V. R. PAIVERNEKER AND A. C. FORSYTH
3736
ments of dipoles in the substrate surface layer, changes in bonding of the adsorbed head group to substrate, and changes in lateral interactions between adsorbed molecules. Each of these terms can be expected to vary when the chain length and surface coverage change. The apparently constant orientation of the amides, reflected by their constant AV, can perhaps be interpreted as follows. Stuart-Briegleb models show that the aliphatic carbon chain can exist in a spiral vertical form which occupies about one-third greater surface area than the straight chain model. If the amide films were in this configuration, they would be nearly “close
packed” a t 60% coverage as calculated from a straightchain model. At other surface coverages, the amides would be expected to behave like the other homologous series studied. The results of this study show that ellipsometric and surface potential measurenients can be used to obtain reproducible surface dipole moments. More work is needed to separate the various dipole vector terms involved before the data can be analyzed further. Acknowledgment. The authors thank Dr. L. G. Wiedenmann for helpful discussions on molecular structure.
Photodecomposition of a-Lead Azide in the Solid State
by V. R. Pai Verneker’ and A. C. Forsyth Ezplosives Laboratory, Picatinny Arsenal, Dover, New Jersey
(Received March 6, 1967)
The kinetics of nitrogen evolution from a-lead azide during irradiation with ultraviolet light from a low-pressure mercury lamp has been investigated as a function of the intensity, temperature, and time of the irradiation, the method of preparation, and the age of the sample. The data clearly demonstrate the dependence of the initial photolytic decomposition rate on the method of preparation. It is further concluded that, during the aging process, the defects (resulting from the incorporation of impurities) which participate in the photodecomposition disappear irreversibly. The efficiency of the photolytic process is seen to be greater at +lo” than a t +25”. This is discussed in the light of possible electron traps and intermediate free radicals.
Photodecomposition of Lead Azide in the Solid State
Experimental Section
The thermal decomposition of a-lead azide under vacuum and in air has been extensively studiede2 The mechanism, which assumes that the final product of decomposition, lead, catalyzes the reaction, has been shown to be valid by the experiments by Reitzner, et al.3s Acceleration of the thermal decomposition of a-PbN6 as a result of prior irradiation has also been demonstrated experiment all^.^^ Recently Jacobs, et aL,* have shown how the photodecomposition of BaNa is affected by its prior partial thermal decomposition. On the other hand, no work on the photolysis of PbT\’6 is reported in the literature.
PbN6 was contained in a small boat so as to keep constant the surface area exposed to the light source.
The Journal of Physical Chemistry
(1) Research Institute for Advanced Studies, Baltimore, Md. 21227. (2) W. E. Garner and A. S. Gomm, J . C h a . Soc., 2123 (1931); W.E. Garner, A. S. Gomm, and N. R. Hailes, ibid., 1393 (1933); W. E. Garner, Proc. Roy. SOC.(London), A246, 203 (1958); W.E. Garner, “Chemistry of the Solid State,” Butterworth and Co. Ltd., London, 1955, Chapters 7 and 9; G. Todd, Chem. Ind. (London), 1005 (1958); M. Stammler, J. E. A M , and J. V. R. Kaufman, Nature, 185,456 (1960); B. Reitzner, J . Phys. Chem., 65,948 (1961). (3) (a) B. Reitzner, J. V. R. Kaufman, and E. F. Bartell, ibid., 66, 421 (1962); (b) J. V. R. Kaufman, Proc. Roy. SOC.(London), A246, 219 (1958); W.Groocock, ibid., A246, 225 (1958);J. Jach, J. Phys. Chem. Solids,24, 63 (1963).
