T H E DECAY O F PHOSPHORESCEKCE OF ZIXC SULPHIDE BY H. ACSTIN TAYLOR'
The uncertainty attached t,o a study of phosphorescent phenomena, despite the large amount of work already accomplished is multiplied by the diversity of opinion on what would seem to be the very fundamentals. T h t quest,ion, for example, as to whether some exciting element present as impurity in the phosphorescent material is a necessity for phosphorescence does not seem, even at the present time, t,o have been satisfactorily settled. There is evidence on this point from both directions. Again, the question of the cause and rate of decay of phosphorescence has no thoroughly sat'isfactory answer anti both theory and practice have supporters whose ideas are mutually contradictory and conflicting. The cause of such a situation would appear to lie in the fact that the experimental work so far performed has not in many cases been of the highest order and the resulk obtained generalised to too great an extent, t,hereby involving error. The complexity of t.he problem of phosphorescence then, necessit'ates an attack on a small portion of the phenomena first', and it is proposed herein to st,udy the effect of temperature on the rate of decay of phosphorescence using zinc sulphide as tmhephosphorescent material. The similarity between this effect and that of red and infra-red radiation suggests a t once that a similar explanation holds for both. Such a suggestion has already been made by Perrin2 and experiment,al evidence quoted in its favor, t,he whole being taken as evidence in favor of the simple radiation theory of chemical reaction, The doubt recently cast on this theory from numerous points of viex necessitates a critical examination of the evidence from phosphorescence. According to Perrin, phosphorescence is due to the emission of radiation by the phosphorogen ret,urning to the normal state from the modified or excited state into il-hich it, was thrown by absorption of the exciting radiation. That the frequency of t8hephosphorescent radiation is in general lower than that of t'he exciting light3 is explained by the assumption that the return from the excited t o the normal state is a process occurring in numerous stages, each accompanied by its own emission and therefore of a lower order than the excitation which presumably occurs in one step. The cause of the return of the excited molecule to the normal state is to be found in t,he internal radiation present in the system in virtue of its temperature. If the temperature of the system is raised, the rate of return is increased, that is, the phosphorescence is increased. Alternatively if we increase t,he internal radiation Contribution from the Hsvemeyer Chemical Laboratory, New York Vnirersity. Trans. Faraday Soc., 17, j66 (1922). 3 S!okes' 1 . a ~ .
I 16
H. AUSTIS TAYLOR
of the system photochemically, as opposed to t,hermally, me shall again increase the rate of return of the excited molecule to its normal state, again increasing the phosphorescence. Qualitatively the explanation appears to agree with the observed facts. Perrin however proceeds to make the theory quantitativc. The regeneration of the normal phosphorogen is considered as a unimolecular chemical reaction and its rate thus measurable at different temperatures. From the temperature coefficient of this rate of decay of phosphorescence the energy of excitation,-the critical increment,-may be calculated in the usual manner and hence t,he frequency of radiation which should cause the more rapid decay. Perrin writes: "I have done this recently in the case of phosphorescent zinc sulphide containing copper. Measuring the increase in the brightness produced by a known rise of temperat.ure I have succeeded in calculating t,he wave length, approximately ~p of the regenerating radiat,ion. It remains to be seen whet,her t,his infra-red radiation really regenerates the phosphorogen. . . . The spot hit by these rays of wave length 0 . 