2404
NOTES
1 At
0
3
6
9
12
15
t (seconds]
Figure 1. Representative plot of kinetic data for cyclohexane solutions, including a least-squares fit.
0.317 and 0.00025 cm/sec (with average deviation 3y0 and 5%) for cyclohexane and polystyrene as solvent, respectively. We attribute this large difference to the greatly decreased mobility of CO in polystyrene relative to cyclohexane. If this is the correct interpretation, one can visualize the use of polymer films as “solvents” for other similar reactions which would require fast reaction techniques for study in normal solvents.
Acknowledgment. Helpful discussions with P. J. Hart are gratefully acknowledged. I thank G. J. Kallos and E. J. Strojny for analysis of the solvents.
Fluorescence of Liquid Benzene under Proton and Electron Impact’*
room temperature under uv excitation was identified as fluorescence from the lowest excited singlet state of the monomer. At higher concentrations, a broad structureless emission band peaked at 320 nm has been attributed2 to excimers formed from the interaction of excited singlet and ground-state benzene molecules. Under electron bombardment3t4 of liquid benzene, the monomer emission was reported to be absent and the observed fluorescence was assigned to excimers formed directly via ion recombination. Cooper and Thomas5 observed short-lived species via fluorescence a t wavelengths below 400 nm and via absorption in the 500-nm region in the electron pulse radiolysis of liquid benzene and this emission and absorption was attributed5j6 to the lowest singlet state (Big) of the benzene excimer. Phillips and Schug’ observed both singlet monomer and singlet excimer emission in the electron radiolysis of frozen benzene. That the observed emission was not totally excimeric as was that reported by Carter, Christophorou, and Abu-Zeid3b4was explained by the fact that the benzene molecule was bound by the crystalline form and not free to assume the excimer geometry. This note reports an experimental study of the luminescence from benzene at room temperature upon excitation by electrons and protons. In addition to the singlet monomer fluorescence previously unobserved under electron and proton bombardment in liquid benzene, we observed several fluorescing radiolysis products. By use of a flowing target cell, we are able to eliminate most of the radiolysis product emission and thus obtain spectra consistent with those reported for uv excitation. The experimental setup has been previously described.* A Van de Graaff accelerator provided 1.7MeV protons and beam currents up to 50 nA. An electron accelerator provided 80-keV electrons and beam currents up to 10 pA. Protons (or electrons) entered the front of the cell through a 2.8 X cm thick nickel foil and were totally absorbed in less than 40 p (100 p ) of liquid. Estimates of beam heating based on calculations of heat conductivity give temperature changes of less than 1” for the data presented in this note. Fluorescence was observed through a quartz
by M. L. West and L. L. Nichols Battelle Memorial Institufe, Pacific Northwest Laboratory, Richland, Washington 90566 (Receined November 18, 1969)
Extensive studies have been conducted in recent years on the radiation physics and chemistry of aromatic molecules and because it is the simplest of the aromatics, benzene has perhaps received the most attention. The reported optical emission spectra of benzene obtained under various modes and conditions of excitation have led to several theories of reaction kinetics. The reported emission’b from dilute solutions of benzene a t The Journal of Physical Chemistry, Vol. 74, No. 11, 1970
(1) (a) This paper is based on work performed under United States Atomic Energy Commission Contract AT(45-1)-1830; (b) I. B. Berlman, “Handbook of Fluorescence Spectra of Aromatic Molecules,” Academic Press, Inc., New York, N. Y., 1967. (2) J. B. Birks, C. L. Braga, and M. D. Lumb, Proc. Roy. SOC.,Ser. A , 283,83 (1965). (3) J . G. Carter, L. G. Christophorou, and M-E. M. Abu-Zeid, J . Chem. Phys., 47, 3879 (1967). (4) L. G. Christophorou, M-E. M. Abu-Zeid, and J. G. Carter, ibid., 49, 3775 (1968). (5) R. Cooper and J. K. Thomas, ibid., 48, 5097 (1968). (6) J. B. Birks, Chem. Phys. Lett., 1 , 625 (1968). (7) D. H. Phillips and J. C.Schug, J . Chem. Phys., 50, 3297 (1969). (8) L. L. Nichols and W. E. Wilson, A p p l . Opt., 7, 167 (1968).