PHOTODECOMPOSITION OF LEAD AZIDE
The boat was in turn placed in a transparent silica cell with a flat window connected via a B-24 standard joint to a vacuum line. This consisted, in sequence, of a trap immersed in liquid nitrogen, a standard volume, a Pirani gauge, a McLeod gauge, a cutoff, a second trap, a diff usion pump, and a backing pump. The temperature of the azide was measured with a calibrated copper-constantan thermocouple in the sample. The intensity of the light (X -2537 A) was changed whenever desired by changing its distance from the sample. The P b S 6 used throughout the course of the work, except when the effect of different preparations was being studied, was prepared as follows. PbO as supplied by Jlessrs. Johnson and Illatthey, United Kingdom, was converted to lead nitrite by bubbling nitrous oxide through an aqueous suspension of PbO. Gaseous HN3 prepared from a Reitzner generator5 was collected in alcohol and the alcoholic solution of HK3 was added to the solution of lead nitrite to obtain PbN6. X-Ray and chemical analyses revealed the product to be a-PhX6. Three other preparations, as follows, were also investigated. Instead of using spectroscopic grade PhO, reagent grade PbO was used as the starting material. X solution of lead acetate was allowed to react with a solution of KaN3. PbXO as supplied by Du Pont [prepared from Pb(KO&] is also used. Fresh PbX6, prepared as described above, was stored in the dark in a vacuum desiccator (pressure mm). Before the start of any photolysis experiment, the pressure was always less than 1 X mm. The low-pressure mercury lamp was allowed to warm up for a minimum of 20 min before a run. The nitrogen gas evolved was measured with a Pirani gauge.
3737
.--
L
0
REAGENT PbO + NoNj PPTD
o PbAc
SPECPURE PbO
E.I. DUPONT
COLLOIDAL AZIDE
.40
t 0'
I
I
40
I I I 80 120 TIME (MINUTES) I
I
I
160
Figure 1. Decomposition rate us. time of irradiation (A -2537 A) for various preparative methods of crPbNs.
Results The rate of nitrogen evolution from a freshly prepared sample of a-PbK6 plotted against the time of irradiation is shown in Figure 1. The rate decreases and then attains a constant value. Figure 1 also shows how different preparations behave when photolyzed. Except for the Du Pont material, the samples show approximately the same final, constant rate although the initial rates vary. If the light is switched off after the photolytic rate has attained a constant value, gas continues to be evolved. The rate of this reaction (termed hereafter the dark rate) falls off gradually and ultimately becomes zero, as shown in Figure 2. The total gas collected after the light is switched off is directly proportional to the total time of irradiation, as can be seen in Figure 3. If the sample is reirradiated after the postirradiation gas evolution stops, Le., the dark rate becomes zero, the rate starts at a lower value. This rate is lower than the constant rate in the initial
0 TIME (MINUTES)
Figure 2. Dark rate us. time.
(4) P. W. M. Jacobs, F. C. Tompkins, and V. R. Pai Vernaker, J. Phye. Chem., 66, 1113 (1962). (5) B. Reitzner and R. Manno, Nature, 198, 991 (1963).
Volume 71. Number 19 November 1967
V. R. PAIVERNEKERAND A. C. FORSYTH
3738
4i' 2
0
"
40 80 120 160 IRRADIATION TIME (MINS)
20 40 TIME (MINUTES)
60
Figure 5. Plot of final rate minus the subsequent starting rate (Rn- R I )us. time.
Figure 3. Dark rate rn a function of irradiation time.
\.
RUN
INTENSITY (ARBITRARY UNITS) Final Rate Represents -1 7 Runs Over a 2 Week Period I
I
20
I
I
I
I
40
60 IRRADIATION TIME (MINS)
I
Figure 6. Final constant rate us. light intensity.
I
100
Figure 4. Subsequent rate-time plots on the same sample of cu-PbNe.
run and increases to attain approximately the same value as that of the constant rate in the initial run. Figure 4 presents plots of these curves. The starting point of the Nth run depends on the time that has elapsed between the Nth and the (N - 1)th runs. If the initial rate is R1 and the final constant rate Rz, then (Rz - R1) is dependent on the time elapsed between the two runs if the time is of the order of 10 min. This is shown in Figure 5 . From Figure 6 it can be The Journal of Physical Chemhtry
seen that the constant rate is directly proportional to the intensity of the light. Finally, the activation energy for the process is found to be negligible between 30 and 80". If 2% of the sample is thermally decomposed prior to photolysis, the behavior of the sample does not change, ie., the photolytic rates are the same as those of an untreated sample, shown in Figure 1. The total decomposition caused by photolysis amounts to less than
%. During the first photolytic run the sample turns brown. If a few millimeters of oxygen is then admitted into the system and kept in contact with the sample for about 24 hr the sample turns white and, on reirradiation, behaves as if it were a fresh sample (Figure 7).