7 to I . j p emits a bright phosphorescence and very soon afterwards that spot turns (lark and is then easily distinguished again from the faintly luminous background." The evidence seems unequivocal. When we a&lxse the (lata recorded before that of Perrin however, we are immediately struck by some apparent anomalies if the theory suggested is the correct one. The experiments of Becquerel more than fifty yea1.s ago showed definitely that the radiation causing the more rapid decay of phosphorescence was not confined to a single frequency nor even a narrow band of frequencies such as the radiation theory mould demand, but was general throughout the red and infra-red regions of the spect,rum. More quantitative experiments of Sichols and JIerritt,' on the decay of zinc sulphide phosphorescence at, different, temperatures showed t,hat the effect of temperature over large ranges is a complex effect antl not at all in agreement with the unimolccularity assumed by Perrin. It was found then, that the rate of decaytemperature curve showed various niasima, one at some temperature above zoo(', another, very pronounced, a t - 4 o T antl a t,hird a t -16ooC,'. The existence of such maxima necessarily means that the temperature coefficient of decay is negative for certain ranges; a fact hardly capable of reconciliation with Perrin's view. That such behavior could be explained on the basis of complexity of phosphorescence hands does not seem possible in view of the work of Pierce? in the same year, wherein he finds no change in the phosphorescent band position, either with progressive decay, under tmhcaction of infrared radiation, or x i t h a temperature change. antl it ivoultl not seem probable that the effect of either infra-red or temperature would be the same on each component of n complex band. Ives and Luckiesha, too, have shown that the effect of infra-red is one of total emission and not, of a separate band or part 'Phys. Rev., 32, 38 (1911). *Phys. Rev., 32, 115 (1911). 3.1strophys. J , 34, 173 (1911,; ibid.. 3 6 . 330 (1912)
DECAY OF PHOGPHORESCETCE O F ZISC SULPHIDE
117
of the radiation, although they point, out that the intensity of the phosphorescence is directly proportional t o that of the red or infra-red radiation causing the ahnormal decay. Here again the general evidence so far forthcoming is conflicting and a t times contradictory. Lenard' anti his eo-workers have shown definitely that, in the phosphorescence, three different processes may be distinguished, one of very short duration, one of medium duration and one of very long duration. Such being the case it is not to be espected that the direct observations as made .should hear of a i-ery simple esplanation. It was proposed therefore to study again the effect of temperature, oT-er a limited range, on the rate of decay of zinc sulphide phosphorescence and to compare this rigorously with the action of radiation as Perrin suggests, Experimental and Results The method adopted in obtaining the temperature coefficient of phosphorescent decay was to measure the time taken for a sample of zinc sulphide, saturated with the exciting radiation, a t a known fised temperature, to decay to a fised photometric standard. The comparison between the intensity of the zinc sulphide screen and that of the standard was made possible by use of a photometer in xhich the screen was viewed directly through a circular glass plate which carried at its center a small right-angled prism, the latter reflecting the light of the phot'ometric standard. The field of view thus obtained was a square of fised color and intensity surrounded by the light from the phosphorescing scyeen. K!ien the field presented an even appearance the intensit'y of the phosphorescence was identical x i t h that of the stantlard. The arrival at this point was accurately observable by the disappearance of the line of demarcation between the two lights. The arrangemcnt of the apparatus was as follows: the zinc sulphide screen n-as enclosed in an oven, the temperature of which was automatically regulated The oi-en carried two windows, one of glass through Tvhich the to *O.I'('. phosphorescence was ohseryed and one of quartz eo that the exciting radiation when necessary could be in the ultra-violet region. -1shutter \vas placed immediately in front of this latter window and served to cut off the exciting light as desired. Observation of the phosphorescence was made as stated, through the photometer using a solution of suitable color illuminated by a small electric light as standard for comparison. The samples of phosphorescent zinc sulphide were five in number. Three, containing copper, exhibited a bluish green phosphorescence; the fourth was yellow, whilst the fifth had a distinctly red phosphorescence. The samples were obtained from various sources and their composition and method of preparation was not known. h s sourcee of exciting radiation the Kromayer quartz mercury k m p was used for ultra-violet alone, the visible being excluded by means of a nickel glass screen. For visible radiation as exciting source two tungsten filament
' .Inn.
Physik, 1904 et seq.
