Thermal and Photochemical Conversion of F-Centers in Colored Alkali Halide Crystals, Lecture notes of Physics

Download Thermal and Photochemical Conversion of F-Centers in Colored Alkali Halide Crystals and more Lecture notes Physics in PDF only on Docsity! STUDIES ON ~HE THERMAL AND OPTICAL COAGULATION OF F-CENTERS IN KCl by PLOYD EARL THEISEN A. THESIS submitted to OREGON STATE COLLEGE in partial fulfillment of the requirements for the degree or MASTER OF SCIEN'CE June 1952 r TBffiBDT Redacted for Privacy Amoclrtr Xro{tirs*. sf, Ghrulrsf Ia.0hr,rgp of,.IrJor Redacted for Privacy Urrd ol Bqnrtnrut cf 0hrntrtry Redacted for Privacy Chrlrurn of, trhool 0radurtl Om!,ttrr Drra of Orrdurtc S*hao1 Redacted for Privacy htr t&m!,; i.r pnurntod ,--ectobcr Ig. IgSl ftp.d F trn EUrbr 1 • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • GRAPHS Graph No. Title Page 1 . Linearit~ of monochromator compared with Beckman B" ••••••••••••••••• 16 2 . Absorption curves f or col ored crys tals us ed in exper•iments . • • • • • • • • • • • • 36 3. Absgrption s pe·ctra of crystals treated at 100 c•• •••••••••••••••••• 37 4 . Absorption spectra of crystals treated at 250° c.. . • . . . • . • . . . j . • • • • • • 38 5 . Absorption spectr a of crystals treated at 275° c•••••••••••••••• • ••• 39 a. Absorption spectra of crystals treated at 300° c•••••••••••••••••••• 40 Conversion of F- centers by irradiation at 25° c. • • • • • • • • • • • • • • • • • .. • 41 a. Conversion of F- centers at 100° c. • 42 Conversion of F- centers at 250° c. without illum. 43 10 . Conversion or F- centers at 250° c. 1th 560 mp illum•• • • • • • 4'4 11. Conversion of F- center.s at 275° c. without il1um. • • • • • • • • • • • • • • • • • • • 45 12. Conversion of F- centera at 275° c. with 560 mp illum•• • • • • • • • • • • • 46 1 3 . Conversion of - centers at 300° c. without il1um. • • • • • • • • • • • • • • • • • • • 47 14 . Conversion of F- centers at 300° c. with 48560 .lllJl i l lum •• • • • • • • • • • • • • • • • 15. Conversion of F- centers at 350° c. without " i1lum. • • • • • • • • • • • • • • • • • • 49 GRAPHS (Cont. ) Graph ro . Title Page 16-A . First order plot for the thel')mal con­ ver ion of F-centers • • • • • • • • • • • • 50 16- B. First order plots for the thermal con­ version of F- centers • • • • • • • • • • • • 51 17-A . Second orQ~r plots for the thermal con­ version of F- centers (250°) ••••••••• 52 17- B. Second order plots for the thermal con- · · version of P• centers (275°) ••••••••• 53 17- C. Second order plots for thg thermal con- . version or F-centers (300 ) . • • • • • • • • 54 17- D. ~econd order plots for the thermal con­ version of F-centers (350° )••••••••• 55 18- A. Second order plots for thermal plus ~60 m? ill . conversion of F- centers (100 ) • • • 5o 18..:B. Second order plots for thermal plus ~60 m~ ill . conver ion of F- centers (250 ) • • • 57 1s- c. Second order plots for ther mal p l us g60 mp 111 . conversion of F- centers (275 ) • • • 58 18- D. Second order plots for thermal plus 560 m~ ill. conversion of F- centers (300°) ••• 59 19 , Plot of log k vs • l / T for thermal F-center conversion . • • • • • • • • • • • • 60 STUDIES ON THE THERMAL AND OPTICAL COAGULATION OF F· CENTERS IN KCl I . INTRODUCTION The first observations on colored alkali halide crystals appear to be the work or Goldstein (4); later work as then done by Pohl (11,12 , 13) , Molnar (7,8), Klein­ achrod (5), and Glaser (2) as notable examples, and the theory of color centers in alkali halides has been reviewed compreh naively by Mott and Gurney (9, pp . l09-151) and by · Seitz (15) . Coloring can be induced in alkali halide crystals by x- ray irradiation or by· exposure to an atmosphere ot the alkali metal vapor at various temperatures (additive coloring); the resulting color dependa on the alkali halide crystal , and the color intensity depends on treatment his­ tory factors such aa duration of x-ray, exposure or tempera­ ture employed . KCl , which was uaed exclusively throughout the experiments undertaken in this work, is colored magenta . The absorption spectrum of additively colored KCl crystals shows a strong absorption peak around 560 ~ wave length which is theoretically