THE ABSORPTION of ULTRAVIOLET Radiation by DIAMOND Bv K

THE ABSORPTION OF ULTRAVIOLET RADIATiON BY DIAMOND Bv K. G. RAMANATHAN (From the Department of Physics, Indian l, stitute of Science, Bangalor•)

Received May 18, 1946 (Communicated by Sir C. V. Raman, Kt., F.R.S., N.L.)

1. INTRODUCTION THE early work of Miller (1862) and the later investigation by Peter (1923) showed that while the majority of diamonds completely cut off the ultraviolet radiation below 3000 A, there were a few which possessed high transparency up to 2250 A. This finding has been confirmed by Robertson, Fox and MartŸ (1934) who made extensive investigations on the absorption spectra of botb the absorbing and non-absorbing diamonds at 8ifferent temperatures and discovered several interesting facts. On the basis of their work they also classified diamonds into two categories, the absorbing diamonds showing complete cut-off below 3000 A and the non-absorbing diamonds which possess high transparency up to 2250 A. Whereas the previous workers were of opinion that opacity below 3000 A sets in because of the presence of impurities, these writer.s attributed ir to the strain which they assumed to be present in such crystals. It is however very unlikely that the presence of mechanical strain in any substance can give rise to observable changes in its spectroscopic behaviour. Further, it has been shown very clearly by the investigations carried out recently in this laboratory [Raman and Rendall (1944) and Ramachandran (1944)] that it is precisely those diamonds which absorb strongly in the ultraviolet that ate free from any mechanical strain and that form the nearest approach to crystal perfection. The transparent diamonds on the other hand have been found to possess a readily observable mosaic structure. On the basis of tbese facts it is clear that the observed difference in absorption spectra can be ascribed neither to the presence of chemical impurities, nor to the presence of mechanical strain, but to basie difference in crystal structure. The recent work of P. G. N. Nayar (1941,' 42) and Miss Sunanda Bai (1944) on the above subject, carried out in this laboratory, have established that the results are of a more diversified character and cannot be described in terms of two alterrtatives only, as suggested by earlier observers. Work- ing with thin specimens, the thirmest of them being 0.76 millimetre thick, .~ 10 137 138 K. G. Ramanathan and by suitably increasing the photographic exposures, Nayar showed that for the so-called ultraviolet opaque diamonds, there was no complete cut-off at 3000 • and the spectrum could be progressively extended upto 2700 A. Sunanda Bai found out that for blue-lumir~escent diamonds the extension ofthe spectrum transmitted in the ultraviolet depended not only on the thickness of the specime~l and the pkotographic exposures employed but also on the intensity of luminescence. For short photographic exposures, the diamonds showing the mixed type of luminescence transmitted up to 2700/~, butas the exposure time was increased they showed transmission upto 2250 A, the strength of absorption hacreasing rapidly as the limit was approached. The spectrum of the transmitted radiation, lar from being continuous, is crossed by a number of absorption fines and bands ttlroughout the recorded range. Some of these were discovered by Robertson, Fox and Martin, many others by P. G. N. Nayar, while some more were added to the list by Sunanda Bai. The behaviour of these lirles and bands depended greatly on tke strength of luminescence of the diamond. Nayar found that the absorption bands in the region 3000 to 3500 A were strongest in weakly blue-fluorescent diamonds (i.e, diamonds showing the visible absorption weakly) and rice versa. On the other hand, in diamonds showing the mixed fluorescence, Sunanda Bai showed that the bands near 2360 A behaved in just the opposite manner, appearing strong in strongly luminescent diamonds and weak when the visible luminescence was weak. The fact that even the most higbly absorbing diamonds show a definite photoconductivity with a maximum for exciting wavelength at 2300 A just as for the highly transparent diamonds, suggests that these should also transmit the ultraviolet upto 2250 A provided that sufficiently thin specimens are used. The present investigation was undertaken (a) to investigate the :ultraviolet absorption by the thirmest plates of diamond, (b) to trace the variation of the absorption coefIicient with wavelengtk in the range 3100 A to 2570 A of a typical blue-luminescent diamond and (c) to determine the ultraviolet transmission limits of a number of diamonds in order to correlate this pro- perty with the behaviour of the diamonds in the infra-red dealt with in another paper appearing elsewkere in the symposium. Ir may at once be stated here that the fllinnest diamonds of the so-called ultraviolet opaque type have been found, in this iavestigatiou, to exhibit transmission upto 2240 )k which is also. the limit for the so-called ultraviolet transparent type. Other results which have emerged out of this investigation will be dealt with in the eourse of the paper. TAe A6sorŸ243 of Ul•raviolet Radiation [~y Diamond 139