2405
NOTES window a t the rear of the cell. All measurements were made with a scanning spectrometer (grating of 1180 lines/mm, blazed at 250 nm; reciprocal dispersion of approximately 2 nm/mm; a typical slit width was 400 p ) . Maximum dose rates were lo6 rads/sec and lo7 rads/sec for protons and electrons, respectively. The irradiation cell had a relatively small volume (1.0 cm2 X 0.14 cm thick) to reduce self-absorption in the liquid and was also designed to operate under static or flowing conditions. Static conditions refer to a closed system of benzene in which radiolysis products are allowed to accumulate. In the flowing mode, a reservoir of deaerated benzene continuously replaced irradiated liquid in the target volume. “Nanograde” (Trademark of RIallinckrodt Chemical Corp.) benzene was used without further purification. Nitrogen gas from a liquid nitrogen dewar flask was bubbled through the samples to remove dissolved oxygen . Emission spectra of benzene excited by electrons or protons are shown in Figure 1. Emission from dilute solutions of benzene in cyclohexane (Figure l a ) is identical with that observed under uv excitation’ and corresponds to monomer emission from the first excited singlet state. Figure l b shows a typical emission spectrum of liquid benzene for proton or electron bombardment under static conditions where radiolysis products are allowed to accumulate in the microcell. The two less intense peaks (279 and 286 nm) are attributed to monomer emission as they agree in wavelength with the two most intense peaks of monomer fluorescence. The 272-nm peak was not observed because of its lower intensity and because of increased absorption in the sample at this wavelength. The intense band near 320 nm is at the proper wavelength for excimer fluorescence but a large portion was found to be stable radiolysis product emission. Figure IC illustrates the time behavior of fluorescence following steady-state proton or electron irradiation (1 X lo6 rads/sec) of liquid benzene in the static cell. If monomer (curve I) and excimer (curve 11) fluorescence were the only sources of emission a t 279 nm and 370 nm, respectively, then curves I and I1 would rise to the maximum intensity in approximately 0.04 sec as determined by the time constant of the phototube. The emission a t 279 nm (curve I) reaches maximum intensity in about 0.04 sec. The emission at 320 nm (curve 11) shows an initial rapid rise followed by a slow buildup to maximum intensity. The slower rising portion of curve I1 indicates a buildup of an emission from a stable radiolysis product. This build-up emission can be reduced by flowing the liquid through the target cell. This is illustrated in Figure Id which shows a typical spectrum of benzene emission under proton or electron impact when liquid benzene was allowed to flow through the microcell a t a rate of 2 cm3 min-’. Assuming a uniform flow, the average dose
I
-
- 778
330
390
270
I 390
330
270
330
270
Wavelength inrn)
0
0.8
1.6
Time i s e c )
2.4
390
Wavelength i n m )
Figure 1. Optical spectra of benzene under proton and electron impact: (a) benzene (57,) in cyclohexane; (b) emission of liquid benzene when radiolysis products are allowed to accumulate in the microcell (static system); (c) time variation of liquid benzene emission under proton or electron impact (dose rate -1 X lo6 rads/sec; static system); (d) emission of liquid benzene under proton or electron impact (average dose -106 rads; flowed system).
was 105 rads. Under these conditions, the emission a t 279 nm and 320 nm reaches maximum intensity in about 0.04 sec following exposure to a steady-state irradiation. Radiolysis products contribute a significant portion of the emitted light in the proton and electron bombardment of liquid benzene. The intense emission spectrum near 320 nm (Figure lb) from the static sample is similar to the reported‘ emission of biphenyl. A CI~HIO fraction has been foundgin the complex polymer products of irradiated benzene and one of the proposed mechanisms of hydrogen formation in irradiated benzene involves biphenyl as a by-product.lO~l’ We have conclusively shown that emission spectra from liquid benzene under proton and electron bombardment includes monomer fluorescence. We have not conclusively established that the enhanced emission in the long wavelength portion of benzene emission in the flowing cell is excimer fluorescence. However, the similarity of this emission to the reported emission under uv excitation2 and the observed rapid rise to maximum intensity following steady-state irradiation suggest that
(9) S. Gordon, A . R. Van Dyken, and T. F. Doumani, J . Phys. Chem., 62, 20 (1968). (10) W. G. Burns, Trans. Faraday Soc., 58, 961 (1962). (11) W. G. Burns and C. R. V. Reed, ibid., 59, 101 (1963).
The Journal of Physical Chemistry, Vol. 7.4, No. 11, 1970
2406 the primary emission before the accumulation of radiolysis products is the same from the various modes of excitation (i-e., via proton, electron, and photon irradiation). The reaction mechanism of direct excimer formation via ion recombination is not needed to explain the presently reported emission of liquid benzene under electron and proton bombardment.