considered to be due to an absorbing center consisting of an electron that migrated into the crystal from a surface- adsorbed alkali metal atom and became trapped in a halide ion vacancy (9 , pp . lll- 113) 4 formation of the longer ave length bands and the de­ struction of all the F-centers. The foregoing processes and longer wave length bands ill now be considered in more detail. Molnar first observed the development of a band ( M~band) around 820 mp in addition to the F-band and an absorption band in the ultraviolet region when KCl was colored by x-rays (7, p.55). This M-band and the F-band were interconvertible by irradiation ith light absorbed by the respective band ; but some of the prominence of the F- and M- bands was lost during interconversion and ap­ peared as two new bands at about 675 and 730 mp; these were called R-bands because of absorption in the red region of the spectrum. Scott and Bupp (16, pp.343-344) found that 85 minutes of white arc irradiation combined with air jet cooling of an additively colored KCl crystal resulted in a noticeable conversion of F- and M- centers to R~centers in accordance with -olnar's work. and that two minutes further treatment ithout the air jet caused the formation of a broad band, the R1 -band, similar to that found by Glaser. The ab orption spectrum of additively colored KCl crystals handled in total darkness or under red light and stored in light-tight containers was observe by Scott and Smith to show the t. -band, whi ch previou ly had been thought to occur only 1n optically treated or illuminated crystals, and in 5 some cases a quite narro symmetrical band in the neighbor ­ hood of 760 mp. 1hich as termed the ttcolloidal band" (18 , p . 982} . Heating an additiv~ly colored KCl crystal with a sufficient concentration of F- centers cause conversion ot F-centers to the colloidal band. For each temperature of heating there is a concentration of -centers in equilib­ rium with the colloidal band which is independent of the original .~.~ - center concentration (provided the original F­ center concentration exceeds the equilibrium value) (18 , pp . 985-986) . The colloidal band maximum shifts toward longer wave lengths at higher treatment temperatures , and R- bands have not been observed either accompanying the colloidal band nor follo ing its reconversion to the P­ band . Soott and Smith found that the attainment of equil­ ibrium. in he~t treating varied from 90 minutes at 300° to 45 seconds at 500° . They were able to calculate the latent heat for th dissociation of the coagulated F- cen­ ters by plotting the logarithm of the equilibrium concen­ tration against 1/T, and found a value of a.o .:t o.3 kcal/mole of F~centers formed at 350° . Seott and Bupp found the absorption curves of crys­ tals trent d by heating alone and by heating combined ith irradiation to show considerable difference in the result­ ing band formation . The irradiation could possibly give 6 --------------------~---- rise to the R-bands which would overlap the colloidal band considerably, but in both cases a pronounced shift in the maximum of the bands in the rod region to"Ward longer wave lengths for higher crystal treatment temperatures was ob­ served . Possible mechanisms for the formation of absorbing centers other than the F'-centers are the aggregation of F­ oenters into larger clumps; the ionization of F-centers and the recapture of the r lea3ed electron by a different type of trapping center, such as by a pair or a quartet of vacancies; the aggregation of alkali metal atoms diapersed throughout the crystal into colloidal clumps; or a combina­ tion of any of the above . In the aggregation of F-centers into larger clumps, Seitz suggests that the M-center of Molnar is among the first products and may be a F-center attached to a pair of lattice vacancies; he also suggests thAt tho R-eenters may be first aggregates of F-centers such as pairs or singly ionized pairs . The shifting of the maximum of the R'-band and the red absorption peak tOlard longer wave lengths for higher treatment temperatures indicates a change in the nature or relative abundance of absorbing centers composing the band (16, p.344) . This shifting is undoubtedly related to the increasing mobility of negative-ion vacancies with temperatures which should lead to the rapid formation of 9 constant at several temperatures •ere determined and com­ pared with those found for heating alone . 