2. ULTRAVIOLET ABSORPTION SPECTRUM OF THE THINNEST DIAMONDS One of the two blue-fluorescent specimens which were used for this work, was originally a flat, polished circular cleavage plate of thickneas 1.39 millimetres. It was made into a wedge of angle 10 ~ 40' by the firm of Surajmal. The thinnest portion of this wedge was only 0-12 millimetre thick and so could be used for absorption work to take advantage of its thinness. The diamond, previous to making into a wedge had been shown to have a mean coefficient of absorption of 10.2 per centimetre in the 8.0~91 region of the infla-red. (Se~ paper on variatiol~.s in the infla-red absorption spectrum of diamond appearing elsewhere in the symposium.) It was also absorbing the ultraviolet below 3000 A very highly so that spectra obtained with even long exposures e,xtended little below 3000 •. The other piece which was used for this work is a rectangular plate N.C. 62 which is ah intensely blue-fluorescent diamond with its thickness diminishing from 1.5 millimetre at the centre to nearly 0-2 millimetre at the edges by steps. The edges of the plate are in the form of knife-edges with an angle of about 30 o. For photographing the absorption spectra through the thinnest portions ~f a wedge, continuous ultraviotet radiation from a hydrogen discharge tube w• first focussed on the diamond attached to the moveable jaw of a slit, the screw cap of which carried a micrometer head. The radiation, de~iated by the prismatic wedge, was again focussed on the slit of a medium quartz spectrograph so that a focussed image of the wedge was formed in the plane of the slit. By turning the micrometer head, different portions of the diamond could be brought to focus on the slit. The spectrograph had to be kept tilted by an angle equal to the deviation caused by the wedge. The advantage in having a micrometer head is that absorption spectra can be obtained through different portions of the diamoad wedge, begin~.ing from one end and iricreasing of decreasing the thickriess of the absorption path by kaown amounts. Results.--Transmission spectra obtairted with N.C. 78 through its thinnest portion show clear trar~smission upto 2240 A, both at room temperature of the diamond and when it is cooled with liquid air. Beyond 2240 A there is complete cut-off of the radiation. All efforts to record the spectrum below 2240 ]k with long exposures failed to extend the observable transmission beyond that limit. It may be remarked here that the diamond N.C. 78 previous to being made into a wedge (when ir was a cleavage plate of .thickness 1.37 millimetres) gave transmission spcctrum extending very litfle below 3000 A. Transmission spectra obtained with the intensely biue-fluorescent diamond N.C. 62 through the centre of the plate 1.5 millimetres thick extend 140 K. G. Ramanathan upto 2400 A only, even with very long exposures. The spectra with different times of exposure show, as has been pointed out by Sunanda Bai, a series of step-like falls of intensity a.t 2900 A, 2715 A and 2570 A, indicating steep rises in the absorption curve. Of all these, the one at 2715 is the most pro- minent one. The absorption spectrum of the same diamond (N.C. 62) obtained through the thinnest portion of the prismatic edge (about -25 mm.) shows a transmission upto 2250 A.

3. ULTRAVIOLET ABSORPTION BANDS OF DIAMOND The absorption spectra of the two diamonds N.C. 78 and N.C. 62 obtained with the specimens kept at room temperature show very intensely the absorption bands at 2364 A, 2359/~, 2310 A and 2298 A. The former two have been recorded by Sunanda Bai (1944) in the room temperature pictures and all the fouv in spectra obtained at -180 ~ C. In the present investigation, the room temperature pictures of N.C. 78 in which the conditions were most favourable, have been found to show in addition to the above four bands, two others at 2258 A and 2246 A. On cooling the diamond with liquid air, the band at 2298 A mldergoes resolution into two lines, a fainter component appearing on the shorter wavelength side. The two new diffuse

TABLE I Ultraviolet Absorption Bands of Diamond

Robertson, Fox Sunanda Bai (1944) X in ~.U. Present investigation X in A.U. and MartŸ k (193.4) in A.U. 29 ~ C, - 180 ~ C. 27 ~ C. - 180 ~ C.

