Y. P. VIRMANI, J. D. ZIMBRICK,AND E. J. ZELLER
1936
lcinetic scheme obtains
+ Cs(SU) %- 02.Cs(SU)+ Y2eo2+ C ~ ( S U )+ o ~ - c ~ ( -2s u )2cso2+ y3eO2
0 2*
CS(SU)
-%csoz
(17) (18) (19)
The steady-state condition is identical to that derived before, but for I one obtains the expression
I
= aY,iczipcso(i
- e)
+
[ u Y ~ ~ ~ / c ~ ~-~e ~) yc/ s , , ~ ( I [IC,'
+ /C3pcSo(l - e ) ]
(20)
The major difference, experimentally, between these mechanisms is that, whereas the former predicts that the initial emission when oxygen is admitted to the system will be zero (since initial intermediate concentration is zero), the latter predicts a nonzero initial emission to which would be added the second-order emission
as the concentration of intermediate increased. The first mechanism appears to be much more likely, inasmuch as careful attempts t o observe initial emission upon first admission of oxygen to the system have met with negative results. Because the initial emission may be low, however, and because initial pressure control is difficult to attain, the second mechanism cannot be completely ruled out.
Acknowledgments. This work was supported by a research grant from the Arts and Sciences Research Committee of the American University of Beirut. A portion of the work provided the basis for the M.S. Thesis of A h . lloucharafieh, and another portion was carried out with the assistance of A h . J. Topalian. The Pyrex apparatus was constructed by Mr. D. Srouji of the AUB glassblowing facility. We are also indebted to one of the reviewers for calling our attention to the AuCs reaction and to recent work on the role of CszO in photoemissive mechanisms.
An Electron Paramagnetic Resonance Study of Hydrogen Atoms
Trapped in ?-Irradiated Calcium Phosphates by Y. P. Virmani, John D. Zimbrick,* and E. J. Zeller Departments of Chemistry, Radiation Bwphusks, Geologu, and Physics, Univeraity of Kansas, Lawrence, Kansas 66044 (Receized Auoust 6, 1970)
Pitblieation costs borne completetg by The J o u m a l of Physical Chemistry
Epr studies were carried out on trapped H atoms in e°Co y-irradiated calcium phosphate powder at room temperature. The radiation yield of H atoms was relatively small [G(HT)g 0.0021 and inversely related to the extent of hydration of the phosphate. The H atoms decayed slowly at room temperature with an initial half-time of 4 days. The epr characteristics of the H-atom doublet were very similar to those found for the same species in acidic ices except that the epr line width in phosphate was ody 0.35 G compared with approximately 3.4 gauss in HzSOl glassy ice at 77OK. Paramagnetic relaxation measurements us. radiation dose were made on the H-atom epr lines. These data taken together with the thermal decay and dose saturation data indicated that the H atoms are produced initially in a nonuniform spatial distribution which gradually changes with dose and irradiation time to a spatially more uniform distribution.
I. Introduction The y radiolysis of certain calcium phosphate polycryPtalline powders has been reported to yield trapped hydrogen atoms (€IT) which are comparatively stable a t room temperature.'I2 Only the g factors and splitting of the HT have been given and these appear to be very (,lose to the corresponding values for HTin various rare gas matrices3 and frozen acidic ices.4 In the present jvorli n e have studied the radiation yields, epr l'hr Journal of Phpieal Chemistw, Vnl. 76.,\To. l d . 1.971
line widths, dose saturation, and paramagnetic relaxation characteristics of the HT in several different prep(1) I?. W. Atkins, N. Keen, M.C. R . Symons, and H. W. Wardale, J . Chem. Soc., 5594 (1963). (2) S. Ogawa and R . W. Fessenden, J . Chem. Phys., 41, 1516 (1964). (3) 5. N.Foner, E. L. Cochran, V. A. Bowers, and C. K . Jen, ibid., 32, 963 (1960). (4) R. Livingston, H. Zeldes, and E. H. Taylor, Discuss. Faraday SOC., 19, 166 (1955).
Em STUDYOR TRAPPED H ATOMS
1937
Figure I . IGpr spectrum of irradiated Matheson brand calcium phosphate at room temperature: B°Coy dose = 1.84 X los rads; microwave power = 4 x 10-2 W; H , = 1.2 G; P, = 100 kHz.
arations of tribasic. calcium phosphate [Calo(l'O&(0H)zI.
11. Experimental Section Reagent grade tribasic calcium phosphate [Calo(P01)6(OH)z]as polycrystalline powder was obtained from thc following suppliers: Jlatheson, Fisher, Baker, and Rlallinr.l
