V. R. PAIVERNEKER AND J. N. MAYCOCK
2798
The Photochemical Decomposition of Silver Perchlorate by V. R. Pai Verneker and J. N. Maycock Research Institute for Advanced Studies, Martin Marietta Corporation, Baltimore, Maryland (Received December 91, 1967)
B1%,87
The kinetics of gas evolution from anhydrous silver perchlorate irradiated with light from a low-pressure mercury lamp has been investigated as a function of intensity, temperature, time of irradiation, age of tlhe material, and doping with multivalent impurities. The data indicate that three different mechanisms are operating: (1) creation of defect centers leading to an acceleration in the rate of photolysis, (2) consumption of defect centers leading to deceleration, and (3) a steady-state rate resulting from the initiation of the reaction from silver metal specks. The doping experiments suggest that the consumable defect centers are anion vacancies, since an increase in anion-vacancy concentration, achieved by doping with S042-ions, increases the rate of photodecomposition.
Introduction Anhydrous silver perchlorate is known to be light sensitive; however, no work is reported on its photochemical decomposition in the solid state. The importance of a study of the photolysis of silver perchlorate can be realized from the work of Maycock, et aL,l who showed a correlation between the electronic spectra of metastable materials and their instability. The instability of silver perchlorate has resulted in explosion from grinding or breaking up cakes of the salt. The first study of the photolysis of solid perchlorates was reported by RiIaycock, et who postulated a mechanism involving electron traps to explain the photochemical decomposition of nitronium perchlorate. This earlier study involved a perchlorate having an inorganic nonmetallic cation. The present work deals with the study of the kinetics of the photodecomposition of a solid metal perchlorate, i.e., silver perchlorate. Experimental Section Silver perchlorate is known to be very deliquescent and forms a monohydrate which can be dehydrated at 43". The starting material, obtained from K & K Laboratories, was dissolved in double-distilled water, and the recrystallized material was dried in vacuo at 70" for 48 hr before being stored in a vacuum desiccator in the dark. The doping was achieved by adding lop2 mol % of Ca(C104)2 or AgzS04 to AgC104 before crystallization. The decomposition vacuum line and methods of gas analysis and rate monitoring are the same as those described in the study of the photochemical decomposition of nitronium perchlorate.2 A typical sample size weighed about 200 mg with the area of irradiation being about 3 emB. The sample was photolytically decomposed into a working volume of 1.3 1. Before the first photolysis run, the fresh sample of silver perchlorate, contained in the quartz cell connected to the vacuum line, was heated for 2 hr a t 70" The Journal of Physical Chemistry
to remove any possible moisture adsorbed on the surface. The outgassing rate was checked with the Pirani gauge and the mass spectrometer. The irradiation source was a Hanovia low-pressure mercury lamp whose intensity was varied by placing calibratjed neutral density filters between the sample cell and the mercury lamp. The transmittance of these filters was calibrated by replacing the cell with a thermopile. For studying the activation energy, different temperatures were attained by use of a water bath whose temperature was controlled thermostatically. All temperature measurements were made with a copperConstantine thermocouple. Although a provision was made for using a cold trap, in these experiments no cold trap was used in the working volume.
Results When anhydrous silver perchlorate is irradiated with light from a low-pressure mercury lamp, its color changes from white to pink to brown to brown-black. When the irradiated salt is dissolved in water, an insoluble white residue remains. Qualitat,ive tests indicate the presence of silver chloride. The gaseous products of the photodecomposition consist mainly of oxygen, with very small amounts of chlorine and oxides of chlorine. The rate of evolution of oxygen, as measured with a mass spectrometer, and the total rate of evolution of all gases, measured with a Pirani gauge, from the irradiation of silver perchlorate with a low-pressure mercury lamp as a function of time of irradiation is shown in Figure 1, curve A. It is apparent that after the initial periods of acceleration and deceleration this rate is constant with time. Long irradiations of the order of 4 hr have been performed with no change in rate; so apparently the rate will remain constant until all of (1) J. N. Maycock, V. R.Pai Verneker, and C. 8.Gorzynski, Spectrochim. Acta, A23, 2849 (1967). (2) J. N. Maycook, V. R. Pai Verneker, and L. Witten, J. Phu8. Chem., 71, 2107 (1967).
