effect of particle-size distribution on the thermal decomposition of .alpha.-lead azide

870 R. W . Hutchinson, S. Kleinberg, and F. P. Stein

Effect of Particle-Size Distribution on the Thermal Decomposition of a-Lead Azide

Richard W. Hutchinson, Sidney Kleinberg, and Fred P. Stein*

Chemical Engineering Department, Lehigh University, Bethlehem, Pennsylvania 78015

Publication costs assisted by the U. S. Army Research Office-Durham

(Received November 73, 19721

Samples of a-lead azide powder having known, but different, particle-size distribution were thermally decomposed at temperatures between 210 and 270". The finer powders (mean equivalent-spherical radius, 8 g ) began to decompose sooner and exhibited a maximum rate greater than twice that of the coarser powders (mean equivalent-spherical radius, 24 p ) . A single-equation mathematical model, which contains two parameters and incorporates the independently measured particle-size distribution, gave an excellent fit of the sigmoid-shaped decomposition curves. One of the parameters extracted from the data via the model was shown to be the rate of penetration of the reaction front into a single crystal. Thermal decoinposition data for a-lead azide powder of a given particle-size distribution can be transformed to another size and distribution and, thereby, permit study of other effects, such as purposely added impurities, in the absence of the significant particle-size effects.

Introduction a-Lead azide is a metastable, white crystalline com-

pound which decomposes a t temperatures in the range of approximately 200 to 270".

PbN,-Pb + 3Nz At somewhat higher temperatures, or when subjected to shock or friction, it epplodes.

Several of the previous studiesl-4 of the thermal decomposition or explosioii of lead azide mentioned qualitative observations of the effect of particle size, but none seri- ously studied nor quantitatively reported on more than one size of material of the same type. Garner and Gomm,l and other~,~qS also observed that thermal decomposition began at the surface of the particles as evidenced by the rapid darkening over the whole surface. It can, therefore, be expected that the rate of thermal decomposition would be significantly related to the surface area per unit mass of particles, and, furthermore, that the particle-size distribution as well as the total surface area would be impor- tant.

Because the state of the art does not permit the preparation of different batches of high-purity a-lead azide powder with well-controlled, identical particle-size distributions, quantitative comparison of thermal decomposition data cannot be accomplished in the absence of particle-size information If one wants to study, for example, the effect of certa in impurities on the thermal decomposition of a-lead azide, as we do, the effect of the impurity is compounded iInd, in some cases, completely masked by the effect of particle :;ize.

The purpose of this work was to demonstrate, measure, and correlate the effect of particle size on the thermal decomposition of a-lead azide powders in vacuo. In this study powders of a-lead azide which had known, but different, particle-size distributions were thermally decomposed. The per cent of decomposition was monitored continuously throughout each run, and the per cent of total conversion was meamred. These data were correlated with a new model which approximated the decomposition data very well and which produced descriptive parameters that were independent of particle size.

Most of the studies of the thermal decomposition of powders of a-lead azide either considered the reaction to

be autocatalytic in nature6-8 or fit the resulting sigmoid- shaped decomposition curves with various functions of reaction time.4.9-11 However, measurement of the thermal decomposition of single crystals of a-lead azidelJ2 did not produce sigmoid-shaped decomposition curves, although there were various delays in the start of reactions of some of the single crystals. The model developed hereinafter, based on particle-size distribution, describes the sigmoid- shaped curves observed during decomposition of lead azide powders but does not require the autocatalytic ki- netics which are apparently not observed in single crystals.

Experimental Section Apparatus. The apparatus used in this work has been

described in detail e1~ewhere.l~ The decomposition of a- lead azide was monitored by collecting the released nitrogen on activated charcoal which was maintained a t the temperature of liquid nitrogen and by continuously re- cording the weight of the charcoal as given by a Cahn In- strument Co. electrobalance. The entire decomposition was performed in vacuo at a pressure of less than 5 X

Torr. The decomposition was carried out in a constant-temperature furnace which was controlled to within 10.05" and which was designed such that the decompos- ing sample could "see" only surfaces that were at the con-

(1) W. E. Garner and A. S. Go", J. Chem. SOC., 2123 (1931). (2) A. S. Hawkes and C. A. Winkler, Can. J. Res., Sect E , 25, 548

(1947). (3) F. P. Bowden and K. Singh, Proc. Roy. SOC., Ser. A , 227, 22

(1 954). (4) P. J. F. Griffiths and J. M. Groocock, J. Chem. SOC., 3380 (1957). (5) 0. H. Hill, Appendix B(DRL-A 126) to D. R . L. Accoustical Report

No. 128, U. S. Army Contract DA-44-099-ENG-2566, University of Texas, 1957.

