J. Phys. Chem. 1980, 84, 1295-1296
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Solvated Positron Chemistry. Competitive Positron Reactions with Halides in Water J. R. Andemen,*+ N. J. Pedersen,?P. Christensen,' and 0. E. Mogenssnt Chemistry and Nectronics Departments, Risd National Laboratory, DK-4000 RoskiMe, Denmark (Received July 17, 1979) Publication costs assisted by Risd National Laboratory
By means of the angular correlation technique it is shown that, the binding of positrons to halides is strongly influenced by solvation effects. For aqueous solutions we find increasing values for the binding energies between the halide and the positron with increasing mass of the halide. This is contrary to the values calculated by Cade and Farazdel for the vacuum case. The discrepancy is explained by invoking the solvation energies of the halides.
In previous studies at Ris#'p2 the reactions of hydrated positrons, eaqt.,with the halides F-, C1-, Br-, and I-, and the pseudohalide CN-, were examined. It was shown by means of the angular correlation technique that all but F reacted with eaq+yielding bound states, [X-, e']. The angular correlation curves for [Cl-, e+], [Br-, e+], and [I-, et] states were found to be in good agreement with the theoretical ones calculated by Cade and Farazde13g4for the vacuum case. The latter authors also calculated the theoretical binding energies of [X-, e+] and found them to decrease with increasing atomic number of the halide. In this work we have studied the relative binding energies of positrons on the halides Cl-., Br-, and I- in water. The rationale of our experiments was to let the halides compete for tlhe positron in aqueous medium. We found, in contrast to the theoretical results for the vacuum case, that the bindiing energy of the [X-, e+] state increases in the series C1-, Br-, and I-. The hydrated positron reactions were studied by means of the angular correlation technique. A given state of the electron-posiitron system gives a particular angular correlation curve. The basic idea of the method is the following: The pure water angular correlation curve is a sum of three components due to annihilation from the eaq+, p-Ps, and 0-Ps states. Addition of the Ps inhibitor PPS (= pyridiniun1-l-propane-3'-sulfonate) reduces the annihilation from the Ps states considerably. The small components resulting from noninhibited Ps are subtracted, and we are left with the hydrated positron angular correlation curve. When a halide is added too (all counterions in this study are Na'), the corrected curve is a sum of the e, and the bound state [X-, e+] curves. By analyzing curves for various X-. concentrations, we previously determined the shape of the [X-, e+] curves and the amount of bound state formed as a function of the concentration of In this work two different halides were added to the PPS solution, hence the corrected curve may now be assumed to consist of three components: the eaq+component and the components caused by the bound states of the two kind of ions. The intensity of the two bound state components depends on the amount of each bound state formed, and this in turn is determined by the binding energy between X- and e+. A standard linear-dit angular correlation setup was used.s The detectors covered a solid angle of 0.96 X 178 mrad. The full width at half-maximum (fwhm) of the angular resolution curve was roughly 1.15 mrad. The solutions were contained in a glass cell covered with a
SUA +
x-.
t Chemistry Department. t Electronics Department.
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polymer foil through which the positrons from an external 10 mCi 22NaC1source entered. Further experimentall details can be found in ref l. The experimental angular correlation curves were first corrected for a Ps content of 3.23% as previously describedS2Then the curves were analyzed by means of the PAACFIT program6in terms of three Gaussians, and finally the H parameters were extracted. H is the number of counts at the top of the curve divided by the area between A13 mrad as calculated from the fitting curves. It was shown earlier to be largest for solutions of I-, intermediate for solutions of Br-, and lowest for solutions of C1-.2 In Figure 1 we show the H parameter for 1 M solutions of C1-, Br-, and I- as a function of the content of the competing halide. A visual inspection of the corrected angular correlation curves revealed that they could be reasonably well described as a linear combination of an eaq+component and the [X-, e+] components of the halides in solution. To confirm this observation in detail it is necessary to employ more sophisticated and rather tedious PAACFITanalyses of a large number of curves. On the qualitative level, however, the trends of the measurements are clear. By inspection of Figure l a it is seen that the addition of Br- or C1- to a 1 M solution of I- has a very slight impact only on H, meaning that the positron is more strongly bound to I- than to either Br- or C1-. This interpretatioin is corroboratedfurther by the finding (Figure lb, c) that even small amounts of I- added to 1 M solutions of Br- or C1are accompanied by a pronounced increase in H. In a like manner it is found that the positron is more strongly bound to Br- than to Cl-, The results obtained here for an aqueous medium are in clear disagreement with those calculated by Cade and Farazdel for the vacuum case. They found a decrease in the binding energy in going from F- to I-. A very likely explanation for this discrepancy is found in solvation effects. It is well known that as F- is a small anion it has a high solvation energy in water (113kcal m01-l~).Furthermore, F participates in hydrogen bonding. As at least part of the solvation energy, including the energy gained by hydrogen bonding, is lost upon bound state formation with a positron, it appears rather conceivable that the net energy gained by the positron on participating in a bound state [X-, e+] should be the smallest for F- among the halides. As experiments indicate no bound state formation at all for F-, this energy gain may be negative. When the larger halides are considered, the solvation energies in water decrease from 81 kcal mol-l for C1- to 73 kcal mol-l for Br- to 62 kcal mol-l for I-.7 A decrease in solvation energy should make bound state formation more favorable, and this is indeed what we have observed. 0 1980 American Chemical Society
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J. Phys. Chem. 1980,84, 1296-1298
0.0
0.5
1.0
MOLAR CONCENTRATION OF X -
Figure 1. The Hparameter vs. the concentration of one of the halides in an aqueous solution of two halides (and the Ps inhibitor PPS). The concentration of one of the halides is fixed at 1 M, while the concentration of the other is varied (‘I = I-; X = Br-, and 0 = CI-).
