The transient response of the triplet self-trapped exciton luminescence      intensity to a pulsed microwave excitation was measured for 1.3 K < T < 4.2 K in NaCl, KCl, RbCl, KBr, RbBr and CsBr. The results are quantitatively explained with a theoretical model describing the spin relaxation phenomena as 1-phonon direct processes. Good agreement was obtained as a function of temperature and magnetic field

Aegerter, Michel A.

Mori, Y.

Von der Weid, J. P.

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Optical detection of spin relaxation processes in the triplet state of the self trapped exciton in alkali halides

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Solld State Communlcations, Vol. 26, pp. 181-185. 0038-i098/78/0415-018~ $02.00/0 ~Pergamon Press Ltd. 1978 Printed mn Great Britamn OPTICAL DETECTION OF SPIN RELAXATION PROCESSES IN THE TRIPLET STATE OF THE SELF TRAPPED EXCITON IN ALKALI HALIDES Y. Mor~ t, J.P. yon der We~d t% and M. Aegerter Inst~tut de Physique, Un~vers~t~ de Neuch~tel, Rue A.-L. Breguet l, CH - 2000 Neuch~tel, Switzerland The transient response of the triplet self trapped exc~ton lumlnes- cence ~ntenslty to a pulsed m~crowave excitation has been measured for 1.3 K < T < h.2 K ~n NaC1, KC1, RbC1, KBr, RbBr and CsBr. The results are quantitatively explained w~th a theoretical model describing the spln relaxation phenomena as one phonon d~rect processes. Good agreement ~s obtained as a function of temperature and magnetic f~eld. The recomblnatlon of self trapped holes (V K centres) and conductlon electrons In pure alkalz halzdes leads among other processes to the observatlon of self trapped exclton (aTE) lumlnescence emlsslons. The STE, characte- rlzed as relaxed (V K + e-) centres are allgned along the <ll0> axls in crystals havlng the NaC1 type structurel, 2 and along the <100> ax~s zn crystals havzng the CsC1 type struc- ture 3. T~me resolved spectroscopyW, 5, optzcal polar~zat~onW,5, 6 and magneto-opt~calT, 8 data brought much ~nformat~on about the STE emit- ting states and allowed the ~dent~f~cat~on of the varlous luminescence bands as arzs~ng from the radiative decay of elther s~nglet or triplet relaxed states of the STE, electron sp~n resonance (ESR) measurements have indeed been performed ~n triplet states of the STE ~n several alkal~ hal~des9,10,11 . In th~s letter, we present for the fzrst t~me results concerning the spzn relaxation processes zn the trzplet states of the STE ~n some alkalz hallde crystals. The ma~n features which descrzbe the trz- plet states of the STE ~n alkall hal~des are the followzng. In crystals wzth the NaC1 type structure, the poznt group which describes the local symmetry ~s D2h so that at zero m~gnet~c fzeld, the trzplet level ~s splztted ~nto three levels havzng respectzvely Alu , B2u and B3u symmetries. The radiative transition from the Alu level to the ground state Is forbzd- den, whereas ~t zs allowed from the B2u and B3u levels. In crystals wzth the CsC1 type structure, the symmetry point ~roup zs D~h. At zero magnetzc fzeld, the sztuat~on zs therefore mostly the same as zn the other alkall halldes except that the two emlttlng levels are now degenerated into a E u doublet. In the steady state condltlon under X irradlatlon, these levels are constantly popu- lated. Since the Alu level has no allowed transltlon to the ground state, one can under- stand that if the spln lattlce relaxatlon tlmes are not fast enough, the three trlplet levels of the STE will be away from thermal equlllbrlum, In such a case, the populatlon of the Alu level can be much greater than the populatlon of the other two. Magneto-optlcal measurements in KI and Cs112 conflrm thls fact