Spin—lattice relaxation of linear triplet excitons in 1,4 dibromonaphthalene

Vol;l+~2.Tl~m~cr 1,

_’,: ,.. .-. I. .: ,,: ,’ :’ -: .,

CHEMICAL PHYSICS. LElT$s

:

.:.

.. .

.; .

.:,-., :-

.’ ..

..

I’

.,. -..

SPIN-L/&-WE

1 April 1975

...

‘.‘. .’

..

;. .’

REiAXATION Ok LniVti .’ :

: ,m,“i;4 DIDROMONAPHTtiLENE’ :

.’

.,-

TRIPLET EXCITONS :

-.

R. SCHrYriDkEkGER~and H.C. WOLF, Ph_vsikolisclrfs Imtitr~t, T$mirut Rcccivcd,l5..Scptcmbcr

3,~Lhir~&t~t S&t!gart, Stutrgart, Gerhy

1974

‘.

:

,_

‘.. ‘.

.‘, spin-lattjce relaxation (SLR) of linear triplet excitons in I,4 dibromonaphthalcne (1.4 D?N) sin& crystals at liquid ~~,eliktemperatures and at room temperature. SLR was found to be temperature dcptidcnt but isotropkrvith respect ‘to the T;;ternal magnetic field.

Wereport

mcasuriincnts

on

,’ 1. Introduction

..

I

Recently we reported ESR measuiemetits on linear triplet cxcitons in I,4 DEN. From an atialyss lincsl~agc we go: detailed ktfti!mation on exciton dy- ’ namics [I ,2]. In this paper’we report spin-lattice relaxotion (SLR) measurements of these excitons at dilfercni tempizratures. The SLR of’excitons in DBN was found to be faster than that of X-tra’ps.(isol&ed molcculcs). Fro.m this fact we conclude that S&R of excitons in I,4 DBNis a specific excitonir, process.

2. ~~perinenti31

.

:

We used t:.lio different methods for investigating SlR of triplet excitons in 1,4 DBN.‘The first method ‘. Microwave atten;ation I dB is that of measuring the product T1 T2 by~q&itinuous : ‘. . saturation of a? ESR transition. The interpretatiori of___ Fie 1. Contir.uous saturation of an exciton ESR transition in 1,4 DhN at vzious orientations and temperat’dres: linewidth those measurements be&es difficult if the ESR Lines are not homogekotisly broadened. ’ : ,. -. and ESR intclsity as a function of microwave power a’t’1.3 K ‘(kft) md robrn temperature (rightj. The full circles arc measAs a direct methgd for, measuring SLR times we ured.valks, t% solid line is’calculated for homogeneous .:_’ _us.xl pulse-saturation experiments with optical delec-.. _’ broaddned lines from Bloch’s equations. : tion. For this purpose we i&d the niic+aves’with 1. ,.. :, ,’ a fkrit modulator (50 dB in 3 11s)and p5sqved the ., .. : :. ...,_~ ‘. 1, ‘, :‘. ::. change in phbsphb;esc&-ide intensity’ of one ” .. ., ,’ o$icailjr resolved, &eman.&mporien’t. .. :. _ .‘decay [ 11.so that the recqveiy of. the phospho&ende -It will be shown in’th‘is$per that the SLI? if ;he : ,‘,,alter r $@tcting off the sFtur$ion pulse’directly.$ves .. . , .. ... .‘. ,:cxcito,ns in .i,4 DBN‘~s.muc;li,fast& t,Qan thti opticzil, .’ :: :the’SLk tke. ,: .:“__

.-.,.

.; .” .. ‘,_. j ,‘...

:’

.,;, _ ‘.,I.

I.

.’

,, ; ;, .:. .:

:;>

-..:.,. :

:.

‘.

; ‘_.,):

.:-

. ..: ‘. .,

..:. ...‘... -,

.’

-.

... ,,

.. -,

-,

..:.:.,;.: .: I _; .,., ” ... ., -,-,-,’>

CHEhlICAL PHYSICS LETTERS

Volume 32, number 1

I April 1975

,

1

b ORIENTATION

OF

The values were measured by pulse saturation with optical detection. The orientations II z nnd 1 z correspond to the directionsx* andy” of the exciton tine structure tensor in nnthracek or naphthalcne [3].

3. Results Fig. 1 shows the results of continuous saturation measurements of triplet excitons in I,4 DEN at low and room temperature and for different orientations .with respect to the magnetic field. The solid lines, are calculated for.homogeneously broadened lines from Bloch’s theory. It is evident from fig. 1 that at low the ESR lines are not purely

II,,

5 10 20

,

homoge-

,I

513 100

TEMi=fERATURE

Ho

Fig. 2. Spin-lattice relasation time r, of triplet excitons in 1.4 DBN for the mqnetic field in the oc crystal plane at 1.3 K.