A final piece of information lending itself to the above explanation is found in preliminary angular correlation measurements on KF solutions in benzene-crown ether mixtures. In this medium, where F is only weakly solvated as compared to the water case, experiments indicate that bound state formation is taking placeas It is important to realize that the kinetics of positron trapping on the halides may be fairly complicated. A
-
slowly moving ion with a diffusion constant of D 2 x 10” cm2 moves approximately 13 A during the positron lifetime of roughly 400 ps, and the average distance between ions in a l M solution is ca. 12 A. Because of these facts, and the encounter pair effect,2 tunneling, etc., a positron trapped on, e.g., a Br- will transfer rather rapidly to the deeper trap on I- in a solution 1M in I-. The small hump seen at low bromide concentrations in Figure l a may be caused by increased trapping on I- due to initial trapping on Br- followed by transfer to I-. Contrary to the observed results, however, we would have expected to see an increase in the H parameter at higher Br- concentrations as well; compare the discussion in section 4.6 in ref 2. On the other hand, preliminary results obtained from 0.03 M solutions of I- (N= 0.0961 mrad-l) with additions of small amounts of Br- or C1- indicate that a simple competition model for the positron is obeyed a t these concentrations. For example, addition of Br- (Cl-) to the 0.03 M solution of I- increases (decreases) the H value as expected.
Acknowledgment. It is a pleasure to acknowledge the technical assistance of Mrs. Anne B. Nielsen.
References and Notes (1) 0. E. Mogensen andV. P. Shantarovich, Chem. phys., 6, lOO(1974). (2) 0.E. Mogensen, Chem. Phys., 37, 139 (1979). (3) P. E. Cade and A. Farazdel, J. Chem. Phys., 68, 2598 (1977). (4) A. Farazdel and P. E Cade, J. Chem. Phys., 66, 2612 (1977). (5) R. N. West, Adv. Phys., 22, 263 (1973). (6) P. Kirkegaard and 0. E. Mogensen, RistLM-1615, 1973. (7) H. J. Emel6us and A. G. Sharpe, “Modern Aspects of Inorganic Chemistry”, Routledge and Kegan Paul, London, 1973,p 109. (8) The other halides also form bound states with positrons in benz-
ene-crown ether mixtures, and the crown ethers themselves act as Ps inhibitors: J. R. Andersen, N. J. Pedersen, P. Christensen, and 0. E. Mogensen, manuscript In preparation.
Positronium Formation Studies of Weakly Bound Electrons on Electron Scavengers 0. E. Mogensen* and B. L6vayt Chemistty Department, Risd National Laboratory, DK-4000 Roskilde, Denmark (Received July 17, 1979) Publications costs assisted by Riso National Laboratory
Positronium (Ps) is formed by a positron reaction with one of the excess electrons created during the slowing down of the positron in the terminal spur of the positron track. Thus the scavenging of the excess electrons influences the Ps yields. A very short summary and discussion of recent results of Ps yield studies of excess electron reactions with molecules, which bind the electrons in fairly shallow traps (e.g., CS2,SF,, CsFs,benzene, naphthalene, and biphenyl) is given. Measured minima in the Ps yield vs. scavenger concentration found in the CS2and SF, cases @re explained in terms of the onset of partial delocalization of electrons on several scavenger molecules at roughly 0.8 M scavenger concentration. Enhancement of the Ps yield was found on addition of “very shallow trap” electron scavengers to hexane, and on addition of -0.2 M of a hole scavenger to cyclohexane and decalin, where the hole is rapidly moving.
According to the spur model of positronium (Ps) formation,l Ps is formed by a reaction between a mainly thermalized positron and one of the excess electrons in the terminal positron spur, i.e., the last part of the positron track. As the binding energy of Ps is 6.8 eV the ther+On leave from the Department of Physical Chemistry and Radiology, Eotvos University of Budapest, Hungary. 0022-3654/80/2084-1296$01 .OO/O
malized positron cannot pull an electron out of normal molecules. The Ps formation process competes with the other spur reactions, such as electron-ion recombination, electron or positron out diffusion, and electron or positron reactions with solvent molecules or added species. In general, the spur model explains well the measured Ps yields, and many new ps Yield results have recently been predicted by means of the model. The purpose of the 0 1980 American Chemical Society