I
I8
H. A U S T I S TAYLOR
lamps ( 2 5 0 watts) were used, one with a plain glass bulb antl the obher with a blue glass bulb, the familiar day-light lamp) which transmitted therefore less red light than the former. The experimental procedure consisted first in allowing the sulphide screen at a fixed temperature to become saturated nith the exciting radiation, which was then cut off by means of the shutter, at the samc instant a stop-watch being released. The moment that an even field was obtained in the photometer, v a s registered on the watch antl the interval for the decay of phosphorescence noted. This measurement was then repeatcd from ten t,o twenty times and the average d u e at that temperature taken. The temperature of the oyen was then raised a known amount antl the process repeated using the same intensities of radiation for both the exciting source and photometric standard as at the lower temperature. I n this manner the times of deca Jyere obtained at a series of temperatures from room temperature to I 10°C The same process was then repeated wing the other sources of excitation antl the temperature coefficients found under the different conditions. Other samples of zinc wlphide were then substituted and similar measurements made. Table I gives the times of decay to standard intensity for the various samples at the temperatures indica tetl. The graph accompanying these. Fig. I, is typical of the others. TABLE I Times of Decay in Seconds Zinc Sulphides Green 13 Green CJ Ycllow Red Green A Cirern A TeGmp. Green A 7
C
Iiromayer Lamp
Tungsten Tungsten Tungsten Lamp Lamp Lamp
10.60
Tungsten Lamp 24.00
23.00
11.80
8.40
16.60
9.00
20,oo
20.40
IO.60
j .lo
14.20
7.60
'j.05
18.30
9.60
6.60
IZ.Ij
6.30
14.40
16.40
8.60
j.80
I C .2 0
j.IC
12.20
14.40
7.80
5.00
8.30
4.00
10.00
12.60
6.90
4.40
6.45
3.IO
8.00
I O , 80
6.20
3.80
4.90
2.30
6.20
9.20
j.60
3.40
3.40
1.60
4.60
7.80
j . 2 0
3.00
Tungsten Lamp
Daylight Lamp
19.00 7.40
6.31 5.40 4.5.5
3 .so 3 . IO 2.55
2.00
I .60
The average temperature coefficients correFponding t o these times of decay and calculated directly from (hem are given in the following table.
DECAY O F PHOSPHORESCENCE O F ZINC SULPHIDE
TABLE
119
11 Mean T f m p . Coef.
Green .A. Kromayer Lamp. 1.20 Green A. Tungsten Lamp. 1.24 Green X. Daylight Lamp. I . 2 j Green €3. Tungsten Lamp. I . 2 j Green C. Tungsten Lamp. I.Ij Tellow, Tungst,en Lamp. I . I2 Red. Tungsten Lamp. 1.14 The reliability of these mean yalues may be seen from the detailed calculations shown in Table I11 which gives the temperature coefficients per 10°C throughout the range of temperatures studied. ,b; I
$
ld~t
s1;
--a
IWb
8s
I
I
o, \o
K. I
\o
;:
~2:
50
lI 1i -
~
~
I
I
1
i l
I
o% .
I
I
IF P
6
1
~ & s p ~ , G ? s mpun fpmp.ror//= /.?4
~
\\o
1
I
rrMP
bO
A0
'c.
i
1 1
L p g
!
BO
Inn
.
1''lC. I
T A B L E 111 ZnS. Green A. Kromayer Lamp as Excitor
Temp. Rang? in "C 30-40
1.17
Temp. Range in 'C 70-80
40-jo
I . I j
80-90
1.21
jo-60 60-70
I . 19
90-100
1.28
I .2 0
100-I10
Temp. Coef.
Temp. Coef. 1.23
I.2j
__
Nean 1 . 2 0 The variation of these values from the mean may he either real or apparent, that is to say, there niay actually he a variation in the temperature coefficient, or, the error involved in the measurement of the t,imes of decay a t the higher temperatures, which is of the order of two seconds, may account for it That the former is more probably the case is suggested first hy the results themselves, in the general rising trend of the temperature coefficients with a rise in temperature, shown not only in the instance quoted but in every case studied. Secondly, the work of Lenard already mentioned, having shown the complexity of the total phosphoreecent process, ~voultlsuggest a change in the temperature coefficient with a change in temperature since in all probability the long and short phosphorescences will not have t'he same temperature coefficient and one or ot'her would predominate at the higher temperatures.