for T < l0 K. Since for STE havlng thelr axls parallel to an external magnetlc fleld, the A1u state does not mlx wlth the other two states ESR transltlons can be induced from thls level to the two emlttlng ones, and detected as an increase of clrcular polarlzed (o + or o-) emltted llght due to the greater populatlon of the AIu level. In the present experlments, we measured the tranalent response of the STE lumlnescence intenslty to a pulsed mlcrowave excltatlon at temperatures between 1.3 K and 4.2 K. The effect was analysed elther in o + or o- clrcu- lar polarlzatlon in the followlng ultra pure crystals NaC1, KC1, RbC1, KBr, RbBr and CsBr. The cleaved or cut samples were annea- led at about 450°C in order to ellmlnate in- ternal stralns, then placed in a TE01 ! X band mlcrowave cavlty and imerged in a superconduc- tlve llquld Hellum cryostat (Oxford Instru- ments Spectro Mag SMh). The samples were placed wlth a <llO> axls parallel to the external magnetlc fleld except for CsBr, whlch was placed wlth a <100> axls parallel to the TOn leave from the Department of Applled Physlcs, Osaka Czty Unlverslty, Japan %TOn leave from the Physlcs Department of the Pontlfzcla Unlversldade Cat611ca, Rio de Janelro, Brazll 181 182 OPTICAL DETECTION OF SPIN RELAXATION PROCESSES field. The STE emxssIons were obtained by X zrrad~atlon from a 150 kV, l0 mA Tungsten target X ray tube and the emitted l~ght ana- lysed with broad band optical f~lters, quartz ~/h plate and detected by an EMI 9558 QB photomultiplier. Its output was ampl~fled and finally analysed w~th a HP 5480 A (150 kHz) s~gnal averager (typically w~th l0 ~ passages). A typxcal electron sp~n resonance spec- trum of the triplet STE ~s shown in figure 1 for RbBr. The microwave modulatxon frequency used for th~s recording was 1 kHz. The ESR transxtions due to the three ~nequivalent STE orxentations are ~dent~f~ed according to the angle between the STE symmetry ax~s and the external magnetxc f~eld. Data for the other crystals have been found s~m~lar to those already published9,10, II Vol 26, No whlch characterlze the approach to the steady state 32 and 33 . When the microwave fleld is shut off, the translent response is flrst characterlzed by a rapld decrease of the luml- nescence intensity to a value lower than the initlal one, followed by a slow recovery to the steady state, also characterlzed by two response tlmes. We shall call them respec- tively 3{, 3~ and 3~ Values of these response times for RbC1, RbBr and CsBr are shown in Table I, together wlth the radlatlve llfet~me 3 of the B2u or Bsu levels ~n RbC1 and RbBr and ~ levels ~n CsBr. The s~mlla- rlty between 3[ and the l~fetLme 3 for all crystals measured ~s remarkable, and ~ndeed shows that th~s particular behav~our ~s due to the fact that the ESR transitions occur between the forbidden Alu level and the I . . . . . .  I I I I B3u. + +I Rb Br 1.3 K B 2u ~"'-,,~ 0 / I I I B [kG] 7 6 5 4 3 2 I 0 Fzg ,  i ESR spectrum of the trlplet STE in RbBr at 1 3 K, taken in o- polarlzatlon. The magnetlc fleld is parallel to a <ll0> dlrectlon of the crystal. The arrow indlcates the 0 o tran- sition shown ~n the level scheme ~n the inset which was used for the recordlng of the translent response shown in flgure 2. The mlcrowave frequency was 9.1 GHz and its amplltude modulatlon frequency 1 KHz. The llnes are classlfled accor- dlng to the angle between the STE axls and the magnetlc fleld. The transient response of the STE spln system to the microwave fleld Is quite dlffe- rent from other sp~n systems as for example close F centre palrs In the same crystals IS A typlcal transient response to pulse mlcro- wave excitation is shown In figure 2b, for