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THE DESTRUCTION OF PHOSPHORESCENT ZINC SULFIDES

BY ULTRA VIOLET LIGHT

BY LEONARD B. LOEB AND LLOYD SCHMIEDESKAMP

RYERSON PHYSICAL LABORATORY, UNIVERSITY OF CHICAGO

Communicated by R. A. Millikan, June 12, 1921

It has long been known that the phosphorescence exhibited by certain alkaline earth sulfides is gradually destroyed when they are subjected to the action of alpha particles from radio-active matter.' It has also been found that a permanent destruction of the phosphorescent properties of such sulfides is caused by the bombardment of the sulfides with canal3 rays or with cathode2 and beta1 rays. In this process of destruction, the sulfides are caused to phosphoresce brilliantly by the destroying agents, and the destruction is accompanied by a change in color of the sulfide, generally a darkening.

In 1919 J. Perrin1 and more recently R. W. Woods have shown that in the process of fluorescence, a process assumed to be somewhat similar to phosphorescence, the fluorescing molecules undergo chemical changes which result in the loss of the fluorescent power. In other words light of a given wave-length falling upon a molecule of fluorescent substance causes it to emit light of a different wave-length, and in this process the molecule is chemically changed so that it can no longer fluoresce.

The detection of the destruction of the fluorescent properties of certain substances is complicated by the fact that the fraction of molecules that are destroyed per second is relatively so minute that the time of exposure required to produce a measurable effect must be very long. As Perrin showed, this time may be much reduced by using very thin films of the solutions of fluorescing molecules. Wood has shown that it may also be much reduced by using intense sources of light. In view of the destruction of the phosphorescence of zinc sulfides by other agents which cause them to phosphoresce, and in view of the powerful technique developed by Perrin and Wood for the destruction of the fluorescent property by light, it occurred to one of the writers to attempt to detect a destruction of the phosphorescent property of the zinc sulfides through the action of light. The only reference to observations on such a destruction yielded by a survey of the literature consisted of the following quotation taken from a long paper by Baerwald3 on the effect of canal rays on phosphorBaerwald states "It is remarkable that intense ultra violet light from a mercury arc also destroys the phosphorescence of zinc sulfide. As it becomes dark in the process we may suspect a chemical change of the sulfide into an allotropic form,....

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For the preliminary experiments which are being reported here, three

different samples of phosphorescent zinc sulfides were used. One was a sample obtained from the Radium Standard Chemical Company in 1918, the others were two samples obtained in France from MM. Guntz and Ziegler of the powder works at Bouchet in 1918. The specimens to be studied were prepared by sifting the fine crystals upon small circular plane glass plates, 7 mm. in diameter which had been moistened with a very dilute solution of collodion in amyl acetate (about 3 drops of an ether collodion solution of the consistency of "New Skin" in 100 cc. of amyl acetate). Microscopic examination of the specimens showed that they were covered with a uniform layer of the sulfide occupying about 99% of the surface. The glass plate was fastened to a plate of thin transparent quartz, with the layer of sulfide pressed firmly against the quartz. The back side of the glass plate was then painted with optical black to permit the photometry of the sample. At the same time that the specimen was mounted on the quartz plate a control specimen as nearly the same as possible was similarly mounted on a glass microscope slide.

The quartz-mounted specimen was then placed in the image of a quartz Hareus mercury arc thrown upon the wall by a quartz lens of 3 cm. aperture and 15 cm. focal length. The arc operated continuously at about 2.5 amperes in a light-tight box. A blackened thermometer bulb placed in the image of the arc showed a rise in temperature of but 0.5° C. above that of the room, so that the heating of the specimen produced by the arc was not of importance. The glass-mounted control sample was kept in a light-tight box and was removed from this only at the time of photometric measurement.

The phosphorescent intensities of the quartz-mounted specimen and the control were compared by photometric measurements on the phosphorescence exhibited by them when studied in a simple phosphoroscope. The specimens were mounted on the window of a small photometer with the blackened side towards the window. This window was diffusely illuminated from behind by a small automobile headlight lamp. The intensity of illumination of the window was varied, and hence the photometry was accomplished, by changing the distance of the lamp from the window until the phosphorescent specimen faded against the window which served as its background. A color match between the lamp and the specimen was obtained by the use of absorbing screens of colored glass. In the photometry the specimens were excited to phosphorescence by the light from a 75-watt lamp focussed on them. The exciting light was put on and cut off in the usual way between the intervals of observation by means of slits in a metal disc rotated at a high speed, the procedure being in all respects the standard procedure used in phosphorescent measurements. After the quartz sample had been exposed to ultra violet light for a suitable interval of time, its intensity was compared with the

control as above. The results obtained on three samples are tabulated below.