2799
THEPHOTOCHEMICAL DECOMPOSITION OF SILVER PERCHLORATE
I
I
40
I
80
l
l
1
120
TIME (min)
Figure 1. Rate of photolysis as a function of time of irradiation with a low-pressure mercury lamp: 0, 0, as measured by a Pirani gauge; 0 , +, rate of 0 2 evolution as measured by mass spectrometer. Curves A (0,0 ) and B (0, +) represent 3 day old and 3 week old samples, respectively.
5 '0
2.51
9: 1.4
TIME (min)
Figure 3. Rate us. time plot to show the effect of doping on the photolysis of AgClO4: AI, B1, CI, D1 represent AgClOa doped with 10-2 mol % Caz+; Az, Bz,CZ, Dz represent undoped AgC104; AB,BsJCsJDSrepresent AgClO4 doped with 10-2 mol % Soda-. I
~
Light on gases
pumped off
TIME (mln)
Figure 2. Rate-time plot to show the photolytic behavior of a preirradiated sample. This also shows the effect of pumping off the gas during photolysis and illustrates the possibility of a back reaction.
INTENSITY (Arbitrory Units)
Figure 4. Rate of gas evolution as a function of the first power of the intensity of the light source.
the salt is transformed. When a shutter is placed between the light source and the sample, photolysis immediately ceases and there is no evidence of any ' I dark rate." A careful analysis of the postirradiation measurements suggests that some back reaction is occurring, resulting in a slight decrease in the over-all pressure. This is so small that a systematic investigation of the back reaction is not possible. Upon reirradiation of the same sample, the same constant rate (as in the first run) is achieved after a small deceleratory region. This is shown in Figure 2 . If now one pumps off the gases evolved during the photolysis in the constant-rate region for a period of 40 min and again starts the measurements, one observes the same deceleratory region leading to the same constant rate. This is also shown in Figure 2. Photolysis of an irradiated sample left in an atmosphere of dry oxygen for 24 hr, after which the oxygen was pumped off, is similar to that of a sample which has not been exposed to oxygen. This is not shown separately, as the rate-time curve can be superimposed on the one in Figure 2. When a fresh sample of silver perchlorate (nonirradiated) stored in a desiccator in the dark for 3 weeks is photolyzed, its behavior is much different from that
0.0
1031~OK
Figure 5. Logarithm of the rate of gas evolution us. 10B/T(OK) for the photochemical decomposition of pure silver perchlorate in the constant-rate region.
of a sample just 3 days old. This is shown in Figure 1, curve B. The behavior is similar to that of an irradiated sample when the initial acceleratory and deceleratory regions are absent. Figure 3 is a rate-time plot showing a comparison between the photolysis of pure silver perchlorate, silver perchlorate doped with mol yoof Ca2+, and silver Volume 72, Number 8 August 1968
V. R. PAIVERNEKER AND J. N. MAYCOCK
2800
perchlorate doped with mol % of S042-. The 502- ions cause an increase in the rate of decomposition in the early portion of the curve, whereas the Ca2+ ions produce a decrease in the rate. At longer times, the dopant appears to have no effect. In the constant-rate region, the rate of the product formation is proportional to the first power of the intensity of light (Figure 4), the dependence being valid from 0 to 92". The temperature dependence of the rate of gas evolution is shown to obey an Arrhenius type of plot quite accurately from 0 to 92" (Figure 5 ) , the activation energy of the photochemical decomposition being 4.5 kcal mol-l.