the 0 ° ESR llne of the STE in RbBr shown in flgure 1 When the mlcrowave fleld ~s applled, the transient response shows a very steep increase of the lumlnescence intenslty followed by a slow decrease to the steady state. We shall call the rlse time 31, and the two fall tlmes allowed levels havlng the radlatlve llfetlme 3 It should be polnted out that for a glven ESR llne, the same klnd of translent response was observed elther in o + or ~n o- polar~- zatlon In order to understand the translent res- ponse, we assume that the exclton created by the X rays relax in each trlplet level wlth equal probab111ty R, and that one phonon non radlatlve transltlons occur between the tri- plet STE levels Due to the dlfferent spatlal wave function of the A1u , the B2u , B3u (resp Vol. 26, No. 3 OPTICAL DETECTION OF SPIN RELAXATION PROCESSES 183 (o) r~ - ' I  r~  f . . .  l=} 1 ,1  Z -7 -  L J  >-  Z i , i  Z (b) RbBr 1.3 K 590 nm " __ I t .  i L - -  _ _ . . . . .  . _  L (el L.--- I I I I I I I I I 0 10 20 30 time [msec] I I emission 50 Fig. 2 Translent response of the o- STE emlsslon intenslty (b) to a pulse mlcrowave excltatlon (a), correspondlng to the M S 0 * - i ESR llne shown In flgure i. The response shown in (b) is the output of the slgnal averager after 10 4 passages. Response (c) is a computer slmulatlon of (b) by uslng equa- tlons (I). The recovery tlmes T 3 and ~ are too long to be observed ~n the t~me scale of thls fxgure. Table I. Values of the response tlmes for the hlgh fleld O ° llne of the trlplet STE in RbCI, RbBr and CsBr, measured In u- polarization at 1.3 K. Emission energxes and llfe- times taken from ref. 4 (a) and ref. 14 (b) are also shown. RbCI RbBr CsBr Emlsszon peak Llfetzme (ev) (m sec) (a) Ca} 2.27 5.50 2.1o (a) o.18 (a) (b) (b) 3.55 0.015 • I T2 ~s T{ T~ T~ (g sec) (m sec) (m sec) (m sec) (m sec) (m sec) 60 62 5 130 <_ 40 2.6 55 0.2 5.4 93 i0 0.36 > 6 0.02 0.8 > i0 E u) leve ls ,  we shal l  assume that the frequency factor  (F) for the one phonon t rans i t ion  rates between leve ls  which xnclude the Alu leve l  1.e. Alu+~B2u and Alu~-~B3u (resp. Alu~+E u) is  d i f fe rent  of that (F ')  which describes the one phonon t rans i t ion  rates  between B2u~-+Bsu (resp Eu~+E u) levels. The energy dependence of all these one phonon transitxon rates wxll be taken as proportzonal to the cube of the levels energy difference. The populations n z of the three levels wzll then be connected by the followlng rate e~uatlons, dn i 3 dt = R + Z cxj nj (i) j=l 184 where the rate constants Czj involve the ra- diative • and non radiative decays vzj, the microwave induced transition rates Mete  Indexing the levels Alu , B2u and BSu of the STEzn  RbBr respectively as l, 2 and 3, typl- cal CIO rate constants used for the analysis of the transient response w~ll be 1 1 CII = - ~ (n12 +i) - - -  n13 - M TI2 T13 1 1 C22 = T T I2  1 1 C3~ = T TI3 OPTICAL DETECTION OF SPIN RELAXATION PROCESSES Vol 26, No 3 (2) i C12 = TI 2 n12 + M - -n12  - T23 n23 - M - -  (n13 + i )  - l-l-- (n23 + i )  T23 where the ni~ is g~ven by the Planck function and the one phonon transition rates Tij are  given by j = 2,3 (4) 1 = F ie  I -e j l  3 T I j  1 - F ' I~-~I  ~ ~23 The computer s~mulatzon of the transient response of the triplet STE corresponding to the ESR line Indicated in figure 1 zs shown in figure 2c As observed experlmentally, the model predicts the same kind of behavlour for both q+ or o- polarization A model zn which the frequency factors F and F' are identical cannot explain this last observation. For the case shown zn figure 2c, the best flt for the transition rates are TI2 ~ 160 msec, ~13 = 14 msec, ~23 = 0 12 msec The tempera- ture