(6) W. E. Garner, "Chemistry of the Solid State," Butterworths, London, 1955, pp 184, 238.

(7) 6. Reitzner, J . Phys. Chem., 65, 948 (1961). (8) B. Reitzner, J. V. R . Kaufman, and E. F. Bartell, J. Phys. Chem.,

66, 421 (1962). (9) W. E. Garner, A. S. Go", and H. R. Hailes, J. Chem. SOC., 1393

(1933). (10) J. Jach, Trans. Faraday Soc.. 57, 947 (1963) (11) D. Young, J. Chem. SOC., 3141 (1964). (12) P. G. Fox, J . Solidstate Chem., 2, 491 (1970). (13) S. Kleinberg and F. P. Stein, lnd. Eng. Chem., Fundam., 1 1 , 134

(1972).

The Journal of Physical Chemistry, Vol. 77, No. 7, 7973

Thermal Decomposition of a-Lead Azide a71

TABLE I: Parameters lor Normal Size Distribution

Sample

R , mean equivalent spherical S, standard radius, p deviation, p

Pure a-Lead Azide Fine 8.3 2.6 Intermediate 80% 13.6 3.3

20% 22.9 8.4 Coarse 23.8 6.8

Fe2+ Doped a-Lead Azide Fine 6.6 i .a Intermediate 8.3 2.5 Coarse 20.5 7.1

trolled temperature. Approximately 10-mg samples were used for each run.

Materials. Two batches of a-lead azide were prepared by bubbling WN3 through an aqueous solution of lead ni- t,rite which had been prepared from Johnson, Matthey, & Co., Ltd. Specpure lead oxide (< lo ppm impufity), 99.995% pure NCP-NO2 gas mixture, and triple-distilled, deionized water. One batch was pure a-lead azide, and the other was a-lead azide which contained a known amount (of Fez++. (This paper concerns the effect of particle size on the decomposition data of these two batches. Data and discussion of the effect of Fe2+ and other impurities will be offered in a subsequent paper.) X-Ray analysis showed that the lead azide was totally of the a form. Elemental analysis by spark-source mass specto- graph showed overall undesired impurities to be approximately 270 ppm by weight.

The particles in bfoth batches varied greatly in size from small needle .shaped particles approximately 60 p long to the largest particles which appeared in the shapes of both needles and platlets with a length of 400 to 600 p. Both batches were classified according to particle size by an hydraulic settling technique which is described in detail elsewhere.l4 'Three particle-size fractions were obtained for each of the two lbatches. The volume particle-size distribution of each sample was measured with a Coulter counter. The distribution of each size fraction, with the exception of the intermediate sample of the pure material, was essentially normal. The exception was approximated as the sum of two normal distributions. The parameters for the normal size diatributions are listed in Table I.

The samples of lead azide were stored in vacuo arid in the dark. These precautions were taken to avoid buildup of surface contaminants and photodecomposition which could interfere with the experimental results. Photograph- ic darkroom lights were used when handling the lead aside.

Results The decomposition data, recorded as weight of released

nitrogen adsorbed on the charcoal us. time, were recast in tlre form of cumulative-weight-per cent-decomposed us. time.

The isothermal decomposition data for the three size cuts of the pure a-leiid azide are shown in Figures 1 and 2 for the temperatures of 240 and 230°, respectively. It is readily observed that, the small particles decomposed at a faster rate than did thle large ones.

toor- 80

n VI

a 60

n

FINE SAMPLE

ERMEDIAlE SAMPLE

-COARSE SAMPLE RUN 1 . - RUN 6 , A

TEMPERATURE: 240'C.

I M 200 300 400 500 ' 660 ' 700 ' 800

TIME, MINUTES

Figure 1. Effect of particle size on the isothermal decomposition of pure a-lead azide at 240".

k W

K W

60

40

20

FINE SAMPLE

INTERMEDIATE SAMPLE

COARSE SAMPLE

TEMPERATURE: 230' C.

I O k L o

TIME, MINUTES

Figure 2. Effect of particle size on the isothermal decomposition of pure a-lead azide at 230a.