Discussion When freshly prepared silver perchlorate is irradiated with light from a low-pressure mercury lamp, the rate of gas evolution initially increases to a maximum and then falls to a constant value (Figure 1, curve A). One possible explanation of the rate increase during the initial stages of the photolysis above the final steadystate rate is that it is associated with outgassing or desorption. Outgassing is unlikely owing to the long (18 hr) pumping treatment the sample has undergone. Since an aged sample (Figure 1, curve B) does not show this behavior, outgassing and desorption processes are unlikely. Another argument against desorption is that after exposure of an irradiated sample to dry oxygen for 24 hr it does not show the initial acceleratory behavior. An irradiated sample has many more nuclei on the surface to adsorb oxygen, and if the initial behavior were due to desorption, this behavior should have been much more pronounced. One could argue that some gas other than oxygen is adsorbed; however, our mass spectrometric data show that the gas being evolved during that period is oxygen and that the plot of the rate of oxygen evolution us. time is similar to that of the rate of the total gas evolution us. time. One could possibly attribute this behavior to the diffusion of the decomposition species from within a few layers of the crystal to vacuum, but once again one should have seen this behavior with the aged sample. With the available data it is not possible to explain this behavior of accelerating rate, other than to speculate that some sort of defect centers are being created faster than they are being destroyed. Any mechanism describing this early rapid increase in rate must also be dependent on one photon. With these data, the first mechanism proposed is
*
ClO, f 0 c104*+ @ C104*4products f 0 Ag' -I- @ -+ Ago f 0
(1) (2)
(3)
With an aged sample these centers presumably are destroyed, by annealing processes, and hence one does not see the acceleration and deceleration. The small The JOUTTUL~ of Physical Chemistry
deceleration seen with an aged sample is similar to the deceleration observed when a preirradiated sample is being photolyzed and should not be confused with the pronounced deceleration seen in a fresh sample. The small deceleration seems to originate because of a back reaction. The inference of a back reaction is that mass spectrometrically only 0 2 is observed as a decomposition product. The products of eq 2 will undoubtedly include Clz, which can readily react with the atomic silver of eq 3 to form AgC1, as observed in the qualitative analysis test. Following the acceleratory region in a fresh sample is the pronounced deceleratory region leading to a constant-rate region. This type of deceleration has been observed in the azidesa and also in nitronium perchlorate.2 Pai Verneker and Forsytha and Maycock, et aL12have explained this behavior by postulating that the mechanism depends on the existence of trapping centers which are consumed in carrying out their part of the reaction mechanism. Our results on the ('doping" experiments (Figure 3) are consistent with this speculation and enable us to guess the nature of the trapping centers. Doping silver perchlorate with Ca2+ will enhance the concentration of cation and, likewise, doping with 502- will vacancies enhance the anion-vacancy concentration [ 01. The law of mass action requires that at any given tempera= constant. This requires that an inture, crease in cation-vacancy concentration will result in a decrease in anion-vacancy concentration; ie., doping with Ca2+leads to a suppression of anion vacancies and doping with 502- produces more anion vacancies. Our results (Figure 3) show that doping with Ca2+ suppresses the maximum rate of photolysis and doping with 502- enhances the maximum rate with respect to an undoped sample. Therefore, the trapping center is likely to be an anion vacancy. As the behavior of nitronium perchlorate is similar to that of silver perchlorate in this region before attaining the constant rate,2 the photolysis of nitronium perchlorate may also involve anion vacancies as the initial trapping centers. The mechanism which consumes the traps during photolysis should lead to a rate which is decreasing, ultimately becoming zero; the results, however, indicate that a finite constant rate is attained. This finding suggests that a new mechanism is now in operation, which does not involve consumable trapping centers and presumably continues until the crystal is consumed. During this mechanism the rate of gas evolution is proportional to the first power of the intensity of light (Figure 4) and has an activation energy of 4.5 kcal mol-1 (Figure 5). Presumably silver metal formed as a result of the repetition of reactions 1-3 acts as the reaction initiator by photoemitting an electron
[e]
[@][a]
(3) V. R. Pai Verneker and A. C. Forsyth, J . Phys. Chen., 71.3736
(1967).