dependence due to the change of'the Planck function ~mj and the magnetic field dependence predicted by the model are also in good agreement with the experimental observa- tion The latter process can be easily tested in CsBr crystal, where the zero field split- ting D is large (D = 6240 Gauss) Marked change In the transient response times are observed between the low field and high field 0 o resonance lines due to the ~zfference zn -i 3 the phonon transition rates ~ij ~ lej -Ezl (eq 4) The transient response to pulse microwave excztatlon was also measured zn the ESR lines of STE having their axis at 60 o or 90 o with respect to the external magnetic fleld The situation is somewhat different and more com- plicated because under magnetic field the levels are mixed and the radiative transition to the Agground state become partially allowed ~rom all states. Nevertheless zn all measured transitions there are six charac- teristic response tlmes, which depend on the particular ESR line considered. The model prediction of the shape of the transient res- ponse at a given ESR hne  actually depends mainly on the field mixing between the Alu, B2u and B3u (respectively Alu and E u) states through the one phonon transition frequency factors F and F' In concluslon, we should point out that although quite simple, the model presented in this report glves reasonable qualitative and quantitative description of the unusual expe- rimental spin relaxation processes in the STE relaxed triplet state of alkall halide crys- tals. We therefore confirm that these pro- cesses occur mainly via one phonon direct transitions between the triplet levels, the Alu level being weakly coupled to the other two levels. Further experiments are under way and the complete results will be published in a forthcoming paper. Acknowledgments - We are grateful to the Swiss National Sclence Foundation for finan- cial support. (JFW) is grateful to the Physics Department of the Pontlfzcla Unlversz- dade Cat61zca do Rio de Janezro and (YM) to the Department of Applied Physlcs of the Osaka City Unzverslty for a partlal flnanclal aid REFERENCES i. KABLER M N., Physical Review 136, 5A, 1296 (1964) 2 MURRAY R.B and KELLER F J , Physical Review 137, A942 (1965) 3 SIDLER T., PELLAUX J P , NOUAILHAT A and AEGERTER M., Solid State Communlcatlons 1A3, 479 (1973) 4 KABLER M N and PATTERSON D A , Physical Review Letters 19, 652 (1967) 5 PURDY A.E , MURRAY R B., SONG K S and STONEHAF A M ,Physzcal  Revlew BI5, 2170 (1977) 6 PELLAUX J P , IIDA T. and AEGERTER M., Proceedmngs of the Internatmonal Conference on Defects in Insulatzng Crystals, Gatlznburg, Tennessee, U S A , 336 (1977) 7 MARRONE M J and KABLER M N., Physzcal Revzew Letters 27, 1283 (1971) 8 FALCO L , yon der WEID J P and AEGERTER M ,Proceedzngs of the Internatzonal Conference on Defects mn Insulatlng Crystals, Gatlznburg, Tennessee, U S A , 126 (1977) 9 MARRONE M J , PATTEN F W and KABLER M N., Physmcal Revzew Letters 31, h67 (1973) Vol.  26. No. 3 OPTICAL DETECTION OF SPIN RELAXATION PROCESSES i0. WASIELA A., ASCARELLI E. and MERLE D'AUBIGNE Y., Journal de Physlclue 3h, C9-123 (1973). ii. CALL P.J., HAYES W., HUZIMURA R. and KABLER M.N., Journal of Ph.Tslcs C, 3, 1815 (1970). 12. FOWLER W.B., MARRONE M.J. and KABLER M.N., Physlcal Revlew ]38, 5909 (1973). 13. SCHNEGG P.A., JACCARD D. and AEGERTER M., Physlca Sta%us So116[i (b) 63, 587 (197h). lb. POOLEY D. and RUNCIMAN W.A., Journal of Ph.yslcs C 3, 1815 (1970). 185 

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Optical detection of spin relaxation processes in the triplet state of the self trapped exciton in alkali halides

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