R 346 Philips Res. Repts 1'3, , 1958

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1 R 346 Philips Res. Repts 1'3, , 1958 QUANTUM EFFICIENCY FLUORESCENCE OF F-CENTRE IN KCI Summary by C. Z. van DOORN The quantum efficiency of F-centre fluorescence at 77 ok in X-ray and additively coloured KCI has been measured using an integrating sphere. Values of up to 91% have been found, depending on the concentration of F and M centres. Résumé fl. Introdnction Le rendement quantique de la fluorescence des centres F dans le KCI colorë par rayons X ou par addition a ëtë mesuré à 77 ok au moyen d'une sphère d'intëgration, On a trouvé des valeurs jusqu'à 91%. suivant la concentration des centres F et M. Zusammenfassung Die Quantenausbeute von F-Zentren-lfluoreszenz bei 77 ok von mit Röntgenstrahlen oder additiv gefärbtcm KCI wurde unter Verwendung einer Ulhrichtschen Kugel gemessen.r Werte bis zu 91% wurden in Abhängigkeit von der Konzentration der F- und M-Zentren gefunden, Figures for the quantum efficiency of F-centrè fluorescenee in KCI (À,.x = 1 00 [L), as reported by several investigators 1) 2) 3), vary between 1% and 50%. As the magnitude ofthis efficiency is of interest from a theoretical point of view it was thought worthwhile to repeat the measurement using an integrating sphere to obtain more exact values. 2. Apparatus A cross-section of the integrating sphere is shown in fig. 1. The coloured KCI crystal was mounted on one of the sides of a small whitened (Ti02 paint) cube (1 cm 3 ) of aluminium. The cube was fixed at the centre of a hollow. brass sphere of 10 cm inside diameter by means of a glass tube and ground joint in the wall of the sphere and could be cooled with liquid nitrogen. The sphere could be evacuated to prevent condensation of moisture on the crystal. Its inside was whitened by smoking with burning magnesium, The wall contained two holes, closed with glass windows (fig. 2), one through which the exciting light was allowed to fall on the crystal and the other through which light from the inner wall fell on a lead-sulphide cell. This cell had a constant quantum-sensitivity between 0 5 and 1 2 [L as was ascertain-. ed by comparison with a thermocouple. The crystal was excited with the green mercury line p. = 5461 Á), chopped at 800 cis.

2 ., _ ---~--- QUANTUM EFFICIENCY OF F-CENTRE FLUORESCENCE IN KCt E:22:I brass E;SS;I molybdenum ~CrFe. ~ femléo r::::=j aluminium IE'E glass ';)3934 Fig. 1. Cross-section of the integrating' sphere. 3. Method of measurement The light, radiated in all directions by a sourcè inside an integrating sphere, is diffuselyreflected several times before being absorbed by the wall. 'I'he illumination of the wall, measured through an exit hole, is directly proportional to the totallight flux radiated by the source. The proportionality factor is, however, wavelength dependent. The ratio of the factors for the exciting and the fluorescencelight was evaluated bycomparing the intensities of light of exciting and fluorescence wavelengths entering the sphere with the corresponding intensities of the light coming from the exit hole. Light having approximately the same spectral distribution as the fluorescencewas obtained from the super high-pressure mercury lamp using a Schott UG 6 filter in front of it. This filter, in conjunction with the water used for cooling the lamp, passes a wavelength region around 1 micron.. In order to obtain the quantum efficiency of the fluoreseence three light intensities were.measured with the PbS cell. ~l) The exciting light falling on the crystal: the aluminium cube with the KCI crystal fixed to oneof.its faces was set with one of.its.other faces (~hiténed ~th Ti0 2 ) opposing thé entrance window. AScl;lOtt BG 30