M u t u a l Diffusion Coefficients of Aqueous Copper(I1) Sulfate Solutions a t 25” by L. A. Woolfl and A. W. Hoveling Department of Chemistry, University College of Townsville, Townsville, Queensland,Australia (Received December go, 1060)
This work was undertaken to obtain accurate values of the mutual diffusion coefficient of copper sulfate over the concentration range 0.05-1.4 M . Emanuel and Olander2 had reported an accuracy of about 5% jn their measurements for the range 0.35-1.4 M , but these results did not agree well with earlier data8which extended to lower concentrations. The diffusion coefficients reported here are thought to be accurate to about 1-2%. These results enable a test to be made of the suitability for 2:2 electrolytes of a method used previously to predict diffusion coefficients in concentrated solutions of 1: 1electrolytes. I n addition, viscosity results are presented for the range 0.005-1.4 M .
Experimental Section All solid chemicals used were of analytical reagent quality and were not further purified; before use oncedistilled water was passed through an ion-exchange column. The diffusion measurements were made by allowing copper sulfate solutions to diffuse into water using the magnetically stirred diaphragm cell method of stoke^.^ All copper sulfate solutions were analyzed by determination of copper to *0.1% by standard electrogravimetric methods. The several diffusion cells used were calibrated a t frequent intervals by allowing 0.5 M potassium chloride solutions to diffuse into water a t 25” ; analyses of the resulting solutions generally were made by precise (ca. 0.05%) conductance measurements although some concentrations were determined by potentiometric titration. The overall reproducibility of successive cell constant determinations was 0.1-0.2%. The viscosity measurements were made with an Ubbelohde-type viscometer incorporating flared capillaries5 which eliminated kinetic energy corrections. The Experimental flow times agreed to *O.Ol%. densities measured for the viscosity determinations agreed well with the data reported by Pearce and Pumplina but were of lower precision (ca. 0.001%). The Journal of Physical Chemistry, Vol. 7 4 , No. 11, 1970
NOTES Results The 26 diffusion measurements which were made used solutions which varied in initial concentration from 0.07 to 1.4 M . The resulting integral diffusion coefficients were converted to differential coefficients by the method of Stokes.’ Derivatives necessary for this conversion were obtained numerically by a standard five-point Lagrange method a t intervals of 0.01 in cl”. Smoothed values of these derivatives were obtained by plotting them against cl/’ and reading the required values from a curve drawn through the points. For convenience the diffusion coefficients are reported in Table I a t equal intervals of cl/’; the values a t 0.01 M and 1.440 M (saturation) may be subject to extrapolation errors since they lie outside the range of the experimental measurements. The viscosity measurements covered the range of concentration from 0.005 to 1.42 M and are reported in Table I relative to water a t 25” having an absolute viscosity of 0.8903 cP. At the lower concentrations these results were in excellent agreement with the equation given by Stokes and Millss and a t higher concentrations generally agreed within 0.1% with other literature value^.^ The results reported in Table I were read from a smooth curve drawn through the experimental points.
Discussion The present diffusion coefficients are shown in Figure 1. While slightly lower (except a t saturation) than those of Emanuel and Olander,2 they generally agree within the estimated error of either set of measurements. On the other hand, the present results coincide with those of Eversole, e2 u L , ~only near 0.16 M . Above that concentration our results are lower (in agreement with the data of ref 1) and below 0.16 M they are higher. The diffusion of associated electrolytes has been the subject of papers by Harned and Hudson’O on two 2:2 electrolytes and for some 1 : l electrolytes and a (1) Address inquiries to Diffusion Research Unit, Research School of Physical Sciences, Australian National University, Canberra, A.C.T ., Australia. (2) A. Emanuel and D. R. Olander, J . Chem. Eng. Data, 8, 31 (1962). (3) W.G.Eversole, H. M. Kindswater, and J. D. Peterson, J. Phys. “hem., 46, 370 (1942). (4) R. H. Stokes, J. Amer. Chem. SOC.,72, 703 (1950); 73, 3527 (1951). (5) B.J. Steel, J . Sci. Instrum., 42, 751 (1965). (0) J. N.Pearce and G. G. Pumplin, J . Amer. Chem. SOC.,59, 1221 (1937). (7) R.H. Stokes, ibid.,72, 2243 (1950). (8) R. H. Stokes and R. Mills, “Viscosity of Electrolytes and R e lated Properties,” Pergamon Press, London, 1966,p 88. (9) See, for example, “International Critical Tables,” Vol. 6, 1st ed, McGraw-Hill Publication, New York, N. Y.,1920,p 14. (10) H. S. Harned and R . M. Hudson, J. Amer. Chem. Soc., 73, 3781, 5880 (1951).