2405 ( w ) 2399 (s) 2396 (w) 2388 (w) 236~-2359 2364.5 2359.9 (v.s.) 2364 (v.s.) 2360 (v.s.) 2359.5 2356.5 (v.s.) 2359 (v.s.) 2355"9 (v.s.) 2314 (w) 2312-2300 2309- (s) 2312- (s) 2309- (s) 2306 2308 2306 2300- (s) 2302---(s) 2301 - 2295 (s) 2296 2294 2294 (w) 2271.4 (w) 22~1 -- (s) 226o (~) 2255 2248- (s) 2252.8 (w) 2244 2246-5 (w)

(v. s.) --very ,~trong ; (s.)--strong ; (w.)--weak. The Abso~,Ÿ243 of Ul/raviolet Racliation by Diamond 141 bands at 2258 ,~ and 2246 A have also been found to undergo resolution, a set of four sharp lines appearing in their place. All these features have been reeorded for the first time. However, the five absorption bands between 2405 and 2388 A obtained by Sunanda Bai in diamonds showing the mixed fluoreseence could not be recorded in the absorption spectra of N.C. 78. The results are entered above in the Table I~

4. THE ABSORPTION CURVE OF BLUE-LUMINESCENT DIAMOND Peter (1923) quantitatively determined the absorption coefficient of the highly transparent variety of diamond in the region beginning ti'oro the visible and extending into the ultraviolet upto 2250 A where complete opaeity sets in. The curve reproduced by hito shows that the absorption coefficient increases eontinuously from 0.036 per millimetre at 3130A to 1.477 per millimetre at 2260 A without showing any features. Quantitative measure- ments on the absorption coef¡ of the kighty absorbing blue-fluoreseing diamonds have not been attempted till now. In the present work, for obtaining the variation of the absorption eoefficient with wavelength of a t3,pieal blue-fluorescing diamond, the specimen N.C. 78 already referred to was used. A number of speetra were photographed in the same plate by giving equal exposures and running the hydrogen discharge tube at eonstant eurrent. The diamond was sueeessively moved by means of the micrometer head through equal distanees between suceessive exposures. The distance through which the diamond is moved between two suecessive exposures is aceurately determinable from the known va!ue of the piteh of the mierometer screw. Henee the increment in thickness between successive exposures can also be ealculated. If x and (x + t) centimetres are the thieknesses of the diamond in two suceessive spectra and if Ii and I2 are the intensities of transmitted light in the two cases, the ineident light being I in the spectral region ~, the absorption coeitieient k per centimetre is g~ven by I1/Iz = e kt. The values of I1 and lo required to determine k, were obtained as follows: Absorption speetra were photographed for different thieknesses on a highly contrasting plate (Ilford proeess plates were used) together with a series of in tensity marks given by the method of varying slit-widths. Curves con- neeting photographie density and ]ogarithm of intensity were drawn for different spectral regions between 2550 A and 3030 A. The relative intensities of transmission I1 and I2 eould be then obtained by mierophotometering the speetra and reading off the intensities from the ealibration eurves. Fig. 1 is a graphieal reproduetion of the absorption spectrum obtained in, 142 K. G. Ramanathan

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this manner where the logarithmj0 (Absorption coefficient) is plotted agaŸ wavelength. It can be seen that corresponding to every step-like fall of intensity in the absorption spectrum, there is a steep rise in the absorption curve. 5. ULTRAVIOLET TRANSMISSION LIMITS OF DIAMONDS The transmission spectra were investigated of the diamonds, whose coeflieients of absorption in the infra-red had been determined previouslyo To establish the degree of correlation existing between ultraviolet and infra- red transparency, the spectra of the diamonds were photographed, giving different exposures for the same diamond on the same plate. The ultraviolet transmission limits of the diamonds for tWo of three different exposure timas together w[th th› infra-red mean absorption coefficients in the 8.0/z region and their behaviour in respect of luminescence are reproduced in Table II. Ir was stated in the introd¨ part of this paper that the thickness of a cleavage plate is an important factor in deciding the ultraviolet transmission limit of a diamond. This fact was also brought to light in Section 2 of t.bis paper where it was shown ~at even the most highly absorbing diamond could be made to transmit light upto 2250 A. While looking at Table II, though it shows in a general manner the correlation between mean infra-red coefficient, the ultraviolet transmission limit and luminescence, the thickness factor should always be borne in mind. In conclusion, I wish to record my grateful thanks,~o Sir C. V. Raman under whose inspiring guidance, the present work was done. The diamonds used in this investigation were ~11 from bis personal collection. The AbsorŸ of Ullreviolel Radiagion •y Diemond 143

TABLE II

Ultra-violet Transmission Limits New Colour and Catalogue Thickness t Mea~:2fra" Intensity of Number Coef¡ Luminescence Very short Short Long Exposures Exposures Exposures