3 298 C. Z. van DOORN filter, transmitting the exciting light,only, was used in front of the PhS cell. The signal measured hy the cell was corrected for the transmission and reflection factors of the filter and Ti0 2 respectively. (2).The exciting light not ahsorhed hy the crystal: the cube was turned until the crystal was in the exciting beam, using the BG 30 filter in front of the cell as before, The signal was corrected in the same way as in (1) for the transmission of the filter. (3) The fluorescence: with the crystal in the samë position as in (2) the fluorescence intensity was measured with a Schott RG 5 filter (transmitting the infra-red fluorescence only) instead of the BG 30 filter. As the crystal was in contact with one of the whitened faces of the aluminium cuhe, half of its fluorescence was reflected hy Ti0 2 before entering the sphere. Therefore a correction was applied to the fluoresoenee intensity for the reflectivity of the Ti0 2 In addition, the usual filter correction (for the RG 5 filter) was applied. The transmission factors of the BG 30 and RG 5 filters for the exciting and the fluorescenee light respectively were determined with the filters in the same position as when making a measurement. This was necessary for the following reason: owing to the thickness of the filter and its refractive index heing > 1, the exit hole of the sphere seems to move' closer to the cell, causing a larger fraction of the light to fall on the sensitive surface of the PhS cell. Fig. 2. Schematic diagram of the apparatus. A. Super high-pressure mercury lamp '1000 W. B. Filter combination to isolate 5461 A mercury line (1 5 cm saturated Schort filters BG 35 + GG 11). CuS04 solution + C. Lenses. D. Chopper (800 cis). E. Aluminium cube with KCI crystal. F. Whitel!-edscreen to prevent direètlight froij?the crystal from reaching the exit window. G. Filter transmitting the exciting light (BG 30) or the fluorescence (RG 5) only. H. Lead-sulphide' photoconductive cell..

4 QUANTUM EFFICIENCY OF F-CENTRE FLUORESCENCE IN KCI 299 Owing to the constant quantum sensitivity of the PhS cell the output signal is directly proportional to the incident photon flux. The quantum efficiency can now he calculated from 'YJ = constant X 13/( ) in which 11' 1 2 and 13 are the signals (corrected for filter factors and reflection of Ti0 2 ) ohtained from the PhS cell inthe three cases mentioned above, The constant arises from the wavelength dependence of the proportionality factor hetween light flux radiated hy the crystal and light intensity coming from the exit hole, as was discussed at the heginning of this section, 4. Preparation of the samples All crystals were drawn from the melt according to the K yropoulos method using Merck analytical reagent chemicals. Crystals 1, 2, 3 (tahl~ I) TABLE Quantum efficiencies of colour-centre emission I - Concentration of Crystal F centres (cm- 3 ) Ratio of abs. Quantum constants (%) efficiency Mhand/ F hand (%) I I a b c d e

5 300 c. Z. van DOORN were X-ray coloured at 77 OK and kept at room température in the dark prior to mea~urement. Crystals 4-13 were coloured additively in potassium vapour, quenched to 77 ok immediately after colouring and kept in the dark at room temperature for some time before measurement. The surface of the latter crystals was a little rough due to the heating in potassium vapour, probably resulting in a somewhat lower efficiency; Specimens were kindly supplied to us by Prof. H. Pick of Technische Hochschule, Stuttgart. These were drawn from the melt in platinum crucibles and coloured in potassium vapom;. Prior to measurem'ent they were heated to 600 C for 2 minutes, quenched to 77 OK and stored in the dark at room temperature, Specimen 16 was irradiated in the F band at room température for increasing periods of time (16a-e) in order to develop the M band. 5. Experimental results Some of the results are given in table 1. It is seen that for moderate concentrations of the F centres in both X-ray and additively coloured crystals the quantum efficiency reaches values approaching 100%. The efficiency strongly decreases with increasing concentration of M centres (16a-e), because the F-emission band is replaced by the M-emission band 4). Since the total efficiencies given in the table are for F + M emissions, the F emission must decrease even more strongly, suggesting energy transfer from F to M centres 5).. Eindhoven, November 1957 REFERENCES 1) C. C. Kliek, Phys. Rev. 94, , ) Th. P. J. Botden, C. Z. van Doorn and Y. Haven, Philips Res. Repts 9, , ) K. H. Beeker and H. Pick, Naehr. Ges. Wiss. Göttingen 1956, pp ') C. Z. van Doorn and Y. Haven, Philips Res. Repts 11, , ) J. Lambe and W. D. Compton, Phys. Rev. 106, , 1957.

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