3 . The absorption spectra for crystals which had undergone treatment in 1 and 2 were compared for any noticeable differences . 4. Cleaved crystals - A and B, adjacent to a sens1­ tiv thermocouple ere illuminated at room temperature itb: A : 560 mp wave length light B : white light from a carbon arc . The results were compared to determine whether conversion was due to an ionization process caused by absorption or the 560 mp wave length light, or was mainly due to soma proce s induced by the heating of the crystal 1th infra red radiations from the arc . lO II. EXPERIMENTAL CRYSTAL PTIEPARATI01l. All crystals were prepared by placing KCl stock wrapped in copper foil in the end of a sealed copper tube with a piece of metallic sodium at the other end . Sodium was used in preference to potassium metal because its lower vapor px•es ure resulted in a more uniformly colored crystal batch S.n shorter treatment time. This copper bomb was placed in a muffle furnace for 23 hours at 550° and then rapidly quenched by plunging into ice water . The resulting additively colored crystals averaged 1.5xlo17 F- centers/c . c., and ere handled or ex­ posed only under a ruby aafel1ght . The colored crystals, with dimensions about 0. 25 by 0. 25 by 1. 25 em., were cleaved with a razor bl de into sections as th1n as poa­ aible; only those sections with parallel and unmarred faces were used for the experiments . Any crystal which showed evidence or excessive bleaching around the edges during or at the completion of a run was not used in tabulating re­ aults. APPARATUS . The apparatus used in the experiments is sho n in Plate 1, and consists mainly of the following: 1. Thermostated furnace 2 . Crystal holder 3 . Monochromator 4 . Photomultiplier cirouit s. Filtered carbon arc ll 1. Thermostated ru:rnace: The furnace consisted of an iron pipe 4 inches long and lt inches in diameter; th1a .was wrapped with asbestos pape:r and ten feet of No . 24 Nichrome wire . A brass tube l/2 inch in diameter and fittoo with a brass plug was inserted into one end of the iron pipe ~ A small hole, wbich e,:cted as. an ex.it aperture tor light from the monochromator to pass through the furnace to the. phototube , wa.s drilled thr·ough the center of the braae plug. The furnace waa mounted horizontally in a rigid plasterboard support, and a thermocouple, consisting or ten inches of Copnic wire , was inserted into the furnace through a small hole in the side . The tip of the thermocQuple pro• truded beyond the fUrnace wall only 1/8 ineh, so that it did not obstruet light from the monochromator . Melting ice was used tor the thermocoupl$ cold junction, and tempera• ture was measured by placing the bu,lb of a Centigrade ther­ mometer at the position in the furnace where the crystal would be and noting the seale reading on a millivoltmeter for a given temperature. Temperature wes controlled .man­ ually with a Variac transformer. and was maintained con­ stant within ten degrees !'or the highest temperatures em­ ployed . Experiment indioated that heating the furnace· twenty degrees above the desired temperature before insert... ing the cool metal crystal holder would compensate for the resulting temperatur0 drop . The thermocouple was removabla 14 placed in a posi t1on vJhiah allowed light of about 560 lll)l to pass through. the exit slit at minimum deviation by the glass prism. Tlle spectrum was 1435 em. wide , and the exit slit as o •.004 em. wide., Since the visible speot~um covered about 400 mu from the violet to the red. a wave length band of about 2 mp was isolated if dispersion of light by the prism were perfect ., The 560 nlJl wave length was picked out on the condenser l<nob by comparing tho ab­ sorption cUPvee found with the model B .Beckman spectropho­ tometer for a t-wo percent solution of CuCl2 in ethyl alco• hol and an actual addit1vely colored KCl or.J$tal with ab­ sorption curves .found for the same materials with the :mono­ chromator. 4. Photomul~iplier circuit: A RCA Mo . 