‘temperatures

2

300

TIK

Fi& 3. Temperature dcpenderrce of Si_R time TI of triplet cxcitons in 1,4 DBN. The values were calculated from continuous saturation

mcasurcmcnts;

Pulse-saturntion measureare in agrecmcnl lL
mants a! 2 K and lower temperatures

these results.

Measurements of SLR at room temperature were possible only in orientations with narrow ESR Lines [2] because the microwave power was too Iow to saturate the ESR transition in other orientations. dependence of SLR Fig. 3 shows the temperature in an orien’tation with the mapetic CeId paralIe1 to the b crystal axis. We calculated the SLR probability \v From the SLR

time

T1 using the tionstztionary

solutions

of the

three level fate-equacians and t2ki11i into account [he neously broadened_ This is consistent with the fact that dependence Boltzmann factors 141. The temperature the ESR lines are non-lorentzian a? these temper,atures of w in the range between 1.2 and4.2 Kis \c’0: T2a7‘1.. [l]. Nevertheless, ye treated these measurements like homogeneously broadened lines, i.e., we took the value for T2 from the ESR linewidth”Gd calculated 4. Influence of spectral diffusion. Ti from the microwave power at the saturation maximum. The results are given in fig. 1. In addition we The accordance’between the results of continuousused at low temperatures. the method of pulse saturaand pulse-saturation measurements shows that the detion with optical detection which gives directly‘the viation from:homogeneous broadening of the ESR SLR time. This method cannot beapplied’ at’tcmpe.r-‘. ‘- lines is not very serious. That means that either the atures above 2K due to the’rise time of the ferrit mohomogenep& linewidth is in the order pf the,/nhomoddat& (3 ~5). The result al .1.3K is shown in fig..2 geneous broadening or that there exists a rapid spectral for s&&al orientations of.the mhgnetic field in the s&_diffusion witl$n:the inhomogeneousIy broadened ac plane of the crystal. .’ Withirlee .line. The same cqnclu$on was made from ODMR lineexperimental erroi bqth‘methdds give ~~‘ &p.[5]. : : ... ., “thesame SLR tiink : Frpm our.E$R lineshape tinatysis [2] -we.tiowrhat -, From fi& 2 we-conclude that. there‘is no anisotxopy. exciton mi&tlon &thin the linear chti,ns is coherent ” df S,LR in the ac crystd' plane_-The'satie ijotr?pic{ &$,ht be po&b!e : it loti temperatures.,Therefore;,it :-, behavior of SLR was fo&d ‘& the a& pIa&. :. : ., .. : . ‘_ I...‘., : ;. ‘. ..,‘,,‘, ‘. .. .’ -. 19 : ., ,, : : : ;.. -., ‘. . ~5 .. ._-. : _: .. ., . -. .’ .,:-.‘.__.,_, :.. :;, ‘ ,, :: .. ),.’ _, .: .‘._ - : ., :.-.. .

y&rinc .,.,._

thii

‘.

32;numbdr.l

-,

: CHEMIC+,PHYSICS

LETTERS

1 April 1975 ‘.

.’

‘.

&‘loti

r&dual

kmpe&ure

li&width’[2]

is due

1,4 DBN is not yet understood

’ to

ari’ . . inhbmoginzous broadening as a corkecjuence ‘sf_thi-fict, that different’excitons with difttrent k-

chair,.jumpind the relaxation cene,[3]. -.

: ‘kctdrs have:differerit ESR trallsition frequencies. Exi!:dito‘i+h&& &upling lkads to a k-scatiering of the kitons and thus to a sp&tral.spin diffusion-within ;: i&z inhomogeneously broadened line. In ref. [2] we. :finterireted.the temperature-dependent increase of . ESR linewidth with.hn incrbase of the local flu&&

in’kntinuous

3 d&i?tiqn ;‘_ :.

from

saturatidn

Lomoge~aous

beha*pur

,’

The cr$ls for our experiments were grown in the Stuttgarter Kri:trdlabor. This work was don& with the financial support of the Deutsche Forschungsgemeinschaft. ,’ :.

broadened [5],

experiments

.

.’

&tifetiirie,o[~k = 1/2ro z= iO+ s at this temperature. This’time is shorter than the SLR time so that in whereas

‘of tile excitorij [1,2,4] in analogy to lnechanism of triplet e+citor$in anthra-.