PHOTODECOMPOSITION OF CY-LEAD AZIDE
3739
0 0
0
A
INITIAL RUN
10 Minutes Irradiation 30 Minutes Irradiation 50 Minutes Irrodiation
o RUN AFTER 02
ATMOSPHERE
I
0'
30
I
I
I
60 90 120 TIME (MINUTES)
I
1
150
180
Figure 7. Ratetime plot: 0, fresh sample of or-PbNo; 0,the same photolyzed sample of a-PbNa on exposure to oxygen.
0
7-
INITIAL RATES FOR 50-60°C STORAGE
z
f 2
-I
("C)
Figure 9. Irradiation of cY-PbNe a t -30' and subsequent gas evolution on warm-up a t higher temperatures.
during the first week. A sample which has aged for several months does not show any change in behavior even when exposed to oxygen (compared with Figure 7). Photolysis of PbNa in the temperature range -40 to 0" does not produce any gas during irradiation but, if the irradiation is stopped and the sample warmed up, a burst of gas is given off at 10". On further warming another burst is observed at +25" (Figure 9). The total gas collected at +loo is double that collected at +25". Irradiation of a sample at a constant lamp intensity and for a constant time at both +lo" and also at +25" shows a marked difference. Irradiation at +lo" produces a certain quantity of gas, but on warming to +25" more gas is evolved. The total gas thus collected is about 15% greater than the volume of gas evolved when the irradiation is done only a t +25". Figure 10 is a plot of the gas collected on warm-up vs. the time of irradiation at -40". It can be seen that for short irradiations the gas evolved depends on the time of irradiation but for long irradiations the gas volume is independent of the time of irradiation and is constant. Furthermore, during the initial part of the curve in Figure 10, the rate of gas evolution is proportional to the intensity of the light.
+
6-
cn
s
TEMPERATURE
54-
23 2~ 1
i
' 0
2
4
6
8
1 0 1 2
STORAGE TIME (WEEKS)
Figure 8. Effect of long-term storage a t 50-60" on the photolytic decomposition of a-PbNa.
Figure 8 shows how samples not exposed to any light and preserved in vucuo behave when photolyzed after different storage times. Storage at different temperatures (-196 to +55") does not seem to have an appreciable effect. When kept at - 196" the aging process (the falling off of the initial photolytic rate with storage time) seems to slow down slightly, particularly
Discussion In all azides studies thus far,6 the photolytic rate (rate of gas evolution) decreases and then attains a ~
(6) V. R. Pai Verneker, submitted for publication.
Volume 71, Number 19 November 1967
V. R. PAIVERNEKER AND A. C. FORSYTH
3740
D 5W
30
60
90
120
150
IRRADIATION TIME (MINUTES)
Figure 10. Total gas collected on warming to room temperature w a function of irradiation time a t -40".
constant value. The behavior of a-PbNe is in conformity with the other azides. There are certain defects, e.g., negative ion vacancies, impurity ions acting as electron traps, etc., present in a crystal of PbNs and these defects are assumed to be consumed in the process of photolysis; hence, the original rate decreases. Metal, present in the form of nuclei or small specks and produced during the photolysis presumably initiates B new process. The photolytic reactions in sequence are
+ hv 9(ANs-)* exciton trapped (AN3-)* + NB-+3N2 + (eAe) + 2 vacancies Na-
-
(1) (2)
Pb2+4+ aggregate Of traps (3) Repetition of eq 1, 2, and 3 will produce Pb, having a metallic character.