931-A multi• I . plier phototube with an S4 r .esponse was placed in the path of the light passing through the furnace aperture, and was operated by mean& of a 900 volt battery supply in the oir­ cmit shown in Plate 4. Resistances l to 9 were each 70,000 ob:tna, and lO a,-~ sofooo ohms.. They we:re all wire woun4, precision type resistors of high ohm value in order to out down battery drain and help in ure good linearity of the phototube output. The phototube was placed approximately ten inche$ .from the furnace in order to cut down heating effects, and stray light was eliminated by enclosing the tube in a . ooden box and operating the apparatus in a 15 darkened room. The phototube output was measured with a nicroammeter in parallel wi t h a 50,000 ohm potentiometer, and one hundred percent transmission was measured by adjust­ ing the potentiometer shunt to read full cale on the am­ mater . The linearity of the phototube output as checked by comparing the curves obtained at 560 with the Bockman model B spectrophotometer and with the monochromator for a varying number of thicknesses of a green c llophane and a Kodak ~atten filter No . 82 (Graph 1 howa the curves for the green cellophane) . The result• indicated that tho mono­ chromator and phototube output gave approximately the same linearity as the Beckman model B. 5. Filtered carbon arc: For illumination experi­ ments ith light of 560 wave length, a carbon arc fit ­ ted ith a condensing lens was arranged to one side of the furnace in such a manner that the condensed arc light beam could be ret'lected by a mirror arrang rnent into the furnace opening and onto the crystal . By proper shielding, the arc was operated continuously during the progress of a run, and the arc beam could be cut off from the furnace when it was necessary to take an absorption reading. A circular filter holder , with an extra aperture for allowing passage of the monochromator beam, was set up in front of the fur ­ nace openlng and could be rotated tor selection of th ·proper filter . The 560 mp wave length was isolated by z 0 C/) C/) ~ en z <S:40 a: ~ 10 0 GRAPH LINEARITY OF MONOCHROMATOR COMPARED WITH BECKMAN "B • AT 560 MU o- BECKMAN x- MONOCHROMATOR 2 3 4 THICKNESS OF GREEN CELLOPHANE 19 ammeter ·~ oale read 50 microamperes • Dark current was com.­ pensated for l:ry: A - Measuring 100 perQont transmission, B ... turning of f monochromator lamp , C - adjusting the ammeter needle to read zero transmission, D - repeating the above procedure unti l , when the monochrc.nnator lam_p was turned off., the runmeter reading would fall 'from 100 per.... cent transmission to zero transmission with­ out r~quiring needle ad~ustment. 3 . The crystal was $wung into the path of the light and· sample transmission was read. 4. !n orde-r to cut down errors due to current flue­ tuations in the various circuits , a reading required after a certain time interval (say , for example, five minutea) would be obtained by starting to take readings 1/2 minute before the desired time (4~ minutes, , and continuing to take readings until 1/2 minute pas t the desired time (51~.. minutes) . The values obtained should give a smooth curve, through the one minute interval if there were no serious current fluctuations~ and the midpoint of t he curve would give the d•s:ired value . PLATE S~STEMPERl MENTAL EX <TO~P~V~IE~W~)___ t\) 0 21 III. LEGEND (PLATE 1) 1. Thermostated furnace 2. Crystal holder 3. Monochromator 4. Photomultiplier circuit 5, Carbon are 6. Condensing lena 7. Thermo~ouple a. Ice junction of thermocouple 9. Millivoltmeter 10. Photomultiplier tube 11. Extraneous light shield 12. Mirror 13. Circular filter holder 14. Focusing lens 15. Exit slit 16, 900 volt battery supply 17. Crystal rotator stop 18. Focusing lena 19. Glass prism 20. Condensing lens 21. Monochromator light 22. A. c . ammeter for arc 24 IV. CALCULATION OF RESULTS The F-center concentration was determined by apply­ ing Smakula's equation to transmission data. This equa­ tion, using modern values for the physical constants in­ volved reduced to: here the oscillator strength has been assumed to be unity because of lack of evidence supporting Kleinschrod's value of 0.81 (18, p.984). Since no systematic study ot the dependence of W on tempern.tur at temperature·S much above 25° has been made, it was assumed to be 0.39 for all calcu­ lations, inasmuch as this represents the best estimated value in the neighborhood of 300°. This estimation was based on previous measurements of at room temperature and at -195° (16~ p.342) (18, p.984). The absorption coefficient at the F-band maximum was determined by inserting the transmission data into the optical density formula : 2.303 log I ­ ~ ­ d =crystal thickness in am. The crystal thickness was measured mi croscopically with a calibrated eyepiece micrometer. 