 547-50 (1980). (18) W. Gross and U. Schindewolf, Ber. Bunsenges. Phys. Chen., 85, 112 (1981).
@ 1982 American Chemical Society
