Journal of Molecular Structure, 61 (1980) 5-10 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

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1 Jurnal f Mlecular Structure, 61 (1980) 5-10 Elsevier Scientific Publishing Cmpany, Amsterdam - Printed in The Netherlands 5 APPLICATION OF THE SYNCHROSCAN STREAK CAMERA TO REAL TIME PICOSECOND MEASUREMENTS OF MOLECULAR ENERGY TRANSFER M.C. ADAMS,.J. BRALEY, W. SIBBETT and J.R. TAYLR Optics Sectin, BlacKett Labratry, Imperial Cllege, Lndn, SW7 2BZ CGt. Britain) ABSTRACT Using the cntinuusly perating "Synchrscan" streak camera (time reslutin 3-5 psec) in cnjunctin with the synchrnus excitatin pulses frm a C.W. mde lcked dye laser, singlet - singlet resnance energy transfer frm DODCI (dnr) t malachite green (acceptr) r t DQOCI in ethanlic slutin has been.. ". -4. lnvestlgated. Fr a flxed dnr cncentratln f 10 M, varlous acceptr cncentratins frm 10-5 M t 7.5 x 10-3 M were studied, and the critical transfer distance R determined t have average values f 45.6 ~ and 61.8 ~ fr malachite green and DQOCI respectively. INTRODUCTION In the mde-lcking f the C.W. rhdamine 6G dye laser it has been reprted (1) that the additin f malachite green (4,4' tetramethyldiamintriphenylmethane chlride) r DQOCI (1,3' diethyl 4,2' quinlylxacarbcyanine idide) t the mre cmmnly used saturable absr~er DODCI (3,3' diethylxadicarbcyanine idide) can lead t increased stability f peratin well abve threshld (1,2,3). Resnant transfer f excited electrnic state energy has been suggested as a mechanism t explain the abve bservatin (4), and a study f this is interesting frm the pint f the mde-lcking prcess. The "Synchrscan" streak camera greatly facilitates the carrying ut f these excitatin energy transfer measurements, since cmplete flurescence decay prfiles can be recrded at repetitin rates f ~140 MHz with a time reslutin limit f ~3 psec. In additin, lw peak pwer picsecnd pulses (~200 W) frm a C.W. mde-lcked dye laser perating in synchrnism are utilized fr excitatin, thus aviding nn-linear effects in the sample. EXPERIMENTAL The experimental arrangement is shwn schematically in figure 1. A synchrnusly pumped mde lcked C.W. rhdamine 6G?ye laser (5) was the surce f the picsecnd excitatin pulses. Calibratin and linearity f the streak was checked using the

2 6 Fig. 1. Experimental arrangement. Michelsn interfermeter arrangement f mirrrs pulses was fcussed using lens L1 (f = 7.5 cm) mum peak pwer densities ~5 MW cm-2 were used. respect t the incident beam was passed thrugh M5 and M6' The train f excitatin int the sample cell C. Typical maxi The flurescence detected at 900 with a plarizer P set at t elim- inate any distrtinal effect n the recrded flurescence prfiles due t flurescence deplarizatin arising frm rtatinal diffusin (6). After cllectin and cllimatin the light was passed thrugh a mnchrmatr (bandwidth 1.5 nm) and the selected flurescence was directed with lens system L3 n t the slit S f the streak camera, where it was fcussed n t the S20 phtcathde f the Phtchrn II streak tube in the usual manner (2). Synchrnism f the repetitive sinusidal ramp vltage driving the deflectin plates f the camera was achieved by directing a percentage (~5%) f the mde lcked dye laser utput, ff beam splitter BS1, n t a fast phtdide. The resulting electrical signal triggered a tunnel dide scillatr, the utput f which at 140 MHz, was amplified t prduce the driving vltage. In this way successive streaked images were precisely superimpsed at the 140 MHz repetitin rate while maintaining time reslutin dwn t 3 psec. The deviatin frm linearity is better than 3% and the camera has demnstrated a dynamic range f 4 x 103. Using an ptical multichannel analyser lens cupled t the streak camera phsphr,integratin and strage f the recrded image fr subsequent display n a chart recrder CR was pssible. The excitatin wavelength was set t 575 nm and the peak f the bandpass f the mnchrmatr t 615 nm (just belw the flurescence peak f the nrmal ismeric frm" f DODCI). In the absence f acceptr mlecules a recrded flurescence lifetime f a 10-4 M ethanlic slutin f DODCI gave 1150 ±40 psec which is in gd agreement with previusly reprted values (7,8). Additin f a 10-5 M slutin f malachite green in ethanl t the DODCI reduced the recrded flurescence lifetime t 1060 psec, and further increase in the malachite green cncentratin crrespndingly led t a mre prnunced decrease in the DODCI lifetime. This can be seen in figure 2 and the = recrded lifetimes are listed in table I. Additin f DQOCI led t a similar effect. The shrtening due t DQOCI was greater than with malachite green due t the larger