25 The energy of activation was found graphically by applying a form of Arrhenius'& equation: Log k : - H~ v 1 + C 2.3o R " -r k : velocity constant Ha =activation energy T :; absolute temperature 26 V, ANALYSIS OF ERROR SPECTROPHOTOUETER . The fluctuations of current through the light source were great enough to cause trans­ mission readings to vary by plus or minus five percent . This error was largely eliminated by taking readings in the manner explained previously. Since a number of points were used in plotting the necess~y curvea , the remaining error pro~ably cancelled itself and was certainly not greater than about 2 percent . The narro band of wave lengths passed by the exit slit , and an examination of Graph 1, indicate that negligible error as introduced by incomplete resolution of the spectrum. READING PERCENT TRANSMISSION AND CALCULATING flm • The relative error in I / I 0 was greater for the initial ab­ sorption readings when the F• center concentration was of maximum value , and for cryst&ls wi t h the greatest tb:l.ck­ neaa . The average crystal thickness was around 0 . 085 om., and the error in determining the crystal thickness waa neg­ ligible compared to other error sources . The average error in measuring nbsorbancy, based on twenty- five percent transmission as the average mid· value for a run, was about one percent . The plotting of a large number of points would tend to cancel errors in reading I / 10 • 29 VI- DISCUSSION OF RESULTS The main purpose of the experiments undertaken was to determine if there are any apparent dif'f~rences in the conversion ot F-centers thermally. and the conversion of F•centers by ther~al and optical treatment combined. The experimental results ill be classified as room-tempera­ ture experiments. thermal experiments, and thermal experi­ ments ·combined with illumination for discussion purposea. ROOM-TEMPERATURE EXPERIMENTS . Graph No . 7 aliowa the change in percentage transmission of the F-band with time during white arc light and 560 mp illumination at 25°. It is quite apparent that the whit arc light illumination gave much m.ore rapid conversion or F-oen.tera in comparison with the rate of conversion induced by 560 mp illumination. This increase in rate ith white arc light could be due to absorption processes occurring at wave lengths of light other than 560 mp or infra red wave lengths, heating of the crystal by absorption of infra red radiations, or to an in­ crease in the intensity of the 560 ~ light striking the crystal because of the absence of filters. Thermocouple measurements ot crystal temperature indicated that heating ot crystals illuminated with white light wa$ around nine degrees above room temperature, and the heating of crystals illuminated ith light of 560 ~ wave length was negligible; 30 al·ao., it was stated in the discussion of the apparatus that the filters used t .o isolate the 560 111Jl wave length in the aro beam tran~mittad approximately ten percent of the desired light. The incx-ease in the rate of conversion or F-centers could not possibly be due ·to the small tempera• ture rise during illumination. but the combination of a temperature change .and an increase in illumination inten­ sity could explain it unless the presence of other wave lengths in the white arc light were influencing the conver­ sion rate. This possibility was checked by comparing the absorption curves of crystals treated with white arc light and light of 560 :mp wave, length at 100°. (This tempera­ ture was chosen in order to obtain conversion ith 560 ~ light within a reasonable length of time.) The differ.., enees are shown in Graph