The hump and crossover of the initial portion of the decomposition curve for the coarse particles was an anomaly observed only with the coarse sample. The hump appeared, and was very reproducible, a t all temperature levels studied. The temperature coefficient of the rate for this initial reaction period ( E = 38 & 3 kcal) was similar to the temperature coefficient of'the maximum rate ( E = 35.2 f 0.5 kcal). Thus, it is believed that this initial hump and crossover were the result oE decomposition of lead azide rather than of the decomposition or desorption of some other gross impurity. There was also an experimental safeguard against the measurement of a surface impurity provided by the liquid nitrogen cooled traps in the apparatus ahead of the cold charcoal. Higher boiling materials, such as carbon dioxide and water, would have been removed by the traps before they could have reached the cold charcoal and have been recorded as nitrogen weight gain. All of the pure a-lead azide was prepared in the same batch and all of it was treated identically subsequent to preparation, so that the only difference in the material decomposed to obtain the data in Figures 1 and 2 was particle size. It is suggested that the giant particles, which were present only in the coarse sample, cracked at the onset of decomposition and released nitrogen from the newly formed surfaces in quantities sufficient to cause the hump in the initial portion of the decomposition curve. Such a speculation is not totally unprecedented, for (14) R. W. Hutchinson and F. P. Stein, Ind. Eng. Chem., Fundaim., 10,

635 (1971).

The Journal of Physical Chemisfry, Vol. 77, No. 7, 1973

a72

40

k2 a

20.

R. W. Hutchinson, S. Kieinberg, and F. P. Stein

n y1

B

OARSE SAMPLE If-3)

-3WA ( f - t ) , 62% 11-31

TEMPERATURE 240'C

0 _c__I--

100 200 300

TIME, MINUTES

Figure 3. Isothermal decomposition of mixtures of fine and coarse particles of pure d e a d azide.

t

' O P T ? i r ' 360 ' 460 ' I 560 ' 660

TIME, MINUTES

Figure 4. Effect of particle size on the isothermal decomposition of Fez+ doped d e a d azide at 230".

Hawkes and Winklei.2 observed that their largest crystals (length on the orcler of 0.8 mm) of lead azide broke up abruptly on heating to temperatures below an explosion temperature, whereas the smaller crystals did not. Hill5 also observed that large particles do sometimes fly apart early in the reaction.

The reproducibility of the decomposition data can be assessed by comparing the triangles to the solid lines in Figure 1; the solid line is for one particular run, and the triangles are for a different run at identical conditions on another day. The average reproducibility between two runs was k0.5% decoinposed a t any given time.

Samples consisting of a mixture of the fine and the coarse size cuts of pure a-lead azide were decomposed. The decomposition curves corresponding to different weight fractions of the fine and coarse size cuts are presented in Figure 3. The decomposition curves for the mixtures were, in all cases, bracketed by the curvei3 for the separate fine and coarse samples, and there is quantitative agreement, as well, which is considered in a subsequent section.

The isothermal decomposition curves for the three samples of Fez+ doped of-lead azide are presented in Figure 4 for the tcmperature of 230". The effect of particle size is qualitatively the same as for the pure material.

The per cent of total conversion was obtained for each run by dividing the amount of lead azide decomposed, based on the weight of nitrogen collected, by the amount

TABLE 11: Per Cent of Total Conversiona

Temp, "C Sample % conversion

Pure a-Lead Azide 240 Fine 97.9 240 Intermediate 96.4 240 Coarse 96.3 265 Fine 98.4 Exploded Fine 99.3

Fez+ Doped a-Lead Azide 240 Fine 95.5 235 Intermediate 94.1 240 Coarse 94.7 255 Fine 97.6 Exploded Fine -loo.o

a Reproducibility of data f0.5%

of lead azide introduced into the furnace. The per cents of total conversion are listed in Table 11. The total conver- sions a t 240" were between 94 and 98% for the different size fractions. At higher temperatures the per cent conver- sions increased and approached 100% when the samples exploded. The particle size did not significantly affect the percentages of total conversion.

Model of Lead Azide Decomposition The objectives of the model are twofold. First, to fit the

experimental data of the complete decomposition curve with only one equation. Current m o d e l ~ ~ - ~ O used for the type of data shown in Figures 1 and 2, for example, mod- els derived from nucleation theory, require two or more different equations to fit the several parts of the curve with the attendant disadvantages of several different constants for each equation and the difficulty of matching the solution a t common points between two equations. Sec- ond, to account for the effect of particle size on the observed rate of thermal decomposition using independently measured size-distribution parameters. In addition to the objectives, the curve-fit parameters of the model should be useful and dependent only on temperature and the type of lead azide.