H. .4USTIS TAYLOR
I20
As regards the actual measurement of the time of decay even at, the higher temperatures where the times are short, the general smoot,hness of the curves obtained may be taken as qualititative evidence of their reliability. Actually however the degree of precision for the average of t,he twenty readings taken a t one temperature is =tj percent at the higher t-mperatures and may fall t o as low as *I percent at room temFeratures where the times are much longer. The similarity then, between the t'emperature coefficients of decay in the first three instances in Table I1 where the same sample of zinc sulphide was used and the same photometric standard used for comparison indicates that a change in the source of exciting radiation is without, any appreciable effect on the phosphorescent process. That the phosphorescence itself is complex may be indicated by a study of the same sample using a different standard for comparison. The folloiving table gives the times of (!way for the ,sample Green A using a more fwble intensity in t,he photometer for com1:arison. The times it will be noted are therefore much longer. TABLI:IT' ZnS. Green A. Tungsten Lamp as Escitor Temp. 'C
Time of decay. Seconds
Temp. "C'
25
10, 80
r -
35
9.j 0
8;
4.5
8.,ij
95
I3
Time of decay. Srcclnds
5.35 4.50 3 70 '
55 7.30 I 0j 3.00 65 6.30 The mean t e m p - a t u r e coefficient calculated from these values is 1.18 as compared with the value 1 . 2 0 obtained from a study over a shorter period of the phosphorescence. The same effect namely a slightly lower temperature coefficient x a s ohserved in ti\-o other eases studied when a more feeble stantlard was used for comparison, which would point to the fact that t,he phosphorescent p r o w s of long duration had a lower temperature coefficient t'han the process of shorter duration. This in turn is in line n.ith the increase in temperature coefficient with an increase in tern1:eratui-e since it is the phosphorescence of shorter duration xhich is more affected by an increase in temperature. The temperahre coefficients for the different samples as shown in Table I1 would suggest that there exists some relation between the color of the phosphorescenee and the temperat,ure coefficient. The results 1.12 for the sample phosphorescing yellow and 1.14 for the red, are both lower than the green samples. Remembering that the temperature coefficient is lowered in general by an increase in the period of decay studied there would appear to be a progressive fall in the temperature coefficient as the phosphorescence passes from green to red. It is hoped a t a later date to st'udy this problem further. The feature of prime importance however, which these results have brought out is the order of magnitude of the temperature coefficient of decay. If we take the value 1.20 as a mean value of all the samples, the smallness of
DECAY O F PHOSPHORESCENCE O F ZISC SULPHIDE
I2 1
this as a temperahre coefficient is remarkable. Such a value is a t once suggestive of a phot,ochemical process rather than of a t'hermal process, and is much lower than the figure quoted by Perrin. Using the simple radiation theory in the manner previously suggested we find that the temperature coefficient of 1 . 2 0 corresponds to 4300 calories in the mean temperat,ure range 6;-;j0c, or to a wave length 6 . 6 j p in the short infra-red. The difference between this value 6 . 6 j p and that which Perrin giyes, namely IP, calls immediately for explanation. Perrin tells us that, on esposing an illuminated zinc sulphide screen t o a spectrum of radiation the spot hit by the rays of wave lengths 0 . 7 t,o 1 . j phosphoresces more brilliantly than the rest and eventuallj- becomes dark. The uncertainty of definitely identifying the wave length of a band in such a crude experiment suggests at once that the region was probahly much larger and that even though a w r y narrow hand was responsible for the darkening, the overlapping of spectra would cause an apparent broadening. It seemed advisable therefore t,o try the effects of practically monochromatic radiation at various points in the s p c t r u n i and thereby t o isolate the effective viaye lengt,h, if such exists. The first, wave length tested was that calculated from the temperat,ure coefficient namely, 6 . 6 5 ~ . This was obtained by means of a Hilger Infra-red Spectrometer using a carbon arc as source of illumination and rock salt, prism as dispersing agent. The intensity of the practically monochromatic radiation a t 6 . 6 j p although feeble was still sufficient to cause a more rapid decay of the phosphorescence of the zinc sulphide screen, as shown by a black line-the image of the spectrometer slit-on the faintly phosphorescing background. The result appears then, to be in agreement mit'h the radiat.ion theory. On turning the prism, however, almost' indiscriminately, it was found that the same result was obtainable at any wave length in the short, infra-red. Apparently a single frequency of radiation is not alone active, and it was decided t o t r y the effects of various screens. The first screen was t'he biotite mica screen used previously' which transmitted radiation from about 2 p up t o 8 p only. The zinc sulphide screen was illuminated over it's whole surface, half of it was then covered and the remaining half exposed to the infra-red radiation for a few seconds. I t was found that the half which had been exposed was much darker t,han the unesposed portion. In order to pass further into the infra-red it was necessary to find a screen which absorbed in the near infra-red but which became transparent again in the further region. I t seems remarkable that sphalerite itself is such a substance. Coblentz* st,ates that a crystal 1.j mms. thick is t,ransparent from sp to 1 2 p and is entirely opaque a t I jp. h thin plate of sphalerite was obtained and used as a filter in a manner similar t o the biotite. The result mas again the same, showing that apparently any wave length in the short infrared is capable of causing a more rapid decay of the phosphorescence. J. Am. Chem. Soc., 48, 577 (1926). ,Carnegie Inst. Pub., No 65 (1906).