No. 31 and outside of a loss ot prominence of the M-ba.nd at 820 Dy.t (1o accordance with the work of Glaser and of Scott and Bupp) and a greater losa of F·centers with white light illu.mination, there are no salient differences in the two absorption curves . It could therefore be reasoned that the conversion of F•oenters by arc illumination is a function of the 560 mp wave length intensity, and of heating of the· crystal by in:fra red ab­ sorption. THERMAL EXPERIMENTS . Graphs 8 , 9 1 11 ., 13 , and 15 $how the changt;ts in :F-ce'nter conoentxoations with time for 31 crystals h~ld at various temperatures in the :f'u:tnace; it is apparent from the curves that the ther.mal conversion rate increases with an increase in temperature . The noticeable le.veling ot the curves within 'several minute a for conversion at 360° is due to the higher equilibrium concentration or F- eenters at this temperature , and the rapidity with which equilibrium is reached. Graphs 16- A and l6....B show the first orQ.er plots for these conversion curves, and Graphs 17- A, 17- B, 17- C• and 17- D are second order plots . 5y comparing the two sets of reaction order plots, one may conclude that the reaction is more nearly second order than first , though the distinction is not clear- out . Values for the velocity constant k, calculated from the slopes of the second order curves , are given in the following table : TABLE 1. k X lo19(c . c . )(F-centers) -l (min~ -l Temp ., 250° 275° 300° 350° 1 . 938 1 . 08 1 . 55 3 . 24 2 · . 89 1.14 2. 1 4. 0 3 :82 . o.s . ~h, 09 2 . 5 Ave . . 882 1 . 01 1. 91 3 . 58 34 the reaction caused by illumination is completed within the three minute period not conforming to the second order plot of the reaction curves; if this were the ease, the solid line plots in the saeon.d order :reaction' graphs should yield tho same values of k as were obtau1ed from the corresponding plots without illumination . r:xa.mination of Tabl: 3 shows that this doe• not seem to be th case . TASLE 3 ., k X l019 (o . c.) {F-centere)'"'1 {minJ•1 Temp . 250° 275° 300° 1 5 2.2 ;,77 2 3 11 5-­ 2,.2 ,, 1.e .73 .so Ave . 7 2 . 1 . 74 The negative temperature ooe.ff1c1ent for the rate constant during the later stages or the conversion is a strong indication that the formation of colloid has been completed during the rapid initial stages, and that the later stag~s, consist or a solely optical conve-rsion of F... centere t() R-oenters . If R centers decreaae in stability . with increasing temperature, the negative temperature: coef­ tieient is- readily- explained. 35 COMPARISON OF THERMAL A:ND THERMAL-PLUS-ILLUMINATION EXPERIMENTS . GraphS 4, 5, and. 6 show representative ab­ sorption curves or crystals after thermal# and thermal plus optical treatment. The treatment times for thermal eon­ version wer-e in all cases longer than for combined treat­ ment, and no attempt was made to compare crystals that bad undergone appro.ximate.ly the same amount of conversion.. The treatment times given represent the intervals after which the conversion of -centers was practically complete . The only pronounced differences appear to be the shifting or the absorption peak bet\'leen 600 and 800 Jll}l toward shorter wave lengths (which is probably due to the formation of R­ bands along with the colloidal band ) and diminished protni• nence Of the M- band {possibly due to transmission of light or 820 m)l wave length by the filters, or masking or the M...band by other hand formation) . Also , the height of tho abs.orption peak between 600 and 800 .JtYl is greater in each ease for the crystals which had undergone combined treat.. ment; considering the differences in treatment times, thit is probably due to the increase in F-center conversion rate with illumination• . A possible explanation .for the increased conversion rate or F-oenters with illumination is that the absorption or the 560 m» wave length light ejects electrons into the conduction level banda; the released electrons oan migrate 0 g 0 01 0 CD 0,... zo oco ...... 