The following model met the objectives including an excellent fit of the data with one equation. Equation 2 gives the total fraction decomposed a t any time of interest as the summation of the decomposition of each particle at that time. (Since all particles may not have started to react a t the time of interest, the upper limit is not the largest particle but a variable which will be explained in more detail subsequently.)

d t ) lR 'F'(r)ap(t , r ) dr ( 2 )

where t is the time of interest of the reaction (minutes), r is the initial characteristic dimension (radius) of a particle (p), a( t ) is the total weight-fraction decomposed at time t, F'(r) is the weight fraction of particles that have radius r, and a,(t,r) is the weight-fraction decomposed of an individual particle of radius r a t time t

The function F'(r) was determined to be a normal distribution for the powder samples used in this work.

-(r - R)' (3)

1 F'(r) = - s*


Thermal Decomposition of a-Lead Azide 073

The distribution parameters, R and s , were independently measured quantities. These parameters were always of such a magnitude that for r = 0, F'(r) was less than 10-3 which was insignificant, albeit not zero.

The individual particle was modeled as a sphere which, when it started to react, was immediately covered with a layer of lead that penetrated the particle a t a constant rate of advance in a slirinklng envelope mechanism.

(4)

where p is the crystalline density (moles/micron3), k is the penetration rate constant of the reaction (moles/$ minute), k / p is the penetration rate of the reacting iiiter- face (p/minute), and t b is the time a t which the particle of radius r started to decompose (minutes).

The selection of the shrinking-sphere model was made in the face of contradictory observations. On the positive side, the decomposition curves for single crystals of lead aside reported by Garner and Gomml were very closely approximated by curves for shrinking spheres. Our own photographs of partially decomposed lead azide a t several stages showed tha! certain areas of the crystals darkened very rapidly. A given dark area did not seem to spread much in area indicating that after the initial rapid cover- age, decomposition must have proceeded primarily by penetration. Contrariwise, the particles were not spherical in shape; they have been described hereinbefore as needles and platelets. General model eq 2 could accept any geometry and function for cup(t,r). In addition to spherical geometry, slab anld cylindrical geometries were tried in place of cq 4. Of these, the shrinking-sphere model gave the best fit ofthe data and, hence, was adopted.

The time a t which a particle of radius r started to react, tb, was found to be related to r by the following relation- ship

A is the induction period constant. In the photographs mentioned in the previous paragraph the smaller particles in general appeared to begin to react earlier, thus, lending qualitative support to eq 5 .

Equations 3, 4, and 5 were substituted into eq 2 to obtain the completed model.

The first integral accounts for the particles of radius R1 and less which are coimpletely decomposed at time t. is obtained from the following equation which is a combma- tion of eq 4 and 5 for the special case of complete decomposition,

'The second integral represents the particles larger than RI which are partial1.y decomposed a t time t. The upper bound limits tb to time t .

Because time ( t ) is a constant within the integrals and in the limits, eq 6 must be solved for each time of inter-

TIME, MINUTES

Figure 5. Effect of model parameters A and k / p on the predicted decomposition curves.

0 200 400 600 800 1000 I200 1400 16

TIME, MINUTES

Figure 6. Comparison of decomposition data of pure d e a d azide to the calculated decomposition curves.

est. The model was integrated numerically using a general integration program that utilized the Runge-Kutta-Mer- son integration algorithm.15 Ten to fifteen integrations were required to construct an entire decomposition curve.

The complete model for the decomposition of lead azide, eq 6, contains two adjustable parameters, A and k / p , which are independent of particle size. The particie- size distribution is measured independently and is incor- porated within the model.

Sample decomposition curves generated by the model are shown in Figure 5. It can be observed that the parameter k / p affects the dope of the curve; whereas, A tends to displace the entire curve to the right or to the left. The interaction between these two parameters is slight. The influences of the parameters are consistent with the deri- vation of the model: (1) k / p is the penetration rate and should, therefore, influence the slope of the decomposition curve (rate); and (2) A is the induction period constant which influences the time when the particles start to react.

Figures 6 and 7 show a sample of the fit between the model and the data for pure and Fez+ doped lead azide, respectively. It was not possible to fit the model to the data of the coarse sample of the pure material because of the complication of the hump. The average agreement be-

(15) W E Schtesser "LEANS Ill Programming System for Continuous Systems Simulation, Copyright assigned to Lehig'i University. Bethlehem, P a , 1970


874 R. W. Hutchinson, S. Kieinberg, and F. P. Stein

B e n

4 Y w

d DATA POINTS - MODEL

TEMPERATURE: 230°C.

