THE FLUORESCENCE OF RUBY, SAPPHIRE AND EMERALD. BY C. S. VENKA~'ESWARAN. (F~/om the Department of Physics, Indian Institute of Science, BangaIore.) Received October 12, 1935. (Communicated by Sit" C. V. Raman, Kt., F.R.S., ~r 1. Introduction. THE remarkably brilliant crimson luminescence of ruby was at first observed by Edmond Becquerel 1 by irradiation with sunlight and was later on, the subject of detailed investigations at the hands of several investigators notably Lecoq de Boisbaudran, 2 Du Bois and Elias, 3 Schmidt, 4 and Niendenhall and Wood. 5 As has been observed by these authors, ruby furnishes an interest- ing example of a solid yielding sharp lines of luminescence and its spectrum consists of two intense lines at 6926 and 6942* accompanied by other lines or bands on either sides. The cathodo-luminescence of ruby was investigated by Sir William Crookes 6 and his results agree well with those obtained by visible ]ight. The origin of this luminescence was the subject of prolonged controversy beween Becquerel and Crookes on the one baud and Lecoq on the other. The former authors maintained that the agent causing lumines- cence was alumina while the latter insisted that the small trace of chromium present was responsible "for the same. The detailed investigations of Wied- mann and Schmidt 7 and later by Sehmidt s succeeded in showing that care- fully frac'tionated pure alumina is non-luminescent and a small trace of chromium (1:10,000) is sufficient to make the lines appear intensely. Recently, Tanaka 9 showed that almost all the lines of the ruby coincide with the chromium series discovered byhim and from that he was led to conclude that the small trace of chromic oxide present in ruby is the cause of its luminescence. I Becquerel, E., Am~ de Chim. et Phys., 1859, 57, 125; 1861, 62, 90. 2 Lecoq de Boisbaudran, Compt. Rend., 1887, 103, 1107, 1224; 1824, 104, 330, 478, 554. 3 Du Bols, H., and Elias, J.. Ann. d. Phys., 1908, 27, 233; 1911, 35, 635. 4 Schmidt, G. C., Ann. d. Phys. U. Chem., 1904, 15, 622 5 Mendenhall, E. C, and Wood, R. W., Phil. Mag., 1915, 30, 316. o * The numbers in this and the subsequent pages denote the wavelengths in Angstrom units. 6 Crookes, W., Chem. News, 1887, 55. 25 and 56, 59. 7 Wledmann and Schmidt, ,4nn. d. Phys. U. Chem, 1895, 56 201. s Schmidt, G. C., Loc. cir. o Tanaka, four. Opt. Soc. Amer., 1924, 8, 287. 459 460 C.S. Venkateswaran Alumina in solid solutions with the oxides of other metals like copper, manganese, beryllium, samarium, etc., is also known to exhibit fluorescence, 1~ the colour of which varies with the active metal used. The luminescence of alumina in the form of aluminates or silicates ill the natural state, namely, topas, spinel and smaragd, etc., has also been investigated qualitatively but their complete spectral character and its bearing on the origin of lumines- cence has not been fully studied. In continuation of some unpublished work by Mr. Bhagavantam, the author has investigated the fluorescence spectra of the emerald, sapphire and ruby with a Fuess glass spectrograph of large aperture and having a dispersion of 100.~ per millimetre in the 6900 region. A pyrex mercury arc as well as a carbon arc has been used as sources of illumination. A deep blue solution of cupro-ammonium sulphate surrounding the solid cut off the radiations of tile source beyond 5500, where tile fluorescence of these crystals appear. Ilford hypersensitive panchromatic plates, H and D 2500, are used for photographing the spectrum. The fluorescence of sapphire shows close similarity to the well-known spectrum of ruby ; bnt the emerald, which however belongs to the beryl family, possesses striking dissimilarities in its spectrum though some general features characteristic of the ruby are also noticeable in this case. 2. Results. Emerald.--The most interesting of the substances investigated is tile emerald. It was available in the form of round beads of about 1.5 cm. to 2.5 cm. in diameter and in different tones of colour ranging from greyish green to pale green. These beads possess a fairly constant density of 2-66. A transparent green crystal in the form of a cut jewel with polished faces, which gives ,almost the same spectrum as the beads, has a refractive index of 1.5751 for the D-lines of sodium. A chemical analysis t of one of the beads gives the percentage composition of the mineral as 3 BeO. AI~Oa. 6 SiO~ with 0.3 % of iron and an easily detectable quantity (about 0.25 ~ of chromium as well as small traces of rare earth elements. The fluorescence is very weak in these beads, an exposure of about twenty-four