~
I22
H. AUSTIN TAYLOR
Using a ruby glass filter t,he result was likewise the same, that is there was a more rapid decay of the phosphorescence and the part, exposed to the red light was darker than the unexposed portion. A yellow filter obtained by interposing a solution of picric acid in water between t.he light source and the phosphorescing screen also caused a darkening more rapidly than wit,h the normal rat,e of decay. In all these cases the sample of zinc sulphide ured had been the sample A which phosphoresced green. A green filter was therefore used to see what thc cffecBt would be. The filter was identical wit,h the one which had been used as the comparison Ftantiarti in the phot'ometer in the previous experiments and was a solution of nickel anti copper nitrates in water, It was found that when the light filter was sufficient,ly concentrated to give a color on the non-phosphorescing zinc sulphide, of the same intensity as the normal phosphorescence wit,hout the filter, then practically no phosphorescence of the screen was caused and no increased darkening was observdd with the phosphorescing screen. If this filter is diluted with water, some phosphorescence is caused, presumably owing to t'he shorter wave lengths which are transmitted by the more dilute filter. If however, the filter is made more concentrated, an increased darkening action can be observed with a zinc sulphide screen which is already phosphorescing. From these experiments it is apparent', first, that Stokes' Law is being obeyed, and second, that any radiation of wave length shorter than the mean phosphorescent wave length is capable of causing a more rapid decay of the phosphorescence. This is further confirmed by experiments with the other samples of zinc sulphide which phosphorescenee at different regions. Taking ( I ) the green sample A, ( 2 ) the yellow and (3) the red phosphorescent zinc sulphides and constructing light, filters of ( I ) nickel and copper nitrates ( 2 ) picric acid and (3) potassium dichromate solutions respectively, such that the light transmit,ted by these was of approximately the same intensity as the normal phosphorescence of each of the three samples, it was found that the green filter ( I ) though without action on the green sample was capable of causing phosphorescence of both the yellow and red samples of zinc sulphide. Filter ( 2 ) of picric acid would cause the red zinc sulphide alone to phosphoresce, but also caused a darkening of the phosphorescing green sample more rapidly than normal. Filter (3) of potassium dichromate whiht inactive towards the zinc sulphide which phosphoresced in the red was capable of causing a more rapid decay of the phosphorescence of bot'h the green and yellow samples. It is obvious then that the line of demarcation between ability to cause phosphorescence and ability to cause the decay of phosphorescence is quite sharp. Wave lengths shorter than this line will all cause phosphorescence whilst all wave lengths longer than this line will cause a more rapid decay. The requirements of the simple radiation t,heory are not a t all satisfied. What t,he true explanation of these results is, it is hoped to make the subject of a subsequent paper. The fact that the magnitude of the temperature coefficient would seem to point to a photochemical rather than a thermal
DECAY O F PHOSPHORESCEKCE O F Z I S C SULPHIDE
123
reaction suggests t,hat a physical esplanat,ion is probable, and that purely chemical effects only enter in cases of extreme illumination such as the blackening of zinc sulphide by intense ultra-violet light, as studied in detail by Lenard'. There, the bond between the atoms in the molecule is completely severed anti free zinc remains. That such a profound change is required for phosphorescence does not seem likely, rather woultl an electron shift within the zinc sulphide molecule account for the results.
Summary The contradictory nature of the data on the effect of temperature on the rate of decay of zinc sulphide phosphorescence is pointed out. 2. The temperature coefficient of the rate of decay of phosphorescence has been measured for five different samples of zinc sulphide under varying contli t ions. 3 . Analysis of the data shows the temperat,ure coefficient to be 1 . 2 0 per I O ' ( ' in the neighliorhood of ;o°C corresponding t o a wave length of 6.651-1 on the l m i s of the ratliation theory. The difference between this and Perrin's figure of I F hns been noted. s shovn that the abnormal decaying effect of radiation on 4. It h ~ been zinc sulphide phosphorescence is not confined to a particular frequency. nor t o a particular region of the spectrum. but that all frequencies incapable of causing phosphorescence will cause its more rapid decay. Further work is in progress. I.
S e r rolk, 1
s.Y .
Ann. I'hysik, 68, 553
(1922)
