NOTES
1081
H-
V
0 0
5
I
Figure 1. E.s.r. spectrum of NO2 in irradiated 0.2 M NaNOz at 77°K.
n
tions. Schulte-Frohlinde and Eibenz first detected trapped electrons in irradiated, glassy, highly alkaline ices by electron spin resonance. Hamill and coworkers3 have identified trapped or solvated electrons in a variety of irradiated frozen organic glasses such as tetrahydro-2-methylfuran, ethanol, and 3-methylpentane by ultraviolet spectroscopy. The above-mentioned systems are optically transparent glasses when rapidly frozen a t 77°K. The question arises as to the role of the glassy state in “trapping” the electrons. Is the polycrystalline state equally effective? Other work has shown that irradiation of glassy and polycrystalline matrices often gives quite different radical yields; the yields in the glassy state are usually higher than those in the polycrystalline state. This has been observed in both organic media4 and inorganic media.5s6 In the present note, the effect of glassy and polycrystalline solid phases on the trapped electron yield in alkaline ices is examined.
Experimental
I
50
0
4
Figure 2. E a r . spectrum of NO2 and NOS in irradiated 0.2 M NaNOs at 77°K.
sodium nit,rite ice but due to overlapping spectra the formation of a small amount cannot be excluded. The NOz yields in sodium nitrite ice and in sodium nitrate ice are about t,he same. Since the contribution from NO, in sodium nitrate ice is small, this implies that H 2 0 +has about the same reactivity toward nitrate and nitrite anions in ice. Aclcnowledgm,ent. We thank the U. S. Atomic Energy Commission for support under Contract No. At(l1-1)-1365.
All solutions were made with triply distilled water and reagent grade chemicals. Frozen samples were prepared in the form of cylinders 3 mm. in diameter as previously described’ or in spherical form by dropping the solution into liquid nitrogen. Irradiations were carried out with Co60 y-rays at a dose rate of 0.70 NIrad/hr. a t 77°K. The total dose was 0.30 Mrad. Measurements of the trapped electron concentrations were made with a Varian-4500 spectrometer a t 0.1-mw. power, at which power level saturation effects were negligible. Irradiated samples were warmed at an approximately linear rate from 77 to 140°K. over a 3-min. period. This treatment caused the hydroxyl and other background radicals to decay but left the electron spectrum unchanged. To obtain the yield of trapped electrons, a comparison of the doubly integrated first derivative spectruin of the trapped electron in 2.0 M KOH and that of the hydrogen atoms in 1.0 M NaHZPO4was made for iden-
On Electron Trapping in Polycrystalline and in Glassy Alkaline Ices’ by Larry Kevan Department of Chemistry and Enrico Fermi Inetitute for Nuclear Studies, University of Chicago, Chicago, Illinoie (Receiaed Noatember 10, 1964)
Recently, there has been much interest in the det.ection of trapped electrons in r-irradiated frozen solu-
(1) This is part 111 of a series on the radiolysis of frozen solutions.
(2) D. Schulte-Frohlinde and K. Eiben, 2. Naturforsch., 17a, 445 (1962); H a , 199 (1963); these authors have reversed their original assignment: e - appears at g = 2.002 and 0 - at g = 2.06. (3) M. R. Ronayne, J. P. Guarino, and W. H. Hamill, J. A m . Chem. SOL, 84, 4230 (1962); J. P. Gunrino and W. H. Hamill, ibid., 86, 777 (1964); E.P. Bertin and W. H. Hnmill, ibid.. 86, 1301 (1964). (4) H. W. Fenrick, S. V . Filseth, A. L. Hanson, and J. E. Willard, ibid., 85, 3731 (1963). (5) R. Livingston and A . Weinberger, J . Chem. Phys., 3 3 , 499 (1960). (6) F. S. Dainton and F. T. Jones, Radiation Res., 17, 388 (1962). (7) L. Kevnn, P. N. Moorthv, and J. J. U’eiss, J . A m . Chem. SOC., 86,771 (1964).