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In the first column are given the intensities of the specimens compared to the controls corresponding to the number of hours exposure to the ultra violet light recorded in the second column. In the third column are given the dates on which the measurements were made; while the fourth column is reserved for comments. In every case it may be seen that there is a marked reduction in the phosphorescent intensity with

the increase in time of ultra violet illumination. In fact the French specimen No. 3 is seen to have dropped to a fourth of its initial intensity after 300 hours in the image of the arc. Though the changes are not very regular, the reduction in most cases is continuous with time, until the measurements made on the 14th of April are reached. The irregularities up to that date were doubtless due to the difficulties involved in the photometry of weak sources. On the 13th and 14th of April the air in the room in which the specimens were being exposed was mildly contaminated with chlorine gas used in the course of other experiments which were being performed in the same room. Again, from the 18th to the 22nd of April the air was contaminated by this gas, the contamination being specially strong on the 21st and 22nd. It is seen that the presence of the chlorine gas apparently arrested further decrease in intensity of two of the samples after April 14th; while it actually increased the intensity of the French Sample No. 2 which was subjected to a particulary heavy dose of the gas. On the 22nd, after the exposure to the strong chlorine, it is seen that all the samples showed marked increases in phosphorescent intensity. This was followed by a decrease in intensity on further illumination in the absence of chlorine after that date.

The reduction in intensity of the phosphorescence was in all cases accompanied by a darkening of the sulfide. Microscopic examination showed that the characteristic yellow color of the sulfides had disappeared and was replaced by a slight but uniform blackening throughout the crystal mass. This blackening was much like that of some glasses that have been subjected to cathode ray bombardment. To eliminate the possibility of this blackening having been caused by the collodion, a sample of the collodion film was exposed to the light for many hours. It showed no signs of blackening. A specimen of sulfide made up without the use of the collodion furthermore showed the same decrease of intensity and the same blackening on exposure to the ultra violet light as was shown by the samples using collodion. Following the accidental exposure to the chlorine gas it was observed that the specimens which had been blackened by exposure had regained their original color in a ring about 2 mm. wide at the edges of the specimens, i.e., where the chlorine had diffused inwards. It was these portions with restored color which showed the marked increase in intensity observed. The blackened centers of the specimens remained dull. Finally, to ascertain that it was the chlorine that caused the restoration of the color the French Sample No. 2 was placed in a flask with chlorine gas over night. On examination it was found to have regained its yellow color throughout its mass. On measurement, even though its edges were moist, due to the deliquescent nature of some of the reaction products, the intensity of the specimen was found to have increased markedly, as the table shows.

There is then little doubt in concluding from these experiments that the action of intense ultra violet light causes a destruction of the phosphorescent power of the zinc sulfides. This destruction is accompanied by chemical changes in the sulfides, resulting in a change of color, (possibly in a reduction or partial reduction of the polysulfides of the impurity (here probably Cu) required to produce the phosphorescent centers). It is further obvious that the exposure of the crystals to a powerful oxidizing gas such as chlorine at least in part restores both the original color and the ability to phosphoresce. It may accordingly be quite possible that the action of ultra violet light in this case is similar to that of ordinary light in the photographic processes, only much slower.

The results so far obtained hardly permit one to say much about the form of the curves of decay of phosphorescent intensity with the time of ultra violet illumination. They indicate, however, that the curves may

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either be the sum of a group of curves having the form I = 11⁄2 (1 − e−^1)

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given by Rutherford for destruction of phosphorescence in the case of alpha particles where the intensity of the alpha particle radiation falls off exponentially with the distance below the surface; or they may be the sum of a group of exponential curves having different constants. Such curves would be expected in the case of measurements made on the decay of total phosphorescence with ultra violet illumination, for it is known that the phosphorescent light is composed of groups of different phosphorescent wave-lengths, each group probably having its own susceptibility to destruction, and hence each having its own time rate of destruction by ultra violet light.

The destruction of the phosphorescent power of these sulfides by ultra violet light noted above might point to a close analogy between the phenomena of phosphorescence and fluorescence in the light of the recent work of Perrin. Such an analogy would be in accord with the conclusion arrived at by Rutherford in the case of the destruction of the phosphorescence by alpha particles, viz., that the light emitted under alpha particles bombardment accompanies the dissociation and destruction of the active centers. If this interpretation is correct, and if phosphorescence and fluorescence are closely related, the theory of Schmidt and Wiedemann,7 Lenard, and Merritt,' explaining the mechanism of phosphorescence will have to be modified. This theory ascribes phosphorescence to the return of ionized portions of the active centers (presumably electrons), to the centers from which they were removed by the exciting agent. Such a change does not involve a permanent destruction of the active center. According to Perrin the active center is permanently chemically changed in the process by which the substance emits its fluorescent radiation. It therefore follows that unless the production of phosphorescent light and

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