TIME, MINUTES

Figure 7. Comparislm of decomposition data of Fe2+ doped a- lead azide to the calculated decomposition curves.

TABLE Ill: Model Paraimeters

k / p X 1000, Temp, "C Sample pjmin A, min/p;I3

Pure a-Lead Azide 240 Fine 17.0 f 0.4 38 f 1 240 Interrrietliate 22.7 f 0.6 35 f 1

Fe2+ Doped a-Lead Azide 230 Ftt?I3 17.5 f 0.4 22.5 f 0.6 230 Interniediate 19.6 f 0.5 21.0 * 0.5 230 Coarse 17.5 f 0.4 22.0 f 0.5

tween the model arid the data is f1.0% decomposed at any given time. This number can be compared to the reproducibility of the original decomposition data of *0.5% decomposed. Thus, the model gives a good approximation of the decomposition data.

In addition to fitting the data, it was required that the model parameters k / p and A be independent of particle size. The model parameters are listed in Table 111. The induction period constants for the different size fractions agreed to within :t6% for the pure material and *4% for the Fez+ doped material, and the penetration rate constants varied by i t 4 and fW0, respectively. Thus, the model is capable of discerning the effects of particle size with an accuracy on the order of *lo%. Such a correlation of the effects of particle size is crucial when the effects of other variables, such as impurity levels, are to be considered.

Discussion From the decomposition data it was possible to demon-

strate that the individual particles decomposed independently of each other. Table IV shows the observed per cents decomposed for a 62-3870 mixture of coarse and fine fractions of punt a-lead azide as well as the predicted per cents decomposed. The predicted per cents are based on a linear combination of the respective masses of coarse and Cine samples within the mixture and the decomposition data of the coarse and fine samples. The excellent agreement between the observed and predicted per cents is a demonstration that the coarse and fine particles did not 111 teract during the decomposition.

When the samples exploded, the measured total conver- sions were essentially 100%. Thus, it was demonstrated that the apparatus3 was capable of accounting for all of the

TABLE IV: Observed and Predicted Per Cent Decomposition for Mixtures of Coarse and Fine Particles of Pure a-Lead Azidea

% decomposition

Time, min 0 bserved Predicted

50 150 300 450 600

4.5 31 .O 67.5 86.5 95.5

4.6 31.0 67.9 86.2 95.3

a61.9% coarse (by weight), 38.1% fine.

TABLE V: Temperature Coefficients for Pure a-Lead Azide

AEmrqa A E k i , I ' AEA.' Sample kcal/mol kcal/mol kcal/mol

Fine 35.0 f 0.5 34.9 rt 0.3 35.1 f 1.8 Intermediate 34.6 f 1.1 33.3 f 1.3 36.7 f 0.2 Coarse 35.2 & 0.5

a A€,, is the temperature coefficient of the maximum rate of decom- is the temperature coefficient

AEA is the temper- position (from experimental data). of the penetration rate constant (from the model). ature coefficient of the induction period constant (from the model).

nitrogen which was released, that the lead azide was stoi- chiometric PbNe, and that the decomposition products were lead and nitrogen gas. The material balance obtained should virtually eliminate speculation that lead ni- trides are significant decomposition products in a lead azide explosion. (It is recognized that there are more de- finitive tests for the residue of a thermal decomposition.) To the best of the authors' knowledge no material balance of this type and accuracy has been reported previously for lead azide explosions.

The experimental data for each individual decomposition were curve fit by arbitrarily adjusting the numerical values of the model parameters, k / p and A , such that a statistically best fit was obtained. Two tests of the physical significance ascribed to the model parameter k / p , the rate of penetration of the reaction front into the crystal, are available. One is its agreement with an independent experimental measurement, and the other is the agreement between the Arrhenius temperature coefficient for k / p and the temperature coefficient for the' experimentally measured maximum rate. Fox12 reported experimental rates in units of molecules m-2 sec-l for thermal decomposition of single crystals of a-lead azide which had known dimensions and geometry. For 240" the rate was 2.2 X 1018, which, transformed to units of per minute, is 1.4 X 10-2. The agreement of the model parameter values for pure a-lead azide shown in Table 111 (1.7 X 10-2 and 2.3 x 10-2) with Fox's experimental value i s exception- ally good in view of the nature, origin, and independence of the values, The rather good comparison of Arrhenius temperature coefficient between k / p and i he experimentally measured maximum rate is shown in Table V. The values are also in the 30-38-kcal range of activation ener- gies reported by several authorse%10g12J6 for lead azide, although reported values range widely from about 21x7 to