Volume 69,Number 3
March 1965
NOTES
1082
I
A
2.1
0
S
I
4
7
c
!
8 [OH -1.
12
16
and in Figure 1. Trapped electron yields in KOH (0) NaOH ( 0 )irradiated frozen solutions at 77°K. In region A both solutions are polycrystalline; in region B NaOH solutions are glassy while KOH solutions remain polycrystalline; and in region C both solutions are glassy.
tical cylindrical samples. A power saturation correction was necessary for the hydrogen atom yield. G(H) for 1.0 M NaH2P04was taken as 0.27.5
KOH do not trap electrons a t 77°K. If the samples are annealed a t temperatures around -50" before irradiation, apparent recrystallization is observed and transparent samples become opaque. Subsequent irradiation of annealed samples gives reduced yields of trapped electrons. I t is likely that some phase segregation takes place in the annealing process and that it results in reduced electron yields.s The maximum value of G(e-) in alkaline ices (1.9) is comparable to the values found by the use of scavengers in frozen organic media (1.1 to 3.2)3 and in sulfuric acid ices (2.1).6 Irradiation of pure 9 a O H or KOH pellets produdes no trapped electrons a t 77°K. Acknowledgment. The author wishes to thank Carol Fine for her careful experimental assistance and the U. S. Atomic Energy Commission for support of this research under Contract AT(l1-1)-1365. ~
(8) As this work was completed, Henrikson (Radiation Res., 23, 63 (1964)) reported that trapped electron yields were lower in polycrystalline than in glassy alkaline ices. His conclusion was based on results from annealed polycrystalline samples.
Results and Discussion The trapped electron yields in alkaline ices irradiated a t 77°K. were determined by electron spin resonance as a function of hydroxide in NaOH and KOH ices. The trapped electron appears at g = 2.002. The results are shown in Figure 1. In NaOH solutions of less than 4.5 M , rapid freezing in liquid nitrogen gives an opaque (polycrystalline) solid. Above 5.0 M , rapid freezing gives a clear transparent (glassy) solid. A gradual transition from polycrystalline to glassy appearance occurs over a range of about 0.5 M . In KOH the median transition concentration is 7.1 M. As can be seen in Figure 1 the trapped electron yield increases linearly with hydroxide concentration independently of whether NaOH or KOH is used and, most importantly, it shows no discontinuities in either of the polycrystalline-glassy transition regions. Samples a t identical concentrations within the phase transition regions can be prepared which appear transparent, opaque, or translucent. These samples all show the same trapped electron yield within experimental error. Such results would seem to indicate that the efficiency of electron trapping in irradiated alkaline ices is independent of the nature of the frozen solution. It must he remembered that all samples are prepared by rapid (1-3 sec.) freezing and are thought to be representative of a frozen solution &e., solvated ions). A frozen solution is necessary for the formation of an efficient electron trap; pure ice and pure The Journal of Physical Chemidry
The Effect of Oxygen on the Electron Spin Resonance Spectra of Anthracene and Perylene Adsorbed on Silica-Alumina by Haruo Imai, Yoshio Ono, and Tominaga Keii Department of Chemical Engineering, Tokyo Institute of Technoloyy, Meguro, Tokuo, Japan (Received November 14, 1964)
Electron spin resonance spectra of polynuclear aromatic hydrocarbons adsorbed on silica-alumina catalyst have been observed by many workers. Contradictory observations concerning the effect of oxygen on the e a r . spectra of anthracene and perylene adsorbed on silica-alumina catalyst were reported. Fogol found a large absorption spectrum only when air was admitted, whereas Brouwer2 reported that the positive ion was almost completely destroyed within a few hours upon contact with air. Moreover, Rooney and Pink3 recently observed that the peak height of the spectra of both adsorbed perylene and anthracene decreased with increasing oxygen pressure. The purpose of the pres(1) J. K. Fogo, J . Phye. Chem., 65, 1919 (1961). (2) D. M. Brouwer, Chem. Znd. (London), 77 (1961). (3) J. J. Rooney and R. C . Pink, Trans. Faraday SOC.,58, 1632 (1962).