+ hv +Pb,+ + e Pb,+ + N3Pb, + N3 Na + Ns- +3N2 + e + vacancies Pb,
4
(4) (5)
(6)
As photolysis proceeds more metallic specks are produced and the constant rate might be expected to increase steadily. However, the small specks aggregate and thus have proportionately fewer neighboring azide ions to ionize. Thus, on one hand metallic lead of requisite size is formed and on the other hand its influence is destroyed as larger aggregates develop. The constant rate can, therefore, be considered as the steadystate rate resulting from these opposing reactions. The mechanism described above requires the photolytic rate to be proportional to the intensity. This is indeed the case (Figure 6). Details of the rate-intensity relationship are discussed by Jacobs, et aZ.* The Journal of Physical Chemistry
Different preparations incorporate different impurities into the lead azide crystals and the variation in the initial rates is, therefore, not surprising. The mechanism of the dark rate, however, is difficult to envisage. It is possible that at the end of a photolytic run there are inside the crystal charged lead specks which continue reaction 5 after the light is switched off. Jacobs, et u Z . , ~ have explained the postirradiation gas evolution in BaNBin this way. It is also found that irradiated samples hold within the crystals a considerable amount of gas which can be liberated by di~solution.~It is possible that during the aggregat,ion of small metallic specks some of this gas is released. This process would also show an exponent'ial decay. The fact that the dark rate increases with the fraction of the salt decomposed, however, would favor the mechanism put forth by Jacobs, et aL4 Although the subsequent runs show the same constant photolytic rate, the initial rate of photolysis of the Nth run depends on the time elapsed between the Nth and ( N - 1)th runs. This can also be understood on the basis of the aggregation of metallic specks. If the time elapsed between the runs is more than 10 min the initial rate is relatively independent of the elapsed time. This indicates that the aggregation is a fast process. Experiments on the aging of lead azide samples point out that the defects which are present are steadily destroyed in an irreversible manner. Cunningham and Tompkins* have reported a decrease in the F center concentration in KN3 after aging. A fresh sample of PbNe which has been irradiated and then subjected to oxygen suggests that the defects which were destroyed during the photolysis may be regenerated. However, an aged sample, even on exposure to oxygen, does not regenerate these defects. In the first case, during photolysis metallic lead is produced and it is possible that oxygen reacts with the lead and, in so doing, recreates the original defects (perhaps negative ion vacancies). In the second case no metallic lead is produced and hence the oxygen has nothing with which to react. The low-temperature experiments also revealed some interesting findings which are subject to several speculative interpretations. It is possible that intermediates like N4-and NZ-, which have been found in other azide^,^ are formed. The stability of these intermediates varies from azide to azide. The decomposition of K4- to give N2 at +lo" and of N2- to give Szat f25" would (7) V. R. Pai Verneker, submitted for publication. (8) J. Cunningham and F. C . Tompkinu, PFOC. Roy. Soc. (London), A251, 27 (1959). (9) A. J. Shuskas, C. G. Young, 0. R. Gilliam, and P. W. Levy, J. Chem. Phya., 33, 622 (1960); G. J. King, B. S. Miller, F. F. Carlson, and R. C. McMillan, ibid., 35, 1442 (1961).
PHOTODECOMPOSITION OF CY-LEAD AZIDE
3741
explain the present results. However, it is possible to have different types of traps characterized by different trapping stabilities. If the starting reaction is represented as
+ N3- *(AN3-)* hv + +(BN3-)* trap .4
hv
(7)
trap B
IY3-
(8)
and if one of the traps, say A, empties rapidly above +lo", then the efficiency of the backward reaction
would be greater at higher temperatures. This is a possible reason why there is a 15% difference in the total gas evolution when experiments are done at +10 and +25". The possibility that the two bursts are due to one state and caused by the method of heating cannot be ruled out. Finally, experiments carried out at -40" show that either (1) the traps are limited in number or (2) the crystal has a finite capacity to accommodate the intermediates produced. I n either case one would observe a saturation effect, as observed in these experiments.
volume 71. Number 12 November 1967