(16) F P Bowden and A D Yoffe, "Fast Reactions in Solids, Butter-

(17) H Henkin and R McGtII, Ind Eng Chem 44, la91 (1952) worths, London, 1958, p 150

The Journal of Physicar Chemistry, VoI. 77, No, 7, 7973

Production of Hg(3f1r) from Hg(lP,) 875

65 kcal.2 Such a wide variation is due, as some of the authors themselves point out, to the nature of the original data which is sometimes in the form of explosion data or nonisothermal experiments or to widely varying purity levels of the lead azide. The Arrhenius temperature coefficient for the induct ion period parameter A of the model is not significantly different from that for k / p , which would indicate that the same mechanism is controlling both the initiation of decomposition and the progress of the dt, wom- position reaction. There was no significant effect of particle size on the temperature coefficients, a result which would be expected for a model that was constructed cor- rectly so as to separate and describe particle-size effects independently The model could not be fit to the data for the coarse particles because of the hump in the early part of the curve, presumably caused by the splitting of the very largest particles.

One of the major objectives of this work was met by the ability of the model to discern the effects of particle size with an accuracy on the order of 10%. The thermal decomposition data for d e a d azide powder of a given particle-size distribution can be transformed rather reliably to a different particle-size distribution and, thus, permit comparison and study of other effects, such as those of added impurities, in the absence of particle-size effects of the same order of magnitude. Furthermore, the derived model parameter k / p was demonstrated to have the physical significance of the rate of penetration of the reaction front into the crystal.

Acknowledgment. This work was supported by the U. S. Army &search Office-Durham. The a-lead azide and guidance on its safe handling were provided by the per- sonnel of the Explosives Division of Picatinny Arsenal.

Collisionally lniduced Production of Hg(3P1) from Hg(lP1)'

V. Madhavan, Norman N. Lichtin," and Morton Z. Hoffman

Department of Chemistry, Boston University, Boston, Massachusetts 02215 (Received August 4, 19721

Frriission at 2537 A is observed when mercury vapor is exposed to 1849-h resonance radiation in the presence of each of the following gases: He, Ne, Ar, Kr, DB, CO, Nz, CH4, HzO, DzO, C2H6, CzD4, C3H8, c-c6lf12, and CF4. It was not detected with Hz, NH3, ND3, C2H4, and COZ. The only plausible mechanism of scintillation in the case of the monatomic gases is one-step collisionally induced intersystem crossing from Hg(1PI) to Hg(3P1). This process may possibly occur to some degree with any of the di- and polyatomic gases, but is not obligatory. Analysis of the pressure dependence of the intensity of 2537-A emission for eight of the gases provides values of relative cross sections for quenching of 1849-A emission. Relative cross sections for collisionally induced intersystem crossing can be estimated from limiting values of the intensity of 2537-A emission and relative quenching cross sections if a one-step mechanism for intersystem crossing is assumed.

Introduction

An earlier study2 examined the dependence upon pressure of N2 or CO of the intensity of 2537-A scintillation produced by 1849-A irradiation of mixtures of each of these gases with mercury vapor.3 These data were shown to be equally consistent with induction of intersystem crossing from Hg(lP1) to Hgt3P1) by a single collir;ion, e.g., eq 3 or by a two-step process, e.g. , eq 6 and 8. Two- step processes involving triplet intermediates are energeti- cally possible with both N2 (triplet energy 6.16 eV) and CO (triplet energy 6.01 eV) since Hg(lP1) lies 6.68 eV above Hg(lS0). Induction of 2537-A scintillation by Ne and He was also reported in our earlier paper2 but the significance of this phenomenon was not recognized. Since Hg(lP1) does not contain enough energy to excite eithcr of these atoms, intersystem crossing to Hg(3P1) must occur in a single step. This process is not only optically doubly forbidden (spin and orbital angular momentum) but in- volves a large loss of energy by mercury, 1.82 eV. There seems to be little if any precedent for such a transition in a collision of the second kind.

Research supported by Air Force Office of Scientific Research Con- tract No. AFOSR-765-67 A.Granzow, M. Z. Hoffman, N. N. Lichtin, and S. K. Wason, J. Phys. Chem., 72, 3741 (1968). T. A. Gover and H. G. Bryant, Jr., J. Phys. Chem., 70, 2070 (1966).


effect of particle-size distribution on the thermal decomposition of .alpha.-lead azide

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