Acknowledgement.

tions YO=%.7 X 10m7 lf cm-‘/K. AG.umEg that the ?‘3 .dependence is’valid down .to l;? K we find a

:,pDMR the,line lotiks,homogeneously

in detail. We assume.

that it is goverIled by the non-linear behaviour (i&r-

tlicre

is

(fig. I). References

_. 5. Cc&l&xx

:

‘.

il J R. SchmidbeTger and H.C. Wolf, Chem. Fhys. Letters 16 (1972) 402.

[71 R. Schmidbergr “d I-X. Wolf, Chem. Phys. Letters 25

The measured qalues of T1 are determined by ex-‘&ton motion. This iSshown,by mea&rig SLR of Xtraps (isola’ted molecuk~) in .1,4..DBN:-At 2 c, T of the.triplet state tif X-traps is in the order of lo-. !I s, th@ is’two. orders of magnitude lor.ier ihzn lhat of excitons. ;:, .,.

The mechanisti ; -;:, -’ : :

of TI of trijlet.excitons ;

:.

.-

‘:

51974) 185.

[3] ,D: Haarcr and H.C. Wolf, Mol. Cryst 10 (1970) 359.’ [4] R. Schmidber,;er, Dissertation, Univeqitit Stuttmt(1974): (5 1 R. Schmidbcr.gr and.Hk. Wolf, Chem. Phys. Letters 32 (197ij 21, ,.

in

.,

..

..’

‘.. ., ,:..;

_.

.: _. _:‘..., ‘,

‘.

: _-:_--.

:

‘_ :

.’ ._. .-;

:.,.

. ‘.

.,

“‘,”

...’ ,,.

.-.”

. . .

,,_; .,;. ’ : : ,‘.,- ,‘,I “-I _‘.‘. :..;.-. ‘.,.,_ ,.’ :; .. .: ,: .‘I ‘_: ‘. ‘. .._ . .: . . .. ,. ; .- /,. __ ‘. ‘- .. .-.,_: ._-.‘:- : :.; -_;< . _< ,. .:, :, ; .’ _-*. .‘... ;: _;, ‘. :_ -.. -‘: :.:-. ,. , .._ ._.._ : _‘: .:.:, ‘: : ‘, _, ._> -’ :, ,. ; 1.z.

‘.

. :

_: :, :,

:

‘. : ..,

._

.._

.,

.-. ,. .- .___. 1,.,. :‘, ‘. .. ,,, ‘I....’ _ ..,

,,.‘.

.’

,_

‘..

-’ .’

,:,; /. . -, ..

10 ,,:.::.: .,; ..I:,’‘.::..‘, ‘,<.-._,‘.;:‘:- .,-. ‘:i;;.i, :‘,y: .:.:,C. ..‘. L_, ..::.:.. ::- .:‘_.’ _,- ,: ,;: :.-_‘,,., : :: ,. _..; ,,_: -, :_ :,,:._,‘>,:.,:; .: ~,jj,: :.. . . ;’ --, : ..“_’ :. ,..:. .. _‘.. .., ,;_ ._‘:.~.‘ !., ,,.,.. .-.. ; ,. ,, ‘-

‘.’ ,, ,.‘..

:; ,.

-( ,’

‘.,

., .,‘..

‘. .’ ,’ ,‘!

.,

:

‘,.

._

,.

.’

:.,

.,

., ._ .,

:’ .,

,;’ : - ” .:. : : :ql. ., .;:, : .; _‘_ ‘,,. ‘:

:,

-.

:;, ‘. :: .’ .’ ..(

.-

j:.. :.

..

” :-.

..,

.. .. .._-. ,_ . . . . “._ :: ... . ‘__ ,; ., . .. :‘.‘ . -. ., .,:” : I’ ,. ..’ _‘. .‘. .,” I , -. ,, ,_. ,,,, ; ‘. ;,_ .,._’; ;..-. : ..-. ., ::. : : :. ; _’ :. ; .:; _. ” ‘, . _:. ,-,.‘., ::, ‘. .. .._ :, 1 :.. ,. _, ‘: _.., ‘,, ,: ; . ‘. i:,,_‘.: . . .I._ _.:.:, ,,. .-; ,+,“, ,:.‘ :. _. ::: ..: .: :‘j’,__,’ ;;+., ‘. ., ., ,:,:... ‘,_y.,‘.-;‘,.;I,:, ,; ::.,,;: .._-’ ‘. : :; Y’(. ‘.,::.; . : .. : ,I:,. ,.: :,:,. .I’ I .:_. :....‘. : .. ,;

” ,,,

;.

‘.,

.

:‘.!