hours being required to obtain a fairly intense spectrum under the most favourable circumstances while the ruby has yielded the main lines in a minute. Its spectrum con- sists of two intense and sharp lines at 6806 and 6837, the latter being stronger and broader than the former and a system of diffuse bands on either sides. In order to understand which part of the spectrum of the source is responsible lo Handbuch der Exptl. Phys., 1928, 23, Part 1, p. 425. t The chemical analysis was carried out by Mr. N. Jayaraman of the Department of General Chemistry of this Institute to whom the author's best thanks are due, The Fluorescence of Ru@, Sapphire and Emerald 46i for this fluorescence, the intense lines of the mercury arc have been isolated by suitable filters and used separately for excitation and tile fluorescence appears equally well and unchanged in character for the 4046, 4358 and 5461 radiations of the arc. The wave-lengths of the lines are measured by comparison with the iron arc spectrum and the mercury arc lines and are classified in Table I together with their relative intensities. The wave-lengths given for the two intense lines at 6806 and 6837 are correct to -I- 1 ~x. An enlarged picture of the spectrum is reproduced in Fig. 1 of the accompany- ing Plate. In clew of the characteristic dissimilarity of the spectrum of emerald from that of ruby, the author bus also examined fairly transparent pieces of a bluish green and a deep green (smaragd) beryl. According to chemical analysis, also carried out by Mr. Jayaraman, the bluish green crystal has a structure 3 BeO. AlzOa. 6 SiO~ but contains about 3 % iron and only a very much smaller percentage of chromium than the emerald, barely detect- able during the analysis. It has yielded no fluorescence even on prolonged exposure of more than forty-eight hours though the Raman line n 647 cm.-1 appears in four hours. The deep green crystal possesses an intense fluores- cence between 4950 and 5450 in agreement with the observations of some earlier workers for beryl? 2 The significance of these results are discussed in a later section. Sapphir~.--This was available in the form of light blue beads having a density of 3-95, thus evidently belonging to the corundum family. Blue crystals possessing deeper shades of colour are also examined in the form of cut stones and all of them have given an identical spectrum. The fluorescence, however, being extraordinarily feeble, very long exposures of about forty- eight hours are required to record the spectrum and this, perhaps, accounts for the absence of any investigation on the luminescence of this mineral. The spectrum consists of two intense and sharp lines at 6927 and 6942 and a series of bands, resembling the spectrum of ruby and is illustrated in Figs. 4 and 6. The results are given in Table I along with those of emerald. Ruby.--In order to furnish a comparison for the spectra of the two former substances, the fluorescence of a few beads of natural ruby of density four have also been examined. An exposure of about four hours has yielded a very intense spectrum showing a number of interesting features which have not been observed previously. It may be mentioned that in the previous investigations of ruby instruments of large dispersion of about 3 ~_ per milli- metre but only poor light-gathering power have been used by Mendenhall n Nisi, H., proc. Phys. Math. Soc. Jap., B., 1932, 14, 214. 12 Gmetiffs Handb~tch der /lnorg. Chem., 1930, 26, Be. 21. 462 C.S. Venkateswaran TABLE I. (a) Emerald at 35 ~ C. (b) Sapphire at 35 ~ C. Wavelength I I Wavelength Intensity i in A.U. I ntensi ty in A.U. / I 7130 weak broad band 6946 very weak and diffuse band 7060 very weak band 6992 weak band 690S weak, diffuse band 6942 very strong line 6837 very strong line 6927 strong sharp line 6806 strong sharp line 6802 broad diffuse ban, I 6736 strong band weak band 6633 medium band 6753 6578 weak band 6690 medium band 6592 strongnarrow ban, and Wood and Du Bois and Elias. While the latter authors recorded a large number of lines on either sides of the intense doublet at 18 ~ C., Wood and co-worker ~a were unable to reproduce their results at 23 ~ C. With an exposure of one minute at a temperature of about 35 ~ C. the author has been able to record the two lines reported by Wood and his co-worker with good intensity. At longer exposures many more lines and bands appear, some of which are observed for the first time. That Wood and 3,Iendenhall did not obtain any but the most intense lines, is perhaps due only to the low intensity of the spectrograph and short time of exposure.