ISOTROPIC ROTATIONAL RELAXATION OF PHOTOSELECTED EMITTERS
+ eAgMo + Clod.
hv
AgM+
AgMo--t AgM+
(4)
+ Clod-
(5)
-
C104. +products
+
+
(6)
AgZ Ag+ eAgM+1° (7) AgM+lois representative of the growth of the colloidal metal. The absolute rates for the photochemical decomposition of AgC104 are of the order of 3 X 1013 molecules sec-l, I n contrast, the rates for nitronium perchlorate are of the order of 5 X 10l6 molecules sec-l. A difference between these two rates is to be expected if the Ago atoms are t o control the rate of photodecomposition. Similarly, a mechanism of this type has been postulated for the photodecomposition of metal azide^.^ By analogy with the photodecomposition of metal azides, this mechanism would predict a dark reaction. Our experimental results do not indicate any dark reaction. This, however, does not mean that the dark reaction is not present. The gas evolution during the dark reaction is often small and it is thus possible that the back reaction (such as the reaction of chlorine with colloidal silver specks) overshadows the dark reaction. This is perhaps why we find some silver chloride in the residue.
280 1
Alternatively one could argue that the initial processes lead to the formation of gaseous oxygen and silver chloride and as the mechanism involving the consumption of traps (reactions 1-3) nears its end, the photodecomposition of silver chloride commences. Photolysis of silver chloride leads to the formation of chlorine and metallic silver; metallic silver, in turn, starts the mechanism illustrated in reactions 4-7 and one observes a steady-state rate of gas evolution. In conclusion, the pl otodecomposition of silver perchlorate seems t o involve three mechanisms: (1) creation of defect centers leading to an acceleratory rate, (2) consumption of defect centers resulting in deceleration, and (3) a steady-state rate originating from the initiation of the reaction from the photoemission from silver metal specks. The doping experiments suggest that the trapping centers are anion vacancies. Acknowledgment. We wish to acknowledge the valuable assistance of W. Lochte in the performance of these experiments. (4) P. W. M. Jacobs, F. C . Tompkins, and V. R. Pai Verneker, J. Phys. Chem., 66, 1113 (1962).
Isotropic Rotational Relaxation of Photoselected Emitters and Systematic Errors in Emission Decay Times by A. H. Kalantar Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada (Received December 96, 1967)
Photoselected luminescent molecules, whose first-order decay time, r , and isotropic rotational relaxation time, rR, are of the same order, yield nonexponential decay curves. The systematic errors in the observed lifetime can exceed 12%. The errors depend upon the duration of the excitation, T / T R , the angle between the exciting and viewing directions, the polarization of the exciting light (even if unpolarized), the electronic anisotropy of absorption and emission, and the relation between the molecule axes of absorption and emission, Errors in lifetimes are apt to be more important for fluorescence. They are more likely for larger (e.g., biochemical) emitters or for more viscous solutions. The ways to recognize and minimize such errors are outlined.
Introduction The origin of the errors with which this article is concerned is most simply understood if the lifetime measuring scheme of Levshin and of Perrin is first recalled.lg2 Indirect determination of short luminescence lifetimes ( T ) is possible for molecules whose transition moments for absorption and emission are along
particular axes. Polarized light excites and selects from the randomly oriented ground-state molecules those whose absorbing axes are most favorably ori(1) V. L. Levshin, Z.Phys., 26,274 (1924); F. Perrin, C. E. Acad. Sci., Paris, 182, 928 (1926). (2) P. Pringsheim, “Fluorescence and Phosphorescence,” Interscience Publishers, New York, N. Y.,1949,p 370 ff.
Vohme 78, Number 8 August 1968