0 ~.,., a:: 0 ~i 4 ~~ 0 N 0 0 300 400 ~00 600 700 800 900 GRAPH 2 ABSORPTION CURVES FOR COLORED CRYSTALS USED IN EXPERIMENTS WAVE LENGTH (MILLIMICRONS> 0 m ~ GRAPH 5 ABSORPTION SPECTRA OF CRYSTALS TREATED AT 275• c. z• 0 0 ~0a.., a: 00 C/). /' _,.... '/ / / T..ltMAL CIO ....1 T....AL + HO 1111 ILL . l 10 MI...I 0'~----------------~--------~------~----------------------- ~ 400 600 .00 700 100 <0 WAVE LENGTH CMILL..ICRONS) GRAPH 6 ABSORPTION SPECTRA OF CRYSTALS0 2 TREATED AT 300• c. I \ I "-I I \ \ THERMAL+ 560 MU ILL. ( 10 MIN.) ~ 600 700 800 900 WAVE LENGTH ( Ml LLIMICRONS) 0 01 0 GRAPH 7 CONVERSION OF F-CENTERS BY IRRADIATION AT 25. c . - 0 ~ ~ 0 <D 01/)- z 0­ C/) 0~ C/) - o - 00 cr 0 - - 0 0 W..TI AltO ILL. --- NO MU ILL . o'Lo----------------------------------.~0~----------------~.~0~--------------~=-----e TIME (MINUTES) CJ)­ z ~ .... ~ •- ,., - N -- U)- I 0 0 X m (.) z 0 (.) CD ,.... "' It) 0 ) GRAPH 10 CONVERSION OF F-CENTERS AT 2so· c . WITH 560 MU ILLUM. 0 \ 0 o\ 0 \0 '0 \ 0 'ao, '0-o­O--o­ 0 20 0 20 ~ ~ TIME (MINUTES> GRAPH II CONVERSION OF F- CENTERS AT 275• C. WITHOUT ILLUM . '= !! 0 liD- • ... I 0 ,., X (.) Nz 0 (.) -a:: ­ w 0 1­ 0z w u I LL ~0"' 0'-­CD .... 0 10 IS 0 GRAPH l2 CONVERSION OF F-CENTERS AT 275• C. WITH 560 MU ILLUM . 0 10 IS 0 10 IS TIME (MINUTES) co co II)- -I 0 X • u 0z ,., 0 - 0~ a:: N u 0~ t- ·""' o "-o z w 0 °.::::----- 0 ­= GRAPH 15 CONVERSION OF F-CENTERS AT 3so· c. WITHOUT ILLUM . 0w ~o......_o_u 0 I . u. 0 8 -- 0 - 0 G ~~------~--~--~--~--~--~--~--~--~------------------- 0 2 3 4 7 8 9 10 TIME (MINUTES) THEftWAL (0 ~ (.) z 0 (.) GRAPH 16-A "liST ORDlR PLOTS FOR THE COIIYERSION OF F-CENTERS 2so· c. 20 40 60 80 100 120 0a: w ~ z l.LJ 0 I IJ..-(.!) 0 ..J ...; 275. c. Ci ·o 10 20 30 40 ~0 60 (.11 TIME (MINUTES) 0 N (0 I g (.) a: w 0 t­ z w (.) I lL- (!) 0 _J 0 'o Z ""-o I GRAPH 16- B @ 0~""~~'-.........~~ I FIRST ORDER PLOTS FOR THE THERMAL '----0 ~~ I CONVERSION OF F-CENTERS ~0 ~~ ®~ 0-------0 300" c . 0~ ------0 0 s 10 IS 20 0 G) ­ 0 0> ....__..._----­ 0 ----­ ~-----=:::::::. ~ :=::::::::::::: @ .:::::.==8C:.J 3so· c. 0 2 3 4 s CJI TIME (MINUTES) ~ o--o-- ~-------------------. o--o-­ -o-o- GRAPH 17-C (.) z 0 · ·coND ORDER PLOTS FOR THE THERMAL CD (.) CONVERSION OF F- CENTERS 00:: -w ­ X~ ~--------~~--------~--------~----z-w (.) I LL 3oo• c. 0 10 15 20 TIME CMINUTES) - G» ~r- •0 ,._ (.) qz 0 (.) • a:0­ w ~ ­X -z w- (.) I - lL eft ~- •0. ,._ q 0 0----0 ----- _o ____ ---­ o­ 0 GRAPH 17-D SIECONO OROIER PLOTS trOR THE THERMAL CONVERSION 0, '-CENTERS 0 _____-----;;­ 0 ---~--®­----- 0 _@- 3so· c. ---0 - _@>­ @ I 2 3 4 . :s 6 (11 TIME (MINUTES) C1l < ( ••.. + ..z ~ ... i ~ ... ... %.. 0• ., 0 5 6 . (.) •0 0 0 -- GRAPH 18-D SECOND ORDER PLOTS FOR THERNAL + S60 t.W ILL. CONVERSION OF F -CENTERS c..) z 0 (.) !i! 00:: w ~" 0- 0 0 -@ X~z / o..:t / @ G> w (.) I LL / / / / / / " / / ., -­ "'/ -­ /., / / / , / ® / / / -­/ -­/.-9­ // -­-­ G> G> 300 ° c . -­ / ., -­ -­ / / It) / q 0 s 10 TIME {MINUTE 5) en co GRAPH 19 PLOT OF LOG K vs. 1/T FOR THERMAL F- CENTER CONVERSION 0 • 0 ~~0~0~1~.~~--~~~~~--~0~0-1-8~~~~~--~~~~-0~0~17._~~~--~~~~~~. . . 0 1/T 61 through the crystal lattice until they are re icaptured by an ionized F-center, an alkali metal ion, or some trappins • center, such a4t .colloidal alkali metal , which i.s or lower potential energy than the rest of the crystal structure. A colloidal particle, after -capturing one or more e.lec- · tron•, would possess a negative charge and could then act as a force for the attraction of the negative ion vacan­ cies lYhicb have previously lost an electron, and for I alkali metal ions. The vacancies and ions attracted to tm colloidal particle neutralize part ot its charge, but neutralization of metal ions increases the size of the col­ loidal particle and makea it a center ot still lower poten• tial energy for the eaptur.e of ele otrons. .In this manner the rate of coagulation of tb.e F·centers during combined treatment would be con$ldt'rrably greater than during th&Pinal treatment alone, where the attractive .force for coagula• tion would presum.ably be absent . This mechanism is similar to the one suggested by Gurney and Mott for the formation of the photographic latent image in silver halide emul• sions, in which electrons released by absorbed light o.re attraoteul to sulfide impurities, and silve1:' g;ranules are formed from S·ilver .ions attracted to the negatively charged centers (l).