3 7.. ~ Flg. 2. Recrded flurescence prflle f 10 M -5-4 DODCI in ethanl with (a) 0, (b) 10 M, (c) 10 M, (d) 10-3 M, (e) 2.5 x 10-3 M and (f) 7.5 x 10-3 M malachite green acceptr. verlap between its absrptin prfile with the flurescence prfile f DODCI. M TABLE I Malachite. Green T (psec) R - (~) DQOCI T (psec) 51 xacceptr Where Frster kinetics f energy transfer are applicable (9,10) the mdified frm f the time evlutin f the flurescence intensity is written 1 I ()exp(-t/t)exp(-2y(t/t)2) D

4 8 where T is the flurescence lifetime in the absence f excitatin energy transfer and Y = CA/C, and CA is the acceptr cncentratin with CO the critical cncentratin given by 3000/2TI3/2NR 3. N is Avagadr's number and the. critical transfer distance R is defined as the distance at which non radiative diple-diple energy transfer has the same prbability as the sum 0vey-all ther de-excitatin prcesses 1 f the dnr excited state. A graph f lg10id(t)/if(t) against t2, where IF(t) is the unquenched flurescence prfile and IF(t) = 1I F()exp(-t/T), shuld give a straight line dependence f gradient -2y/2.303 T2 Such a plt is shwn in 7 figure 3 where the number f pints has been reduced fr clarity, and relates 1000 ",,, ~: 1900!' ' lpw:s«rds)!) '0 '0 Fig. 3. LO~10IID(t)/IF(t) I versus t~ fr a 5 X 10-4 M malachite green acceptr c.ncentratln t a 5 x 10 M malachlte green acceptr cncentratln In 10 M DODCI. Slmllar plts were taken fr all the ther cncentr~tins f the acceptrs shwn in table I, and the critical transfer distance R determined, assuming that Frster kinetics were valid ver the cmplete range f acceptr cncentratins used. Frm table I it can be seen that the value f R tends t level ut at higher cncentratins f acceptr. The unrealistically high R values fr the lwer cncentratins may be due t the excitatin energy transfer rate (which varies as R-6) becmlng.. small at these lwer cncentratlns and the decay f the 1 excited state is nt s well defined by the mdifying term (2y(t/T)2) in the Frster expressin. T~e deviatin f the experimentally recrded flurescence prfiles frm the Frster theretically predicted cmputed curves can be seen in figure 4. Fr a 2.5 x 10-3 M malachite g~een acceptr cncentratin, agreement is quite gd. Hwever, at lwer cncentratins deviatin between thery and experiment is quite apparent (see figure 4(b) fr example fr a 10-5 M malachite green acceptr cncentratin), indicating that the crrectin factr intrduced by the Frster thery is nt valid in this regin. Using the five highest values f acceptr cncentratin used, an. average value fr R f 45.6 Rand 61.8 R were btained fr malachite green and DQOCI respectively.

5 9 In additin the results shw that resnance energy transfer shuld nt play a majr rle in the mde lcking prcess at the typical dye cncentratins used in practice DOOCI 2.SX10-'3MMal&chltcGfftn Fig. 4. Experimental and cmputed curves, assuming Frster kinetics, f flurescence decay f 10-4 M DODCI with (a) 2.5 x 10-3 M and (b) 10-5 M malachite green added. The ability f the synchrscan streak camera t reslve small changes in flurescence lifetimes under lw pwer excitatin greatly facilitates these direct measurements f resnance energy transfer with picsecnd reslutin, and shuld therefre find wide applicatin. Further studies are planned in which energy transfer between DODCI and its phtismer will be investigated. A mre detailed accunt f the results presented here will be given in a later publicatin. ACKNOWLEDGEMENT The verall supprt fr this wrk by the Science Research Cuncil ls gratefully acknwledged.

6 10 REFERENCES 1 E.P. Ippen and C.V. Shank, App. Phys. Letters 27 (1975) D.J. Bradley, Tpics in Applied Physics, Vl. 18 (Springer Verlag, 1977) I.S. Ruddck, PhD Thesis, University f Lndn (1976). 4 G. Prter and D.J. Tredwell, Chem. Phys. Letters 56 (1978) J.P. Ryan, L.S. Gldberg and D.J. Bradley, Optics Cmmun. 27 (1978) H.E. Lessing and A. vn Jena, Chem. Phys. Letters 42 (1976) J.C. Mialcq, A.W. Byd, J. Jaraudias and J. Suttn, Chem. Phys. Letters 37 (1976) D. Magde and M.W. Windsr, Chem. Phys. Letters 27 (1974) Th. Frster, Discussins Faraday Sc. 27 (1959) J.B. Birks, Jurn. Phys. B 1 (1968)

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