1Tlm. cm, -1 A A A N. C. 123 0.63 ~0 No,q-Lmninescent 2260 2250 2240 N.C, 60 1 "27 0,2 Do. 2260 2250 2250 N. C. 125 0.60 0.6 Do. 2300 2250 2240 N.C. 89 1.10 1,3 1)o. 2260 2250 2245 N.C. 121 0.54 1,5 Yellow 2260 2240 2240 N. C. 120 0-78 1.7 Do 2300 2255 2250 N. C. 124 0,94 1.7 Non-Luminescem 2260 2250 2250 N C. 122 0.74 1-9 Yellow 2700 2270 2250 N. C. 117 087 3.3 Faint 2680 2310 2250 N C. 127 0-66 4.0 Do. 2660 2300 2260 N. C. 126 0- 78 4.4 Do. 2270 2250 2242 N, C. 62 1-50 5.0 Inten>e Blue 2700 2450 2380 N.C.~ 0.25" ,5.0 [)o. 2250 N.C. 73 0.68 5.4 Weak Blue 2830 2650 2400 N. C, 79 1.20 5-4 Intense Blue 2800 2650 N.C. 70 2.20 6.5 Do. 2950 2400 N.C. 92 0-61 6,7 Weak Blue 2910 2740 2580 N.C. 157 0 -59 7.6 Do. 2870 2730 2650 N.C. 71 0-79 8.2 Medium Blue 2910 2780 N.C. 75 0.94 8-6 Weak Blue 2950 2850 N.C. 77 0.97 9.2 ]Do. 3010 2880 N.C. 78 1-39 10.2 Medium Blue 3100 2940 ~.C. 78 O. 12" 10 -2 Do. -- 2240 2240 N.C. 99 0.58 10.7 Weak Blae 2900 2780 N.C. 94 0-59 15.1 I)o. 3030 2830 N.C. 93 0.56 16.5 Do, 2950 2810

*--Approximate thickness. -- --Transmission limit not determined.

SUMMARY

The thinnest diamonds of the so-ca!led ultraviolet opaque type have been shown to exhibit transmission upto 2240 A which is also the limit for the so-called ultraviolet transparent type. In a diamond showing blue lumineseence, five new absorption bands b.ave been discovered for the first time in tke regiort below 2300 ~. The variation of absorption coefficient with wavelength for a typical blue-luminescent diamond has been determined for the first time in the region 3039 A-2570 A. Finally, the ultraviolet transmission limits for a number of diamonds have been obtained for different photographic exposures and the data correlated with their lumi, nescence and mean infla-red absorption coefficient in the 8 tz region. 144 K. G. Ramanathan

REFERENCES 1. Miller .. (Quoted by Robertson, Fox and Martin.) 2. Nayar, P. G. N. .. Proe. Ind. Acad. Sci., 1941, 14A, 1. 3. .. lbid., 1942, 15, 293. 4. Peter .. Z.f. Physik, 1923, 15, 358. 5. Ramachandran, G. N. .. Proe. Ind. Aead. Sci., 1944, 19, 304. 6. Raman, C. V., and Rendall, G. R. .. Ibid., 1944, 19, 265. 7. Robertson, Fox and MartŸ .. PhiL Trans. Roy. Sor 1934, 232A, 482. 8. Sunanda Bai, K. .. Proe. lnd. Acad. Sei., 1944, 19A, 253. DESCRIPTION OF PLATES PLAT~ XI Fto. 1. Ultraviotet Transmission Limits of Btue-Fluorescent Diamond (N.C. 78) tbr different thicknesses (a) 0- 15 mm. (b) 0.30 mm. and (c) 0.37 mm. F~o.: 2. Ultraviolet Transmission Limits of Strongly Blue-Fluorescent Diamond (N.C. 62) for (a) Very short photographie 91 (b) Short photographic exposure; (c) Long photographic exposur9 and (d) Small absorption path 0.25 mm. pLATE XII Fio. 3. Micro-photometric curve of the Ultraviolet Absorption Spectrum of Blue-Fluore- ~..ent Diamond (N.C. 78) at Liquid Air Temperature. Fio. 4. Ultraviolet Absorption Spectrum of Thin Blue-Fluorescent Diamond (N.C. 78): (a) At Room Temperature; (b) At Liquid Air Temperature. FtQ. 5. Ultraviolet Absorption Sper of (a) Green-Fluorescent Diamond N.C. 151 and (b) Non-Fluorescent Diamond N.C. 126. K, G. Z(ammzrl/JŸ Proc. [Jzd. Acad. Se~., A, yo/. XX/U, P[. XI

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